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Estimated PCAT Reading Comprehension Score: 200
Just know, when you truly want success, you’ll never give up on it. No matter how bad the situation may get. Keep your head up and keep on fighting!
Estimated PCAT Reading Comprehension Score: Less than 350
Just know, when you truly want success, you’ll never give up on it. No matter how bad the situation may get. Keep your head up and keep on fighting!
Estimated PCAT Reading ComprehensionScore: Less than 330
You’re on the right track. Take your time to reflect on your performance and how you can improve your scores the next time around. Carefully review these solutions, learn from your mistakes and understand the intricacies of each question. You’re going in the correct direction and you’ll only go up from here!
Estimated PCAT Reading Comprehension Score: 340
You’re on the right track. Take your time to reflect on your performance and how you can improve your scores the next time around. Carefully review these solutions, learn from your mistakes and understand the intricacies of each question. You’re going in the correct direction and you’ll only go up from here!
Estimated PCAT Reading Comprehension Score: 370
You’re on the right track. Take your time to reflect on your performance and how you can improve your scores the next time around. Carefully review these solutions, learn from your mistakes and understand the intricacies of each question. You’re going in the correct direction and you’ll only go up from here!
Estimated PCAT Reading Comprehension Score: 390
You’re doing a good job! Keep working on it and you’ll soon see your score in the 20’s. Take your time in understanding your mistakes and in carefully reviewing these solutions and learning from the intricacies of each question.
Estimated PCAT Reading Comprehension Score: 410
Good going! You are really getting to where you need to be. Keep it going! Take your time in understanding your mistakes and in carefully reviewing these solutions and understanding the intricacies of each question. Your goal should be to beat your 410 on the next test! Every point you get correct will get you closer to the perfect 600!
Estimated PCAT Reading Comprehension Score: 430
Good going! You are really getting to where you need to be. Keep it going! Keep on working on it and you’ll soon see your score in the 500’s. Take your time in understanding your mistakes and in carefully reviewing these solutions and understanding the intricacies of each question. Your goal should be to beat your 430 on the next test!
Estimated PCAT Reading Comprehension Score: 450
Awesome job! Keep it up and you’ll soon be in the 500’s. Learn from your mistakes and strategize on how you’ll beat your 450!
Estimated PCAT Reading Comprehension Score: 470
Awesome job! You did it! You really outdid yourself today. What can we do differently on the next exam to get yourself up to 500? Lets do it!
Estimated PCAT Reading Comprehension Score: 500
Impressive! You hit 500! Now let’s push you up to the perfect 600!
Estimated PCAT Reading Comprehension Score: 540
You rocked it! That was quite an accomplishment!
Estimated PCAT Reading Comprehension Score: 570 or higher
You are a rockstar! We tip our hats to you!
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
1. Where is the thyroid gland located?
According to paragraph 1, the thyroid gland is “located in the anterior portion of the neck just below and bilaterally to the thyroid cartilage.” Choice A is the correct answer.
According to paragraph 1, the thyroid gland is “located in the anterior portion of the neck just below and bilaterally to the thyroid cartilage.” Choice A is the correct answer.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
2. From what does the thyroid gland develop?
In the first sentence of paragraph 1, it says that the thyroid gland “develops from the thyroglossal duct and portions of the ultimobranchial body.” The best answer is Choice C.
In the first sentence of paragraph 1, it says that the thyroid gland “develops from the thyroglossal duct and portions of the ultimobranchial body.” The best answer is Choice C.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
3. Which is the largest portion of the thyroid gland?
Paragraph 1 states, “The right lobe is normally larger than the left.” Therefore, Choice B is the correct answer.
Paragraph 1 states, “The right lobe is normally larger than the left.” Therefore, Choice B is the correct answer.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
4. The thyroglossal duct is divided by which bone?
Toward the end of paragraph 1, we read that “The thyroglossal duct, as it develops, is “divided” by the hyoid bone, and remnants can become enclosed or surrounded by the bone.” The answer is Choice E.
Toward the end of paragraph 1, we read that “The thyroglossal duct, as it develops, is “divided” by the hyoid bone, and remnants can become enclosed or surrounded by the bone.” The answer is Choice E.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
5. Which of the following is secreted by the thyroid gland?
The first sentence of paragraph 2 says, “The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin.” All of the three answer choices are mentioned, and thus Choice D is correct.
The first sentence of paragraph 2 says, “The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin.” All of the three answer choices are mentioned, and thus Choice D is correct.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
6. Parafollicular cells produce
Paragraph 2 states, “The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin.” Choice A is correct.
Paragraph 2 states, “The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin.” Choice A is correct.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
7. T3 and T4 help to regulate
According to the last sentence of paragraph 2, “T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.” The answer is Choice B.
According to the last sentence of paragraph 2, “T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.” The answer is Choice B.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
8. Which factor does not influence the hypothalmic-pituitary-thyroid axis?
Paragraph 3 discusses about the hypothalamic-pituitary-thyroid axis. It says, “Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.” All but Choice E. Hypothalamic disorder are mentioned here. Therefore, Choice E is the correct answer.
Paragraph 3 discusses about the hypothalamic-pituitary-thyroid axis. It says, “Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.” All but Choice E. Hypothalamic disorder are mentioned here. Therefore, Choice E is the correct answer.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
9. What will modify the hypothalmic-pituitary-thyroid axis?
Paragraph 3 states, “Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system.” Choice C is the answer as you can see.
Paragraph 3 states, “Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system.” Choice C is the answer as you can see.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
10. TRH is release due to
The first sentence of paragraph 4 states, “Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). All of the answer choices are correct.
The first sentence of paragraph 4 states, “Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). All of the answer choices are correct.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
11. What is the function of TRH?
Paragraph 4 says, “TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3.” Clearly, the answer is Choice B.
Paragraph 4 says, “TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3.” Clearly, the answer is Choice B.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
12. Low levels of T3
According to paragraph 5, “T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4.” Low levels of T3 will turn on the release of TSH, and thus Choice A is correct.
According to paragraph 5, “T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4.” Low levels of T3 will turn on the release of TSH, and thus Choice A is correct.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
13. How do the peripheral blood levels of T4 compare to those of T3?
Paragraph 5 states, “The level of T4 in the peripheral blood is 60 times that of T3.” Choice D is the answer without a doubt.
Paragraph 5 states, “The level of T4 in the peripheral blood is 60 times that of T3.” Choice D is the answer without a doubt.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
14. What percent of circulating T3 comes form the conversion of T4 into T3?
The first sentence of paragraph 7 states, “Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4.” Doing the simple math, you should know that “the rest” means 80%-90% of the circulating T3. The answer is Choice C.
The first sentence of paragraph 7 states, “Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4.” Doing the simple math, you should know that “the rest” means 80%-90% of the circulating T3. The answer is Choice C.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
15. All of the following will inhibit the conversion of T4 into T3 except?
The opening sentence of paragraph 8 states, “The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci).” Choice C. Hypertension is not one of the listed items inhibiting the conversion of T4 into T3. Thus, Choice C is the correct answer.
The opening sentence of paragraph 8 states, “The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci).” Choice C. Hypertension is not one of the listed items inhibiting the conversion of T4 into T3. Thus, Choice C is the correct answer.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
16. The minimum daily requirements of iodine is
Paragraph 8 says, “The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine.” Choice B is correct.
Paragraph 8 says, “The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine.” Choice B is correct.
Passage #1: Thyroid Disease
(2) The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. The tissue developing from the ultimobranchial bodies is thought to give rise to the parafollicular cells, which produce calcitonin. T4 and T3 are hormones that affect metabolic processes throughout the body and are involved with oxygen use.
(3) Blood levels of T4 and T3 are controlled through a servofeedback mechanism mediated by the hypothalamic-pituitary-thyroid axis. Increased or decreased metabolic demand appears to be the main modifier of the system. Drugs, illness, thyroid disease, and pituitary disorders can affect the control of this balance. Recent findings also show that age has some effect on the system.
(4) Under normal conditions, thyrotropin-releasing hormone (TRH) is released by the hypothalamus in response to external stimuli (stress, illness, metabolic demand, and low levels of T3 and to a lesser degree T4). TRH stimulates the pituitary to release thyroid-stimulating hormone (TSH), which causes the thyroid gland to secrete T4 and T3/
(5) T4 and T3 also have direct influence on the pituitary. High levels turn off the release of TSH, and low levels turn it on. The effect of T3 on the pituitary is greater than that of T4. T4 is the main hormone secreted by the thyroid. The level of T4 in the peripheral blood is 60 times that of T3. T4 is converted to T3 peripherally by deiodination. T3 is the more active hormone and is the main effector principle. A small amount of an inactive form of T3, called reverse T3 (rT3), is found in circulation.
(6) Goitrins are antithyroid agents that inhibit thyroid hormone synthesis. Foods such as cabbages, turnips, and rutabagas contain them. Thiocyanate, perchlorate, thiourea, methimazole, and propylthiouracil and goitrins. Methimazole and propylthiouracil block both the iodination of tyrosine and the coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) to form iodotyrosine.
(7) Under normal conditions, 10% to 20% of the circulating pool of T3 comes from the thyroid gland and the rest comes from the monodeiodination of T4. In cases of hyperthyroidism, 30% to 40% of circulating T3 comes from the thyroid.
(8) The conversion of T4 to T3 can be inhibited by fasting, illness, steroids, and certain drugs (e.g., propylthiouraci). Iodine must be available for the synthesis of T4 and T3. The inorganic form of iodine as used by the gland comes from the peripheral degradation and deodination of thyroid hormone and the diet. The minimum daily requirement of iodine is about 75 mg, and a typical 2800 calorie daily intake in the Unites States will contain some 700 mg of iodine. It is of interest to note that there has been a gradual increase in the dietary intake of iodine in the United States in recent years. Iodine, which is stored in the thyroid gland, appears to be oxidized to a higher valence by a preoxidase, and then it combines with thyrosyl to form either MIT or DIT. These compounds then, by an oxidative coupling reaction, form either T4 or T3.
(9) The thyroid hormones are stored in the colloid of the thyroid gland. A 3- to-4-month reserve is maintained. Thirty-five percent of the organic iodine content of the thyroid gland is stored as T4, and about 5% to 8% as T3. Phagocytosis of colloid droplets starts the secretion process. The colloid droplets digested by proteases, and, once freed, thyroid globulin (TG), T4, T3, and small amounts of rT3 are secreted into the blood.
(10) In the blood, T4 and T3 are almost entirely bound to plasma proteins. The binding plasma proteins are thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). The most important thyroid hormone-binding serum protein is TBA. TBA binds 70% to 75% of bound T4 and T3. TTR binds only to T4. TTR transports only about 15% of T4, but its contribution to free hormone is comparable to that of TBG die to its higher dissociation constant. TBA binds poorly to both T4 and T3. TBG binds both T4 and T3 but has less affinity for T3. Therefore, the free circulating level of T3 (FT3) is nearly 10 times greater than the free level of T4 (FT4). Only 0.02% to 0.03% of FT4 and about 0.3% of FT3 are in plasma.
17. Which of the following is not a binding plasma protein?
In paragraph 10, the binding plasma proteins are mentioned: thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). Choice D. PDA is not a binding plasma protein.
In paragraph 10, the binding plasma proteins are mentioned: thyroid-binding globulin (TBG), thyroid-binding prealbumin (now referred to as transthyretin [TTR]), and thyroid-binding albumin (TBA). Choice D. PDA is not a binding plasma protein.
