Thyroid Hormones
Thyroid Hormones
Two hormones, triiodothyronine and thyroxine (T3 and T4, respectively) are secreted by the thyroid gland. This large endocrine gland is located in the neck of all vertebrates. The thyroid is composed of thousands of tiny spherelike units, called follicles, where thyroid hormones are synthesized, stored, and released into the bloodstream as needed. The size of the follicles, and amount of stored T3 and T4 they contain, depends on the activity of the gland (Figure 36-8).
A unique characteristic of the thyroid is its high concentration of iodine; in most animals this single gland contains well over half the body store of iodine. Epithelial cells of the thyroid follicles actively trap iodine from the blood and combine it with the amino acid tyrosine, creating the two thyroid hormones. T3 contains three iodine atoms, whereas T4 contains four iodine atoms. T4 is formed in much greater amounts than T3, but in many animals T3 is the more physiologically active hormone. T4 is now considered a precursor to T3. The most important actions of T3 and T4 are to (1) promote normal growth and development of the nervous system of growing animals, and (2) stimulate metabolic rate.
Undersecretion of thyroid hormones in fish, birds, and mammals dramatically impairs growth, especially of the nervous system. The human cretin, a mentally retarded dwarf, is the result of thyroid malfunction from a very early age. Conversely, oversecretion of thyroid hormones causes precocious development in all vertebrates, although its effect is particularly prominent in fish and amphibians. In frogs and toads, transformation from aquatic herbivorous tadpole without lungs or legs to semiterrestrial or terrestrial carnivorous adult with lungs and four legs, occurs when the thyroid gland becomes active at the end of larval development. Stimulated by a rise in thyroid hormone levels of the blood, metamorphosis and climax occur (Figure 36-9). Growth of frogs after metamorphosis is directed by the growth hormone.
In birds and mammals, control of oxygen consumption and heat production is the best known action of thyroid hormones. The thyroid maintains metabolic activity of homeotherms (birds and mammals) at a normal level. Oversecretion of thyroid hormones will speed up body processes by as much as 50%, resulting in irritability, nervousness, fast heart rate, intolerance of warm environments, and loss of body weight despite increased appetite. Undersecretion of thyroid hormones slows metabolic activities, leading to loss of mental alertness, slowing of heart rate, muscular weakness, increased sensitivity to cold, and weight gain. An important function of the thyroid gland is to promote adaptation to cold environments by increasing heat production. Thyroid hormones stimulate cells to produce more heat and store less chemical energy (ATP); in other words, thyroid hormones reduce efficiency of cellular oxidative phosphorylation. Consequently many cold-adapted mammals have heartier appetites and eat more food in winter than in summer although their activity is about the same in both seasons. In winter, a larger portion of the food is being converted directly into body-warming heat.
Synthesis and release of thyroid hormones are governed by thyrotropic hormone (TSH) from the anterior pituitary gland (Table 36-1). TSH is regulated in turn by thyrotropin-releasing hormone (TRH) of the hypothalamus. As noted earlier, TRH is part of a higher regulatory council that controls the tropic hormones of the anterior pituitary. TRH and TSH control thyroid activity in an excellent example of negative feedback). It can be overridden, however, by neural stimuli, such as exposure to cold, which directly stimulates increased release of TRH and thus TSH.
Some years ago, a condition called goiter was common among people living in the Great Lakes region of the United States and Canada, as well as some other parts of the world, such as the Swiss Alps. This type of goiter is an enlargement of the thyroid gland caused by deficiency of iodine in food and water. By striving to produce thyroid hormone with not enough iodine available, the gland hypertrophies, sometimes so much that the entire neck region becomes swollen (Figure 36-10). Goiter caused by iodine deficiency is seldom seen in North America because of the widespread use of iodized salt. However, it is estimated that even today 200 million people worldwide experience varying degrees of goiter, mostly in high mountains of South America, Europe, and Asia.
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| Figure 36-8 Appearance of thyroid gland follicles viewed through the microscope (approximately ×350). When inactive, follicles are distended with colloid, the storage form of thyroid hormones, and epithelial cells are flattened. When active, the colloid disappears as thyroid hormones are secreted into the circulation, and epithelial cells become greatly enlarged. |
Two hormones, triiodothyronine and thyroxine (T3 and T4, respectively) are secreted by the thyroid gland. This large endocrine gland is located in the neck of all vertebrates. The thyroid is composed of thousands of tiny spherelike units, called follicles, where thyroid hormones are synthesized, stored, and released into the bloodstream as needed. The size of the follicles, and amount of stored T3 and T4 they contain, depends on the activity of the gland (Figure 36-8).
A unique characteristic of the thyroid is its high concentration of iodine; in most animals this single gland contains well over half the body store of iodine. Epithelial cells of the thyroid follicles actively trap iodine from the blood and combine it with the amino acid tyrosine, creating the two thyroid hormones. T3 contains three iodine atoms, whereas T4 contains four iodine atoms. T4 is formed in much greater amounts than T3, but in many animals T3 is the more physiologically active hormone. T4 is now considered a precursor to T3. The most important actions of T3 and T4 are to (1) promote normal growth and development of the nervous system of growing animals, and (2) stimulate metabolic rate.
Undersecretion of thyroid hormones in fish, birds, and mammals dramatically impairs growth, especially of the nervous system. The human cretin, a mentally retarded dwarf, is the result of thyroid malfunction from a very early age. Conversely, oversecretion of thyroid hormones causes precocious development in all vertebrates, although its effect is particularly prominent in fish and amphibians. In frogs and toads, transformation from aquatic herbivorous tadpole without lungs or legs to semiterrestrial or terrestrial carnivorous adult with lungs and four legs, occurs when the thyroid gland becomes active at the end of larval development. Stimulated by a rise in thyroid hormone levels of the blood, metamorphosis and climax occur (Figure 36-9). Growth of frogs after metamorphosis is directed by the growth hormone.
In birds and mammals, control of oxygen consumption and heat production is the best known action of thyroid hormones. The thyroid maintains metabolic activity of homeotherms (birds and mammals) at a normal level. Oversecretion of thyroid hormones will speed up body processes by as much as 50%, resulting in irritability, nervousness, fast heart rate, intolerance of warm environments, and loss of body weight despite increased appetite. Undersecretion of thyroid hormones slows metabolic activities, leading to loss of mental alertness, slowing of heart rate, muscular weakness, increased sensitivity to cold, and weight gain. An important function of the thyroid gland is to promote adaptation to cold environments by increasing heat production. Thyroid hormones stimulate cells to produce more heat and store less chemical energy (ATP); in other words, thyroid hormones reduce efficiency of cellular oxidative phosphorylation. Consequently many cold-adapted mammals have heartier appetites and eat more food in winter than in summer although their activity is about the same in both seasons. In winter, a larger portion of the food is being converted directly into body-warming heat.
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| Figure 36-9 Effect of thyroid hormones (T3 and T4) on growth and metamorphosis of a frog. The release of TRH from the hypothalamus at the end of premetamorphosis sets in motion hormonal changes (increased TSH, T3 and T4) leading to metamorphosis. Thyroid hormone levels are maximal at the time forelimbs emerge. |
Synthesis and release of thyroid hormones are governed by thyrotropic hormone (TSH) from the anterior pituitary gland (Table 36-1). TSH is regulated in turn by thyrotropin-releasing hormone (TRH) of the hypothalamus. As noted earlier, TRH is part of a higher regulatory council that controls the tropic hormones of the anterior pituitary. TRH and TSH control thyroid activity in an excellent example of negative feedback). It can be overridden, however, by neural stimuli, such as exposure to cold, which directly stimulates increased release of TRH and thus TSH.
Some years ago, a condition called goiter was common among people living in the Great Lakes region of the United States and Canada, as well as some other parts of the world, such as the Swiss Alps. This type of goiter is an enlargement of the thyroid gland caused by deficiency of iodine in food and water. By striving to produce thyroid hormone with not enough iodine available, the gland hypertrophies, sometimes so much that the entire neck region becomes swollen (Figure 36-10). Goiter caused by iodine deficiency is seldom seen in North America because of the widespread use of iodized salt. However, it is estimated that even today 200 million people worldwide experience varying degrees of goiter, mostly in high mountains of South America, Europe, and Asia.
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