The elevation in body temperature reflects the heat producing or calorigenic

The elevation in body temperature reflects the heat

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The elevation in body temperature reflects the heat-producing or calorigenic effects of thyroid hormones. Regulation of Secretion The regulation of thyroid gland activity is illustrated in Fig. 14.5B. The hypothalamus secretes a hypothalamic releasing hormone, which stimulates the anterior pituitary to secrete TSH. TSH stimulates the thyroid gland to secrete T3 and T4. The Need for Iodine Synthesis of Thyroid Hormone The synthesis of T3 and T4 requires iodine salts called iodides. The iodides come from dietary sources; they are absorbed into the blood and are then actively pumped into the follicular cells of the thyroid gland, where they are used in the synthesis of the thyroid hormones. Tetraiodothyronine, or thyroxine, contains four (tetra-) iodine atoms and therefore is called T4. Triiodothyronine (tri-) contains three iodine atoms and is called T3. Iodine Deficiency Why does an iodine-deficient diet cause the thyroid gland to enlarge? In an iodine-deficient state, the amount of T3 and T4 production decreases because iodine is necessary for the synthesis of the thyroid hormones. Graves’ disease is a hyperthyroid state characterized by bulging eyes, a condition called exophthalmia. The eyes bulge forward because the fat pads behind the eyeball enlarge and push the eyeballs forward in the eye
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With insufficient iodine, thyroid hormones cannot be made in quantities great enough to shut off the secretion of TSH through negative feedback control. Persistent stimulation of the thyroid gland by TSH causes the thyroid gland to enlarge; an enlarged thyroid gland is called a goiter. Calcitonin The parafollicular cells of the thyroid gland secrete a hormone called calcitonin. Although calcitonin is secreted by the thyroid gland, it is not called “thyroid hormone” as are T3 and T4 . The effects of calcitonin are very different from those of T3 and T4 . Calcitonin helps regulate blood levels of calcium and phosphate. Calcitonin is secreted in response to elevated blood levels of calcium and stimulates osteoblastic (bone-making) activity in the bones, thereby moving calcium from the blood into the bone. Calcitonin also increases the excretion of calcium and phosphate in the urine. In general, calcitonin acts as an antagonist to parathyroid hormone. Parathyroid Glands Four tiny parathyroid glands lie along the posterior surface of the thyroid gland. The parathyroid glands secrete PTH.
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The stimulus for the release of PTH is a low blood level of calcium. PTH has three target organs: bone, digestive tract (intestine), and kidneys. The overall effect of PTH is to increase blood calcium levels, which it does in three ways: 1. PTH increases the release of calcium from bone tissue, called resorption.
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