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The thyroid is a large endocrine organ that functions mostly to control metabolism. It is located in the neck between the trachea and laynx and is bi-lobed with a connecting isthmus. The gland is composed of many tiny follicles, that are in effect, each a separately functioning gland with a single-layer epithelial lining. Each follicle accumulates a storage form of the circulating thyroid hormones, thyroglobin. Thyroglobin is a large protein molecule that contains multiple copies of the amino acid tyrosine. The thyroid hormones are very simple modifications of the amino acid tyrosine. Iodide is added to one or two spots on the amino acid and then two of the modified tyrosines are combined to form one of the two thyroid hormones, thyroxin (T4) or triiodothyronine (T3). The thyroid hormones are then cut off the thyroglobin as needed and released into the circulation. The thyroid follicles accumulate iodine by extracting it from the blood and trapping it within the lumen of the follicle. This ability to store homone in a large molecule is unique to the thyroid.
Both T4 and T3 enter cells and bind to an intracellular receptors whereby they increase the metabolic capabilities of the cell. Mitochondria and mitochondrial enzymes are increased thereby influencing cellular metabolism. Thyroid hormones are necessary for normal growth and development. They have metabolic effects on protein synthesis, lipid metabolosm and carbohydrate metabolism.
Also produced by parafollicular cells within the thyroid is the polypeptide hormone calcitonin. It funtions in calcium maintainence to decrease the levels of calcium in the blood. When serum calcium levels are excessive, calcitonin is released. It inhibits bone resorption (by inhibiting osteoclast activity), allows the loss of calcium in the urine and therefore decreases calcium in the blood. It opposes the action of parathyroid hormone and has been used clinically for the treatment of osteoporosis.
Parathyroid
The four parathyroid glands lie on top of the thyroid gland in seperate nodes spread out to the four quandrants of the thyroid. Parathyroid homone is under direct feedback control of circulating levels of calcium. If calcium levels fall, then parathyroid hormone is released. As calcium levels rise, release of the hormone is reduced. Parathyroid homone acts on bones, the kidneys and the intestines to reabsorb calcium.
Both T4 and T3 enter cells and bind to an intracellular receptors whereby they increase the metabolic capabilities of the cell. Mitochondria and mitochondrial enzymes are increased thereby influencing cellular metabolism. Thyroid hormones are necessary for normal growth and development. They have metabolic effects on protein synthesis, lipid metabolosm and carbohydrate metabolism.
Also produced by parafollicular cells within the thyroid is the polypeptide hormone calcitonin. It funtions in calcium maintainence to decrease the levels of calcium in the blood. When serum calcium levels are excessive, calcitonin is released. It inhibits bone resorption (by inhibiting osteoclast activity), allows the loss of calcium in the urine and therefore decreases calcium in the blood. It opposes the action of parathyroid hormone and has been used clinically for the treatment of osteoporosis.
Parathyroid
The four parathyroid glands lie on top of the thyroid gland in seperate nodes spread out to the four quandrants of the thyroid. Parathyroid homone is under direct feedback control of circulating levels of calcium. If calcium levels fall, then parathyroid hormone is released. As calcium levels rise, release of the hormone is reduced. Parathyroid homone acts on bones, the kidneys and the intestines to reabsorb calcium.
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