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a polypeptide hormone secreted by the parafollicular or C cells of the thyroid gland; it is involved in plasma calcium homeostasis and acts to decrease the rate of bone resorption. Preparations of calcitonin are called either calcitonin-human or calcitonin-salmon; the former is a synthetic polypeptide with the same sequence as that occurring naturally in humans, and the latter is either derived from salmon or is a synthetic polypeptide of the same sequence as that found in salmon. They are used in the treatment of severe hypercalcemia, paget's disease of bone, and postmenopausal osteoporosis. Called also thyrocalcitonin.
A peptide hormone, of which eight forms in five species are known; composed of 32 amino acids and produced by the parathyroid, thyroid, and thymus glands; its action is opposite to that of parathyroid hormone in that calcitonin increases deposition of calcium and phosphate in bone and lowers the level of calcium in the blood; its level in the blood is increased by glucagon and by Ca2+ and thus opposes postprandial hypercalcemia.
[calci- + G. tonos, stretching, + -in]