parathyroid hormone

parathyroid hormone or parathormone, a hormone secreted by the parathyroid glands that regulates the metabolism of calcium and phosphate in the body. It has been purified extensively and appears to be a protein containing 84 amino-acid residues, a sequence of which about 33 to 35 are necessary for biological activity. Parathyroid hormone acts to raise the extracellular calcium concentration, that is, the concentration of calcium ions in the spaces between the cells of the body and in the blood plasma; it promotes the absorption of calcium by the intestine, mobilizes calcium salts from the bones, and increases the tendency of the kidney to recover calcium from the urine. The hormone also enhances both the excretion of phosphate by the kidneys and its uptake by the cells. This removes phosphate, which tends to form a relatively insoluble salt with calcium, from the extracellular spaces, allowing more calcium to remain in solution. Calcium is intimately involved not only in the formation of bone, but also in the functioning of the nervous system; thus hypoparathyroidism, the disease associated with a deficiency in parathyroid hormone secretion, is characterized by muscle spasms leading eventually to generalized convulsions and various psychiatric symptoms. This condition is sometimes successfully treated by the administration of the hormone. Hyperparathyroidism, the result of oversecretion of the hormone, often leads to the resorption of bone and can only be treated by the surgical removal of all of the parathyroid glands, which can be found in unusual locations.

parathyroid hormone (parathormone) n. a hormone, synthesized and released by the parathyroid glands, that controls the distribution of calcium and phosphate in the body. A deficiency of the hormone lowers blood calcium levels, causing tetany. Compare calcitonin.

parathyroid hormone The hormone secreted by the parathyroid glands; four glands situated in the neck near to the thyroid gland but not connected with its function. The hormone is secreted in response to a fall in plasma calcium, and acts on the kidney to increase the formation of the active metabolite of vitamin D (calcitriol), leading to an increase in plasma calcium by increasing intestinal absorption and mobilizing the mineral from bones. It also reduces urinary excretion of phosphate.