parathyroid hormone (PTH; parathormone; parathyrin) A peptide hormone secreted by the parathyroid gland in response to low levels of calcium in the blood. It acts to maintain normal blood levels of calcium by (1) increasing the number of osteoclasts, which break down the bone matrix and release calcium into the blood; (2) increasing the reabsorption of calcium and magnesium ions in the kidney tubules, so that their concentration is maintained in the blood; (3) converting vitamin D to its active form, which increases calcium absorption in the intestine. Parathyroid hormone acts in opposition to 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.

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.

parathormone Commonly used as an abbreviation for the parathyroid hormone; correctly a trade name for a pharmaceutical preparation of the hormone.

parathormone (pa-ră-thor-mohn) n. see parathyroid hormone.