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 glands, four small endocrine bodies, located behind the thyroid gland, that govern calcium and phosphorus metabolism. These four masses of tissue (each about the size of a pea) are difficult to distinguish from the thyroid and are often embedded in it. Consequently, before their significance was known they were sometimes accidently removed during thyroid surgery, causing a deficiency in parathormone, the parathyroid hormone. Parathormone increases the concentration of calcium ions in the blood, with accompanying bone absorption and increased reabsorption of calcium ions by the kidneys. The hormone's effect on phosphate ion concentration is the opposite, i.e., phosphate ion concentration in the bloodstream decreases as a result of increased phosphate excretion by the kidneys. Excessive secretion of parathormone, e.g., caused by tumor of the parathyroid glands, is a serious disorder, for excessive blood calcium can cause kidney stones and long-term weakening of the bones. Undersecretion of parathormone, which can be caused by congenital and metabolic disorders, results in too little calcium in the bloodstream, and too much phosphorus. The result is tetany, i.e., violent muscle spasms.

Endocrine gland located close to and behind the thyroid gland. Humans typically have four parathyroid glands. Each gland secretes parathyroid hormone, which regulates blood calcium and phosphate levels. When serum calcium concentrations drop, increased hormone secretion releases calcium from bone into the bloodstream (see calcium deficiency). An increase in parathyroid hormone secretion also increases excretion of phosphate in the urine, thereby lowering serum phosphate concentrations. In addition, the hormone regulates magnesium metabolism by increasing its excretion. When thyroid removal is required, the parathyroid glands must be separated out and left in place to avoid hypoparathyroidism (parathyroid hormone deficiency). Seealso endocrine system.

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