Parathyroid Hormone

Parathyroid hormone

The secretory product of the parathyroid glands. Parathyroid hormone (PTH) is a single-chain polypeptide composed of 84 amino acids. The sequences of human, bovine, and porcine parathyroid hormone are known, and the gene for human parathyroid hormone has been cloned and sequenced.

The major regulator of parathyroid hormone secretion is the serum concentration of calcium ions, to which the parathyroid cells are exquisitely sensitive. Only a limited amount of parathyroid hormone is stored in secretory granules, so that a hypocalcemic stimulus must ultimately influence biosynthesis as well as secretion of the hormone. Parathyroid secretory protein is a large, acidic glycoprotein which is stored and cosecreted with parathyroid hormone in roughly equimolar amounts; the biological function of parathyroid secretory protein is unknown.

Parathyroid hormone is responsible for the fine regulation of serum calcium concentration on a minute-to-minute basis. This is achieved by the acute effects of the hormone on calcium resorption in bone and calcium reabsorption in the kidney. The phosphate mobilized from bone is excreted into the urine by means of the hormone's influence on renal phosphate handling. Parathyroid hormone also stimulates calcium absorption in the intestine, this being mediated indirectly by 1,25-dihydroxyvitamin D. Thus, a hypocalcemic stimulus of parathyroid hormone secretion results in an increased influx of calcium from three sources (bone, kidney, and intestine), resulting in a normalization of the serum calcium concentration without change in the serum phosphate concentration. See Parathyroid gland, Thyrocalcitonin

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

parathyroid hormone

[¦par·ə′thī‚rȯid ′hȯr‚mōn]

(biochemistry)

parathormone

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Parathyroid Hormone

 

(also parathormone), a hormone secreted by the parathyroid glands. It regulates the content of calcium and phosphorus in blood, interstitial fluid, and bone. It was isolated from parathyroid glands of bulls in 1925 by the Canadian biochemist J. Collip and obtained in pure form in 1961 by L. Craig and H. Rasmussen. Chemically it is a polypeptide that consists of 80 amino-acid residues and has a molecular weight of 9,500.

Parathyroid hormone acts to reduce blood phosphate levels and increase blood calcium levels and promotes the mobilization of calcium from bone into the body’s fluids. It also promotes excretion of calcium and phosphates in urine. An excess of parathyroid hormone in animals that receive it by injection or in humans suffering from hyperthyroidism results in resorption of bone, whereas a deficiency of the hormone lowers blood calcium levels with various consequences, including the intensification of neuromuscular excitability (tetany in acute cases) and the development of alkalosis. The amount of parathyroid hormone secreted varies inversely with blood calcium levels; that is, the amount secreted increases as the calcium level decreases.

The mechanism of action of parathyroid hormone has not been completely elucidated. Apparently the hormone acts on both renal and bone tissue and stimulates the absorption of calcium from the intestine and its reabsorption from the glomerular filtrate into the blood. An aqueous extract of parathyroid glands of slaughtered cattle is administered, subcutaneously or intramuscularly, in various forms of tetany, spasmophilia, and certain other diseases.

V. M. SAMSONOVA

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.