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parathyroid glands

parathyroid glands These small but vitally necessary groups of cells, usually four of them, lie on the back of the thyroid gland in the neck. They are endocrine glands — meaning that they deliver their secretion directly into the passing blood. The peptide parathyroid hormone (or parathormone) will therefore reach the whole of the rest of the body, but attaches only to those cells which have receptors for it. The activities which it then modifies in these ‘target’ cells all work towards an increase in the concentration of calcium ions in the extracellular fluid compartment of the body — the blood plasma and the tissue (interstitial) fluid. It is necessary for a great variety of physiological functions — indeed for virtually all of them — that this concentration remains within quite narrow limits.

Where and how, then, would a hormone need to act in order to promote the addition of calcium into the body pool?(i) Calcium from food is absorbed from the intestine: parathyroid hormone promotes conversion of Vitamin D to its active form; this in turn enhances intestinal absorption of calcium.(ii) Calcium is stored in bone mineral: parathyroid hormone stimulates resorption from bone into the blood by activating the osteo-clasts, which break down bone and mobilize the minerals from it.(iii) When the blood passes through the kidneys some of its calcium ions escape into the urine: parathyroid hormone enhances reabsorption by acting on the renal tubule cells.

An appropriate concentration of calcium ions in body fluids is necessary for every transfer of a stimulus from nerve to muscle, and for the contraction of muscle of all types, including the beating of the heart and the regulation of the diameter of blood vessels. The translocation of calcium in and out of cells, and in and out of storage by chemical binding within cells, is also crucial for the secretory activity of glandular cells, producing both external and internal secretions — sweat, milk, saliva, insulin, cortisol, and all the rest, including parathyroid hormone itself.

The parathyroid glands ‘know’ how much hormone to secrete, because their activity is regulated by the concentration of ionized calcium in the blood that flows through them: a rise in calcium inhibits secretion of the hormone and vice versa (a negative feedback mechanism).

Parathyroid hormone thus keeps calcium concentration up — otherwise known as a hypercalcaemic effect. There is also another hormone keeping calcium down: calcitonin, secreted by the ‘C-cells’ within the thyroid gland.

Parathyroid deficiency is rare; it can occur acutely if the glands are inadvertently removed along with the thyroid or with cancerous neck glands. Rarely also, a parathyroid tumour can cause excess of the hormone, resulting in decalcification and cysts in the bones, and kidney problems due to the high concentration of calcium in the filtered blood.

Sheila Jennett


See also calcium; hormones.

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COLIN BLAKEMORE and SHELIA JENNETT. "parathyroid glands." The Oxford Companion to the Body. 2001. Encyclopedia.com. 25 Jun. 2016 <http://www.encyclopedia.com>.

COLIN BLAKEMORE and SHELIA JENNETT. "parathyroid glands." The Oxford Companion to the Body. 2001. Encyclopedia.com. (June 25, 2016). http://www.encyclopedia.com/doc/1O128-parathyroidglands.html

COLIN BLAKEMORE and SHELIA JENNETT. "parathyroid glands." The Oxford Companion to the Body. 2001. Retrieved June 25, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O128-parathyroidglands.html

parathyroid glands

parathyroid glands (pâr´əthī´roid), 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.

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parathyroid glands

parathyroid glands Four small endocrine glands, usually embedded in the back of the thyroid gland, that secrete a hormone to control the level of calcium and phosphorus in the blood. Overproduction of parathyroid hormone causes loss of calcium from the bones to the blood; a deficiency causes tetany (involuntary muscle spasm). See also endocrine system

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"parathyroid glands." World Encyclopedia. 2005. Encyclopedia.com. 25 Jun. 2016 <http://www.encyclopedia.com>.

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parathyroid glands

parathyroid glands (pa-ră-th'y-roid) pl. n. two pairs of yellowish-brown endocrine glands that are situated behind, or sometimes embedded within, the thyroid gland. They are stimulated to produce parathyroid hormone by a decrease in the amount of calcium in the blood.

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"parathyroid glands." A Dictionary of Nursing. 2008. Encyclopedia.com. 25 Jun. 2016 <http://www.encyclopedia.com>.

"parathyroid glands." A Dictionary of Nursing. 2008. Encyclopedia.com. (June 25, 2016). http://www.encyclopedia.com/doc/1O62-parathyroidglands.html

"parathyroid glands." A Dictionary of Nursing. 2008. Retrieved June 25, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O62-parathyroidglands.html

parathyroid glands

parathyroid glands Two pairs of endocrine glands situated behind, or embedded within, the thyroid gland in higher vertebrates. They produce parathyroid hormone, which controls the amount of calcium in the blood. See also C cell.

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"parathyroid glands." A Dictionary of Biology. 2004. Encyclopedia.com. 25 Jun. 2016 <http://www.encyclopedia.com>.

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"parathyroid glands." A Dictionary of Biology. 2004. Retrieved June 25, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O6-parathyroidglands.html

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