Parathyroid Gland

Parathyroid Gland, any of a group of glandular cell aggregations located in the neck region close to the thyroid gland in lizards, some birds, and most mammals. In humans, four such clumps are usually present as distinct, yellowish-brown, encapsulated organs, each about 6 mm (‚ in) long. The combined weight of these glands is no more than 560 mg (1/50 oz). They are located beneath the thyroid gland; one or more of them are occasionally embedded in the thyroid tissue. Sometimes they may appear in association with the thymus, or anywhere between the thyroid and thymus, or even elsewhere in the neck or upper anterior chest. Such glands, which may be present in addition to four normally placed ones, are known as accessory parathyroid tissue.

The parathyroid glands secrete a hormone known as parathormone, which controls the concentration of calcium (calcium ion) and phosphorus (phosphate) in the blood. Calcium and phosphorus normally have a relationship to each other that the body keeps fairly constant. Parathyroid hormone acts to increase the excretion of phosphorus by the kidneys (which tends to lower blood phosphorus levels) and to increase the rate of resorption of calcium from bone (which tends to raise the level of blood calcium). Deficiency of parathormone, which rarely occurs spontaneously and which is sometimes caused by accidental removal of the parathyroid glands during surgical excision of the thyroid, results in reduction of blood calcium, increase of blood phosphorus, and increased nervous excitability leading to rapid involuntary contractions of the muscles, a condition known as tetany. The accessory parathyroid tissue is occasionally sufficient to prevent severe deficiency symptoms when the essential four clumps are removed.

Unless accessory tissue is formed or hormone injections are given, parathormone deficiency results in death. Overactivity of the parathyroid glands, occurring in cases of parathyroid tumors or hyperplasia of the normal glands, results in decreased blood phosphorus and increased blood calcium; the calcium in the bloodstream is withdrawn from the bones, which become soft as a result. Crystallization of excess calcium excreted in the urine in hyperparathyroidism may cause formation of kidney stones. The calcium may be carried to and deposited in the soft tissues of the body by the bloodstream; calcium deposition may also cause dysfunction of various organs, particularly the kidneys.