Parathyroid gland

These are two pairs of ovoid, corn-grain shaped, small, reddish brown, button-like structures located upon the four corners of thyroid gland, weighing 0.05-0.3 grams (in adult) and 6–7 mm × 3–4 mm in size.

Endodermal in origin, these glands are like parafollicular cells of thyroid.

Consists of a solid mass of densely packed polygonal glandular cells, embedded in stroma, enveloped by connective tissue capsule.

These are present in all vertebrates except fishes.

Parathyroid contains two kinds of cells i.e. chief cells or principal cells and oxyphil cells or eosinophil cells. Chief cells are much more

numerous than oxyphil cells. Oxyphil cells are absent in young and appear a little before the age of puberty. Both cells are enclosed by a delicate connective capsule.

Chief cells, the main secretory cells and Oxyntic cells with eosinophilic granules are phugocytic.

Chief cell of parathyroid secrete a hormone called parathyroid hormone (PTH) or parathormone or Collip’s hormone

PTH was discovered by Collip in 1925. Its crystals were first prepared by Craig and Rajmussen (1961).

Its molecular structure was worked by Potts and associates in 1968.

PTH regulates calcium and phosphate balance between blood and other tissues in following ways.

It inhibits collagen synthesis by osteoblasts and bone resorption by osteoclasts.

It mobilises the release of calcium into blood from bones

It increases calcium absorption from intestine.

It increases calcium resorption from nephrons and inhibits phosphate resorption.

Parathyroids are under the feedback control of blood calcium level. A fall in blood calcium stimulates them to secrete parathormone and a rise

in blood calcium inhibits parathormone secretion from them.

PTH and calcitonin are antagonistic in their action.

Increases plasma Ca++ level by increasing bone dissolution (osteoclastic activity) and enhancing its reabsorption through renal tubules and by

increasing Ca++ absorption in the gut.

Thus, maintenance of proper Ca++ level in the body is the synergistic effect of calcitonin, PTH and Vit.D.

Increases phosphate elimination in urine and consequently lowers phosphate concentration in plasma.

Its hyposecretion causes hypoparathyroidism (parathyroid tetany) resulting into muscular pain and cramp due to deficiency of Ca++.

Uncontrolled, painful, prolonged contraction of voluntary muscles and irritability of nerves.

Extreme condition of tetany causes contraction of muscles of larynx, face, hands and feet.

Hypersecretion brings about hyperparathyroidism which causes extensive decalcification of bones resulting in bone deformities and

fractures.