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.