Parathyroid Glands Health Article

Definition

The four parathyroid glands are small, light-colored lumps protruding from the surface of the thyroid gland. They secrete parathyroid hormone, the most important regulator of calcium and phosphorus amounts in the body.

Description

The parathyroid glands are located on the thyroid gland, a butterfly-shaped gland found in the neck on both sides of the windpipe. There are then two parathyroid glands on each side of the neck for a total of four. Parathyroid tissue consists of two major cell types: oxyphil cells, whose function is unknown, and chief cells, which produce parathyroid hormone. The structure of a parathyroid gland is very different from that of a thyroid gland. The chief cells that produce parathyroid hormone are arranged in tightly-packed nests around small blood vessels, quite unlike the thyroid cells that produce thyroid hormones, which are arranged in spheres (thyroid follicles).

The parathyroid glands secrete parathyroid hormone (PTH), a polypeptide consisting of 84 amino acid residues. A hormone is a chemical messenger of the body, produced and secreted by special glands called exocrine glands. It is released directly into the bloodstream and travels to its target cells, often distant, where it binds to a structure called a receptor, that is found either inside or on the surface of the target cells. Receptors bind a specific hormone and the result is a specific physiologic response, meaning a normal response of the body. The activity of all the hormones or growth factors secreted by endocrine glands and circulating in blood is controlled by the exocrine system of the body. PTH finds its major target cells in bone, kidneys, and the gastrointestinal system.

Function

The function of the parathyroid glands is to secrete parathyroid hormone, which causes the release of the calcium present in bone to extracellular fluid (ECF). The ECF is the fluid found outside cells in all body tissues. PTH does this by activating the production of osteoblasts, special cells of the body involved in the production of bone and slowing down osteoclasts, other specialized cells involved in the removal of bone.

Calcitonin, a hormone produced by the thyroid gland that also regulates ECF calcium levels and serves to counteract the calcium-producing effects of PTH. The adult body contains as much as 1 kg of calcium. Most of this calcium is found in bone and teeth, and less than 1 g is found in the ECF, with 50% in the form of ionized calcium (Ca²⁺). Both calcitonin and PTH respond to the circulating levels of Ca²⁺. An increased amount of ECF Ca²⁺ leads to an increased release of calcitonin and a decreased PTH release; similarly, a decreased amount of ECF Ca²⁺ results in a decreased release of calcitonin and an increased PTH release. Overall, calcitonin acts quickly to reduce ECF Ca²⁺ levels, while PTH works more slowly to ensure that adequate Ca²⁺ ECF levels are maintained. PTH action is thus opposed to that of calcitonin.

Three major calcium regulatory processes are affected by PTH:

• Release of calcium from bone: Although the mechanisms remain unclear, it is known that PTH stimulates bone tissue to release calcium into blood.
• Increased calcium absorption in the intestine: Facilitating calcium absorption from the small intestine increases calcium amounts in the blood. PTH also stimulates this process, but indirectly, meaning that it stimulates production of vitamin D in the kidney. Vitamin D in turn facilitates the production of a protein in intestinal cells that binds calcium for its efficient absorption into the blood.
• Suppression of calcium loss in urine: Additionally, PTH slows down the elimination of calcium in urine, thus maintaining calcium levels in blood.