Parathyroid Glands

The parathyroid glands are lumps of tissue about the size of a small pea, which in humans are attached to the thyroid gland or embedded in its tissue. There are usually four parathyroid glands (two at the top and two at the bottom of the thyroid gland), but there may be fewer or more of them.

Although these glands are located near or inside the thyroid gland, they have a completely independent function and a different histological structure. Their cells form a compact mass, not at all similar to the hollow spherical vesicles formed by the cells of the thyroid gland. The parathyroid glands, like the thyroid gland, arise in embryogenesis from outgrowths of the pharynx, and in their origin are the remains of the gill sacs of fish.

Parathyroid hormone is essential for life as it regulates the concentration of calcium and phosphorus in the blood and tissues. After removal of the parathyroid glands from an animal, muscle twitching, spasms and convulsions occur in response to stimuli that do not cause any reaction in a normal animal or cause only weak muscle contractions. This condition, called tetany, is caused by increased irritability of muscles and nerves due to low calcium levels in the blood and tissue fluids.

The calcium concentration in the blood of an animal with the parathyroid glands removed drops to approximately half the normal value. If, during tetanic convulsions, a solution of calcium salt is injected into the vein of an animal, the convulsions immediately stop; further attacks can be prevented by repeated injections or feeding of calcium. The amount of phosphorus in the blood increases as the calcium concentration decreases and decreases with the injection of parathyroid hormone.

Thus, the main role of this hormone is to regulate the content of calcium and phosphorus in the blood and tissue fluids. It promotes the absorption of calcium in the intestine, the release of calcium from bones and its reabsorption from the glomerular filtrate in the renal tubules. The same hormone inhibits the reabsorption of phosphorus in the kidneys, thereby promoting its excretion in the urine.

Parathyroid hormone consists of a single peptide chain with a molecular weight of 8500, containing 77 amino acids. It is inactivated by proteolytic enzymes and should not be administered orally. The amount of hormone secreted by the parathyroid glands is regulated by the calcium level in the blood.

A decrease in calcium concentration stimulates the secretory activity of these glands, and an increase suppresses it. Parathyroid gland failure is rare in humans, although they are sometimes accidentally removed during thyroid surgery and sometimes they degenerate as a result of infection. If the deficiency is mild, then tetany is observed only in cases of overexertion, for example during childbirth, and it is treated, like more severe manifestations of hypoparathyroidism, by administering a hormone or calcium, or both at the same time.

Hyperfunction of the parathyroid glands is a consequence of tumors or enlargement of these glands and is characterized by high levels of calcium in the blood. Because calcium comes at least partly from the bones, hyperparathyroidism is characterized by soft bones that are easily damaged. Muscles are less irritable than normal; they become atrophic and painful.

When calcium levels in the blood rise, the mineral is deposited in unusual places, such as the kidneys. The disease can be treated by removing excess parathyroid tissue surgically or destroying it with X-rays.