Four lentil-sized glands in your neck keep your blood calcium on a knife edge
Tucked behind the thyroid sit usually four tiny glands, each about 6 millimetres long and weighing roughly 30 milligrams. Despite the similar name, they have nothing to do with the thyroid's job. Their task is to hold calcium in the blood within a very narrow range so that nerves and muscles work properly.
The parathyroids are yellowish-brown, flat ovals often compared to lentil seeds, too small to see or feel from outside the neck. Most people have two pairs, upper and lower, but the count varies: some have three, others six, eight or more, and occasionally a gland turns up inside the thyroid, in the chest or within the thymus. Under a microscope they are easy to tell from the thyroid because their cells are tightly packed rather than arranged in follicles. Chief cells make the hormone, and darken when full of it. A second type, the oxyphil cells, become more numerous with age, and nobody yet knows what they do.
Their odd positions come from the embryo. Early in development, a series of pouches form in the throat region, numbered from the head downward. The glands that end up lower in the neck actually start in the third pouch alongside the thymus, and are dragged down as the thymus migrates towards the chest. Those that finish on top grow from the fourth pouch and travel less far. Anatomists name them for where they end up, not where they began, and a third-pouch gland that fails to let go can end up in the chest.
Parathyroid hormone is a small protein that raises blood calcium in several ways. It stimulates bone-building cells, which in turn activate the cells that break bone down and release calcium. It activates vitamin D so the gut absorbs more calcium, and it tells the kidneys to hold on to calcium while getting rid of phosphate. Calcitonin works in the opposite direction.
Too much of the hormone, usually from a benign overgrowth of chief cells in one gland, is called hyperparathyroidism; too little is hypoparathyroidism. Both show up as disturbances in blood calcium and bone.
Source: Parathyroid gland
