The parathyroid glands are four endocrine glands which are situated in the neck, protruding from the back surface of the thyroid gland. The parathyroid glands secrete parathyroid hormone, which is the most important regulator of calcium and phosphorus amounts in the body. Parathyroid glands also control the amount of calcium in the blood and within the bones.
Although the parathyroid glands are named for their proximity to the thyroid, they play a completely different role than the thyroid gland. They are quite easily recognizable from the thyroid because they are composed of densely packed cells, called parathyroid chief cells, in contrast with the follicle structure of the thyroid.
The main function of the parathyroid glands is to keep the body's calcium level within a very narrow range, so that the nervous and muscular systems can function properly. When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated to release hormone into the blood.
The Four Parathyroid Glands (Video)