The parathyroid glands (also known simply as parathyroids) are four glands (sometimes 8) located behind the thyroid gland on the neck. The thyroid gland is in front and the parathyroid glands attach to it. The role of the parathyroid glands is to regulate the level of calcium and phosphorus in the blood and in the bones, thanks to the secretion of a hormone, the parathyroid hormone (or PTH for ParaThyroid Hormone). This role is essential in phosphocalcic metabolism.

PTH will increase the level of calcium in the blood (calcemia, Ca2+ ions). The higher the calcium level, the less PTH will be produced by the para-thyroid glands. PTH will act on the bones, by stimulating the osteoclasts. Osteoclasts are bone cells whose role is to "deconstruct" the bone, transferring the calcium it contains directly into the bloodstream.