The parathyroid glands are four peasized bodies located behind the thyroid gland that produce PTH (Parathyroid hormone). This hormone increases calcium levels in the blood, helping to maintain bone quality and an adequate supply of calcium, which is needed for numerous functions throughout the body (e.g., muscle movement and signal transmission within cells). Specifically, PTH causes reabsorption of calcium from and excretion of phosphate in the urine. PTH also promotes the release of stored calcium from the bones as well as bone resorption, both of which increase calcium levels in the blood. Finally, PTH stimulates the absorption of calcium from the food in the gastrointestinal tract. Consistent with PTH’s central role in calcium metabolism, the release of this hormone is not controlled by pituitary hormones but by the calcium levels in the blood. Thus, low calcium levels stimulate PTH release, whereas high calcium levels suppress it.
Many of the functions of PTH require or are facilitated by a substance called 1,25-dihydroxycholecalciferol, a derivative of vitamin D. In addition, numerous other hormones are involved in regulating the body’s calcium levels and bone metabolism, including estrogens, glucocorticoids, and growth hormone.
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