The parathyroid glands are small pea-sized glands located in the neck just behind the butterfly-shaped thyroid gland. Most people have four parathyroid glands, with two parathyroid glands lying behind each 'wing' of the thyroid gland.
The parathyroid glands produce a hormone called parathyroid hormone.
The parathyroid glands are important in tightly controlling calcium levels in the bloodstream. Because of this, calcium levels are generally very stable. This is important to ensure the nervous system and the body’s muscles can work properly, and also that bones remain strong.
The main target organs where parathyroid hormone exerts its effects are the bones and the kidneys. When needed, parathyroid hormone is released by the parathyroid glands into the blood and causes the bones to release calcium and increase levels in the bloodstream. It also causes the kidneys to stop calcium being lost in urine as well as stimulating the kidneys to regulate vitamin d metabolism (see below).
If someone does not take in enough calcium through their diet or does not have enough vitamin D, the circulating calcium levels fall and the parathyroid glands produce more parathyroid hormone. This brings calcium levels in the bloodstream back up to normal.
Another method that parathyroid hormone uses to increase calcium levels in the bloodstream is activation of vitamin D. This occurs in the kidney too; the activated vitamin D then increases calcium absorption from the gut.
Sometimes the parathyroid glands can make too much parathyroid hormone. In this case, patients develop a blood level of calcium that is too high (hypercalcaemia), which in turn can make them feel generally unwell. The commonest condition which cause this is called primary hyperparathyroidism.
If the high level of parathyroid hormone remains undetected for a long time, it can cause calcium from the bones to be lost into blood and subsequently the urine. This can eventually cause bones to become thin or spongy (osteoporosis). Too much calcium in the urine can also cause calcium stones in the kidney.
Last reviewed: Jan 2015