The parathyroid glands are organs situated behind the thyroid gland and each is about half the size of a grain of rice. They are called the parathyroid glands due to their proximity to the thyroid gland, but the parathyroid glands and the thyroid gland are completely different organs. Normally, there are four parathyroid glands in total, with a pair of two parathyroid glands (one above the other) on each of the two lateral lobes of the thyroid.

The parathyroid glands secrete parathyroid hormone. While parathyroid hormone shares the same name as the parathyroid glands, its function is completely different from thyroid hormone. Parathyroid hormone "regulates the metabolism of calcium."
Calcium is not only a major component of bones, but is also indispensable for blood clotting and for the contraction of every muscle in the body, including the heart. It is also an essential mineral for the proper functioning of brain cells.
In the metabolism of calcium, vitamin D also has an indispensable role to play. Calcium is stored in the bones. Both its release from the bones into the blood and its absorption in the kidneys or intestines are regulated by parathyroid hormone and vitamin D. Thus, parathyroid hormone and vitamin D have the function of increasing calcium levels in the blood.
Calcium levels, in turn, can affect the secretion of parathyroid hormone. Low levels of calcium in the blood induce the secretion of parathyroid hormone, resulting in increased calcium levels. On the other hand, high levels of calcium in the blood inhibit the secretion of parathyroid hormone, leading to decreased calcium levels. In this manner, calcium levels in the blood are regulated.