There are normally 4 parathyroid glands located near or attached to the back surface of the thyroid. Although the thyroid and parathyroid glands are physically near to each other and are both part of your body's endocrine system, their functions are not related. Parathyroid glands secrete a hormone, which controls your blood calcium level.
Calcium is important for muscle and nerve function.
Other cells in the body, especially cells of the bones, kidneys, and small intestine, respond to PTH by increasing the calcium levels in the blood. Under normal conditions, this control is quite accurate. If one or more parathyroid glands enlarge and become overactive, your blood calcium level may increase. This condition is called primary hyperparathyroidism. These tumors are nearly always benign, but they may cause other problems such as kidney stones, mood changes or depression, or weakened or painful bones.