Parathyroid glands are four small glands located on your thyroid. If your parathyroid glands produce insufficient amounts of parathyroid hormone (PTH), it can lead to hypoparathyroidism. The main job of PTH is to regulate the levels of calcium and phosphorus in the blood. In people with hypoparathyroidism, the levels of calcium become too low, and the levels of phosphorus become too high. This imbalance can cause a number of symptoms.
Hypoparathyroidism can be caused by the following:

surgical removal of the glands;
accidental injury to the glands during surgery;
autoimmune diseases in which the immune system attacks the glands;
radiation therapy to the neck and head;
decreased levels of magnesium;
certain inherited disorders.