How the Body Works: The 4 Parathyroid Glands

The four parathyroid glands, shown here as black dots adjacent to the thyroid, secrete parathormone or PTH, which is involved in regulating blood calcium levels and, indirectly, in controlling blood phosphate levels. Low blood calcium stimulates output of PTH, which mobilizes calcium from bones and aids in its absorption from the intestines and kidneys to increase the circulating level. When PTH secretion is inhibited, calcium is deposited in the bones and less calcium is reabsorbed from the intestine and kidneys until the blood level returns to normal.