Endocrine gland , two to eight of which are embedded in each side of the thyroid gland .
It secretes parathyroid hormone , which regulates blood calcium and phosphorus levels. When calcium in body fluids drops, increased hormone secretion releases calcium from bone into the bloodstream (see calcium deficiency ). The hormone also increases excretion of phosphate in the urine, lowering blood concentrations, and may play a role in magnesium metabolism. When thyroid removal is required, the parathyroid glands must be separated out and left in place. See also endocrine system .