 A human being usually has four parathyroid glands located on the back surface of the thyroid in the neck.In order to maintain calcium metabolism the parathyroids secrete parathyroid hormone (PTH) which stimulating bones to release calcium and kidneys to reabsorb it from the urine into the blood therefore increasing its serum level; calcitonin action is just an opposite.When a parathyroid adenoma causes hyperparathyroidism, more parathyroid hormone is secreted, causing the calcium concentration of the blood to rise, resulting in hypercalcemia.