Calcitonin (thyrocalcitonin) is a hormone produced by C cells in the thyroid gland. The main action of calcitonin is on the bone. It increases deposition of calcium and phosphate in the bone and lowers levels in the blood, so effectively it inhibits calcium resorption by binding to a specific receptor on the osteoclasts, and inhibits their action. In the kidneys, it decreases the re-absorption of both calcium and phosphate on the proximal tubules. Its overall effect is to decrease the plasma calcium concentration. Secretion is determined mainly by the plasma calcium concentration. Calcitonin is used to treat hypercalcemia and Paget's disease of the bone.