Calcitonin: A hormone produced by the thyroid gland that lowers the levels of calcium and phosphate in the blood and promotes the formation of bone. Bone is in a constant state of remodeling. Old bone is removed by cells called osteoclasts, and new bone is added by cells called osteoblasts. Calcitonin inhibits bone removal by the osteoclasts and at the same time promotes bone formation by the osteoblasts. Calcitonin is given in hypercalcemia (high blood calcium) to lower the calcium level; in osteoporosis to increase bone density and decrease the risk of a fracture; and in Paget disease to decrease bone turnover and bone pain. Also known as thyrocalcitonin.