calcitonin, a protein hormone synthesized and secreted in humans and other mammals primarily by parafollicular cells (C cells) in the thyroid gland. In birds, fishes, and other nonmammalian vertebrates, calcitonin is secreted by cells of the glandular ultimobranchial bodies. The overall effect of calcitonin is to lower the concentration of calcium in the blood when it rises above the normal value. It also lowers the concentration of phosphorus in the blood when levels exceed normal.
Calcitonin acts to decrease serum calcium concentrations in several ways. For example, it inhibits the activity of the osteoclasts in bone tissue, thereby preventing the resorption (breakdown) of bone, and inhibits the reabsorption of calcium by the cells of the kidney, resulting in increased calcium excretion in the urine. The effects of calcitonin counter the effects of parathyroid hormone (parathormone), a substance secreted by the parathyroid glands that acts to increase serum calcium concentrations.
Both increased calcitonin secretion and increased calcitonin activity are relatively short-lived, lasting only a few days. As a result, patients with chronically high serum calcium concentrations (hypercalcemia) do not have high serum calcitonin concentrations. By contrast, patients with medullary thyroid carcinoma, a cancer of the parafollicular cells that secretes large quantities of calcitonin, have high serum calcitonin concentrations but normal serum calcium concentrations. Calcitonin levels may also be increased in association with lung cancer and certain tumours of the pancreas, namely insulinoma (an insulin-secreting tumour) and VIPoma (a vasoactive intestinal polypeptide-secreting tumour).
Synthetic analogues of calcitonin are of particular interest in the treatment of osteoporosis, a disease characterized by the thinning of bones that arises from increased bone resorption or decreased bone formation. In humans, the calcitonin protein is made up of 32 amino acids.