A peptide hormone in vertebrates that lowers the concentration of calcium (and phosphate) in the blood by inhibiting the action of osteoclasts, thereby suppressing the release of calcium from bone, and promoting excretion of calcium and phosphate by the kidneys. It operates in opposition to parathyroid hormone. Calcitonin is produced by the C cells, which in mammals are located in the thyroid gland. Its precise role in adult humans is unclear, because neither very low nor very high concentrations in the blood seem to have a deleterious effect.