a protein hormone secreted in humans and other mammals by parafollicular cells in the thyroid gland, and in birds, fishes, and other nonmammalian vertebrates by cells of the ultimobranchial bodies, which are discrete calcitonin-secreting glands. Calcitonin lowers the concentration of calcium in the blood when it rises above the normal value. It has the opposite effect of parathyroid hormone (parathormone).