The primary role of calcitonin is to decrease blood calcium levels. It is secreted by the parafollicular cells (C cells) of the thyroid gland in response to high serum calcium. Calcitonin acts mainly on bones by inhibiting osteoclast activity, which reduces bone resorption and calcium release into the bloodstream. It also promotes calcium excretion by the kidneys. This hormone works antagonistically to parathyroid hormone (PTH), helping to maintain calcium homeostasis, especially after meals. Its role is crucial in preventing hypercalcemia and protecting bones from excessive calcium loss.

Reference: Guyton and Hall Textbook of Medical Physiology, Guyton AC, Hall JE, 13th Edition.