Calcitonin is a hormone produced primarily by the thyroid gland that helps regulate calcium levels in the body. It lowers blood calcium levels by inhibiting osteoclast activity, which reduces the release of calcium from bones, and it also decreases calcium reabsorption in the kidneys. Calcitonin is used clinically to treat conditions like osteoporosis and Paget's disease, as well as to lower high blood calcium levels. Its therapeutic forms are often administered via injection or nasal spray.