This document discusses calcitonin, a 32 amino acid polypeptide hormone with a molecular weight of 3400 that is synthesized from procalcitonin. Calcitonin is secreted by parafollicular cells in the thyroid gland, has a normal plasma level of 1-2ng/dl, and a half life of 5-10 minutes, and is degraded and excreted by the liver and kidney.