Calcitonin is an endocrine hormone synthesized and secreted by the parafollicular cells of the thyroid gland (thyroid C cells) located in the neck of the body. Calcitonin, together with parathyroid hormone secreted by the parathyroid glands, and active vitamin D3, also known as calcitriol, regulate the metabolism of calcium and phosphorus in the body, and the stability of blood calcium and phosphorus. The three basic hormones that regulate calcium and phosphorus metabolism, calcitonin, parathyroid hormone and calcitriol, are also known as calcium-regulating hormones. The main role of calcitonin is to lower calcium and phosphorus in the blood. Calcitonin is also a tumor marker. Patients with medullary thyroid cancer have significantly higher levels of calcitonin in their blood. Some patients with lung cancer, breast cancer, cancers of the gastrointestinal tract, and chromophobe cancer may also have elevated calcitonin levels. Occasionally, elevated calcitonin levels can be seen in patients with liver cancer and cirrhosis.