The major source of calcitonin is from the parafollicular or C cells in the thyroid gland, but it is also synthesized in a wide variety of other tissues, including the lung and intestinal tract. The function of calcition is a hormone known to participate in calcium and phosphorus metabolism. In particular, calcitonin has the ability to decrease blood calcium levels by bone and kidney. Besides, malignant tumors arising from thyroid C-cells (medullary thyroid carcinoma: MTC) usually produce elevated levels of calcitonin. Measurement of serum calcitonin is used in the follow-up of patients that have surgical removal of the thyroid gland.