The calcitonin test measures the level of the hormone calcitonin in the blood, which is primarily produced by the parafollicular C-cells of the thyroid gland. Calcitonin's main role is to help regulate calcium and phosphate levels in the blood, opposing the action of parathyroid hormone. The primary clinical utility of this test is to screen for and monitor medullary thyroid cancer (MTC), a rare type of thyroid tumor originating from C-cells. Elevated calcitonin levels are highly indicative of MTC, and the test is also used to assess treatment response and detect recurrence after surgery. It may sometimes be used in conjunction with a calcium stimulation test to enhance diagnostic accuracy.