The Calcitonin (Thyrocalcitonin) test is a blood test used to diagnose and monitor medullary thyroid cancer. Calcitonin is a hormone produced by the thyroid gland that helps regulate calcium levels in the blood. High blood calcitonin levels can be a sign of medullary thyroid cancer or other thyroid conditions. This test is often used to screen for medullary thyroid cancer in people at high risk, such as those with a family history of the disease.