The Calcitonin test measures a hormone produced by your thyroid's C-cells. This is not a routine thyroid test (like TSH); it is a highly specialized tumor marker. It is the primary, gold-standard blood test doctors use to help diagnose a rare type of thyroid cancer (Medullary Thyroid Cancer). It is also the essential test used to monitor patients after treatment to ensure the condition has not returned. A simple blood draw provides this critical, specific information, which is essential for guiding your specialist's care plan.