A Calcitonin test measures the level of calcitonin hormone in your blood, primarily used to diagnose and monitor Medullary Thyroid Cancer (MTC) and C-cell hyperplasia, as high levels signal abnormal growth of thyroid C-cells. It’s a simple blood test, often done with fasting, helping doctors track cancer recurrence or effectiveness of treatment, but it’s not a general screening tool due to other conditions causing elevations, notes MedlinePlus Medical Test and Dr Lal PathLabs.