CALCA and medullary thyroid gland carcinoma: C cells produce calcitonin which is a reasonably sensitive marker of MTC, but its specificity is limited since it can be elevated in other malignant (carcinoid and small cell lung cancers) and nonmalignant conditions (hypercalcemia, pregnancy, renal failure, sepsis, pheochromocytoma, autoimmune thyroiditis) (19).