MTCs in the validation cohort were 38% Bethesda category III, 29% Bethesda category IV, and 33% Bethesda category V. The MTC classifier correctly identified MTCs as small as 7 mm and in patients as young as 18 years old, even when baseline serum calcitonin was <20 ng/L (Table 1). This evidence concerns the gene CALCA and medullary thyroid gland carcinoma.