CALCA and medullary thyroid gland carcinoma: FTCs can be further classified by histologic subtype as follicular (11.9–32.0%), follicular-compact (32.0–58.0%), papillary (3.7–7.1%), or compact/solid (13.6–40.5%) based on the World Health Organization (WHO) scheme4–8, the latter of which can be difficult to distinguish from MTC necessitating immunohistochemistry (i.e., thyroglobulin for FTC and calcitonin for MTC)2,6,8–10.