Most MTCs are well-differentiated NETs, based on the expression of calcitonin and TTF-1; aggressive, poorly differentiated NECs represent less than 1% of MTCs, and include a small cell variant that needs to be distinguished from small cell carcinoma metastatic to the thyroid gland, especially since small cell carcinomas of many anatomic sites express TTF-1 [81, 82]. Here, CALCA is linked to small cell carcinoma.