The thyroid gland on the right can be clearly defined as PTC (Fig. 1C), and the left thyroid gland is confirmed as MTC by immunohistochemistry: CgA(+), Syn(+), CD56(+), CEA(+), TG(+), TTF-1(−) (Fig. 2A–F). Here, CEACAM5 is linked to medullary thyroid gland carcinoma.