In this case, the immunohistochemistry of MTC shows that CT has a focal weak positive expression, which may be related to the above factors, but the patient’s serum CT level is significantly elevated, which is consistent with the disease status.It is worth noting that TTF-1 can be expressed in both MTC and PTC, but its positive intensity in MTC is lower than that in follicular tumors, so it cannot be used to distinguish MTC from PTC (18). The gene discussed is TTF1; the disease is medullary thyroid gland carcinoma.