CEACAM5 and medullary thyroid gland carcinoma: Serum CEA, as a broad‐spectrum tumor marker, may increase in a variety of tumors, including gastrointestinal malignancies, lung cancer, and breast cancer.17, 18 Previous researches had found that MTC can also secrete CEA, and the increased level of serum CEA in MTC patients was synchronous with Ctn.9, 19 This study also showed that there was significant correlation between serum CEA and Ctn in MTC patients (rct‐cea = 0.871).