CEACAM5 and medullary thyroid gland carcinoma: CEA is normally produced in gastrointestinal tissue during fetal development, and present at low levels (<5 ng mL–1) in healthy adults.28 Elevated levels of CEA in the serum of individuals may be related with colorectal cancer, gastric cancer, pancreatic cancer, lung cancer, breast cancer, and medullary thyroid cancer.29 CEA is recognized as a broad‐spectrum tumor marker and is a prerequisite test item in hospitals for cancer screening.