We present the case of persistent CEA elevation after complete remission of CRC and negative 18F-fluorodeoxyglucose (18F-FDG) PET/CT revealing localized MTC diagnoses with 18F-fluorodihydroxyphenylalanine (18F-FDOPA) PET/CT. The gene discussed is CEACAM5; the disease is medullary thyroid gland carcinoma.