We present the case of a 58-year-old woman referred to our outpatient department because of the elevated serum levels of carcinoembryonic antigen (CEA) during routine follow-up 6 years after the diagnosis and therapy of localized rectal cancer. Radiographic and endoscopic examinations did not reveal tumor recurrence or metastases. An asymptomatic hypothyroidism was diagnosed by routine serum marker analysis. Serum CEA level decreased upon adequate substitution of the hypothyroidism. This case illustrates the influence of thyroid function on Serum CEA levels and confirms hypothyroidism as a rare but relevant differential diagnosis of elevated serum CEA levels.