A high level of carcinoembryonic antigen as initial manifestation of medullary thyroid carcinoma in a patient with subclinical hyperthyroidism.

Carcinoembryonic antigen (CEA), a tumor marker with a glycoprotein structure, is frequently used in follow-up gastrointestinal malignancies. CEA levels may also increase in neuroendocrine tumors, including medullary thyroid carcinoma (MTC), and in some benign diseases. Patients whose blood tests show high CEA levels should have additional tests regarding MTC. Although MTC comprises only 3%-11% of all thyroid cancers, it should be tested because it has a poor prognosis and may accompany multiple endocrine neoplasia. We present the case of a 76-year-old man with subclinical hyperthyroidism with sporadic MTC who presented with initial high serum CEA levels. He underwent total thyroidectomy and left modified neck dissection. Pathologic specimens stained strongly for CEA. The patient's blood was analyzed for mutations in exons 10, 11, 13, 14, 15, and 16, but the RET proto-oncogene revealed no mutations. The patient was regularly followed by measurement of serum CEA levels and performance of positron emission tomography-computed tomography. Seventeen months after surgery, the patient has remained well and showed no signs of tumor recurrence.