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Carcinoembryonic antigen (CEA) is commonly raised in patients with colorectal carcinoma (CRC) and has an established role in postoperative monitoring to detect early recurrence. CEA is not recommended as a screening tool for CRC; however, a de novo raised level mandates further investigation with a colonoscopy. This report describes a patient referred from primary care with a raised CEA and functional bowel symptomatology who went on to have a normal colonoscopy. The persistently rising CEA prompted a repeat colonoscopy, upper gastrointestinal and small bowel investigation and a CT of the abdomen and pelvis, without any cause being found. Finally, a CT-positron emission tomography scan revealed localised uptake in the thyroid gland, with a fine-needle aspiration confirming a diagnosis of medullary cell thyroid carcinoma with regional lymph node involvement.