We present a case of a 66-year-old female patient with thyroid papillary carcinoma. On the fluorine-18 fluorodeoxyglucose (F18-FDG) positron emission tomography/computed tomography (PET/CT) performed for thyroglobulin elevation accompanied by negative radioiodine whole-body scan, there was an intense 18F-FDG uptake in the polypoid soft tissue invading the tracheal cartilage and extending into the lumen. Dedifferentiated tumor with high metabolic rate can be easily detected by 18F-FDG PET/CT.