A 66-year-old man with prostate cancer underwent F-fluorocholine PET/CT and thereafter Ga-labeled prostate-specific membrane antigen PET/CT to explore a rising prostate-specific antigen level. Both PET/CT studies showed a thyroid incidentaloma of the right lobe. Neck ultrasound confirmed the presence of a 16-mm right thyroid nodule. The serum calcitonin level was moderately increased at 25 ng/mL (<10). Cytology was non-diagnostic (Bethesda I). A right lobectomy was performed and pathology revealed a 15-mm medullary thyroid cancer. Two months after surgery, the calcitonin level returned to normal at 3.3 ng/mL.