[en] The purpose of this study was to evaluate the usefulness of 18F-FDG PET/CT (PET/CT) in suspicious recurred thyroid cancer before high dose of radioactive iodine therapy (RAI therapy). PET/CT was performed after injecting 370-555MBq of 18F-FDG in 23 patients prior to RAI therapy. RAI therapy (212 58 mCi) was performed, and thyroglobulin (Tg) and anti-thyroglobulin antibody (anti-TG ab) during TSH stimulation was measured. The interval between PET/CT and RAI scan was 41 50 day. 15 patients were confirmed as recurred thyroid cancer (pathologic diagnosis: 10), whereas 8 patients were diagnosed as remission status (pathologic diagnosis: 3). PET/CT had 80.0% (12/15) of sensitivity and 75.0% (6/8) of specificity. TG and RAI scan had 80.0% (12/15) and 75% (6/8), 80% (12/15) and 37.5% (3/8) of sensitivity and specificity, respectively. Combination of PET/CT and TG increased the sensitivity (93.3%; 14/15) and specificity (87.5%; 7/8). There was 25 suspicious lesions (19 malignant lesion, 6 benign lesion) in lesion-based study. PET/CT had 84.5% and 83.3% of sensitivity and specificity but RAI scan had 89.5% and 16.7%, respectively. Five false positive sites of RAI scan were diagnosed as fungus ball of lung, calcified mediastinal LN, and post op induced change such as suture granuloma. PET/CT revealed 80% (3/5) of them as benign lesions. Even though PET/CT had similar sensitivity with high dose of RAI scan, it had a better discriminating nature to detect benign lesion. We could diminish the unnecessary high dose of RAI therapy using FDG PET/CT