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Objective: Evaluate the utility of 18F-FDG PET/CT in the follow-up of differentiated thyroid cancer (DTC) patients with high serum antithyroglobulin antibody (TgAb) but negative serum thyroglobulin (Tg) measurements and negative 131I whole-body scans (WBS)., Material and Method: The present study included 22 consecutive patients with high serum TgAb levels. They had negative serum Tg measurements and negative 131I WBS. PET/CT was performed 60 min after intravenous injection of 227.55-455.47 MBq (6.15-12.31 mCi) of 18F-FDG using a combined PET/CT scanner. Co-registered CT images were used to differentiate physiologic from pathologic tracer uptake. Findings on 18F-FDG PET/CT were correlated with tissue pathology follow-up imaging or clinical follow-up served as a reference., Results: Twenty-two well differentiated thyroid cancerpatients participated. Twelve had positive findings on 18F-FDG PET/CT six were true positives and six were false-positives. 18F-FDG PET/CT results were true negative in 10 patients and the authors found no false-negative patients in the present study. The overall sensitivity, specificity and accuracy of 18F-FDG PET/CT were 100%, 62.5% and 72.7%, respectively. TgAb levels, which are appropriated for sending DTC patient who are 131I WBS negative but have elevated serum TgAb levels to undergo 18F-FDG PET/CT scan, should be more than or equal to 414.6 IU/ml. Results should be highly consider positive if max SUV value is equal or greater than 4.5., Conclusion: 18F-FDG PET/CT is a useful tool for localizing recurrent or metastatic DTC patients, who have negative 131I WBS but elevated serum TgAb levels. The authors recommend its use in clinical management of selected cases regardless of the TgAb level being more than or equal to 415 IU/ml.