Purpose: This retrospective study aims to assess the cut-off value of thyroglobulin (Tg) levels in nux or metastatic well-differentiated thyroid cancers (DTCs) with normal anti-Tg levels using with fluorodeoxyglucose/positron emission tomography/computed tomography (FDG PET/CT). Materials and Methods: We reviewed FDG PET/CT images of 104 patients with well DTC (28 men, 76 women) whose: Iodine-131 ((131) I) whole-body scanning was negative but had elevated Tg with normal anti-Tg levels. Results: The overall sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of florine-18-FDG PET/CT findings were found to be 95.92%, 87.27%, 87.04%, 96.00%, and 91.35%, respectively. The best Tg cut-off value was found to be 10.4 ng/ml. In the Tg level 10.4 ng/ml group, the sensitivity, specificity, PPV, NPV, and accuracy of FDG PET/CT were found to be 94.1%, 91.30%, 88.8%, 95.4%, and 92.5%, respectively. In the other group, which Tg level >= 10.4 ng/ml, sensitivity, specificity, PPV, NPV, and accuracy of FDG PET/CT exams were found to be 96.8%, 84.3%, 86.1%, 96.4%, and 90.6%, respectively. Conclusion: FDG PET/CT imaging is a valuable imaging method in the evaluation of patients with elevated serum Tg levels and normal anti-Tg levels. Furthermore, it has potential utility in the dedifferentiation of active foci that are present, and in assessing optimal decision making during follow-up.