Objective: to determine the most appropriate threshold for thyroid cancer tumor indicators (TG and Anti-TG) among individuals who exhibited negative I131-WBS and rising tumor marker levels during follow-up who have clinical suspicion of de-differentiation and are therefore referred for 18F-FDG PET/CT scan.

Methods: We looked at 31 patients' FDG PET/CT scans of thyroid carcinoma that was well-differentiated at the time of thyroidectomy, but their RAI WBS became negative during follow-up, in spite of rising TG or Anti-TG levels.

Results:
PET/CT was positive in in 21 patients and was negative in 10 patients. Patients whose PET/CT results were positive had TG values that were significantly different from those whose results were negative (p=0.006), but there was no statistically significant difference in the Anti-TG values between those whose PET results were positive and those whose were negative (p=0.29).
A solid predictor of a positive PET/CT result is serum TG level, according to ROC-curve study for the optimal TG level before PET/CT, which showed an area under the curve (AUC) of 81%. The ideal detection limit is 25 ng/ml, which provides a sensitivity of 81%, a specificity of 60% percent and an overall accuracy of 74 %.
An area under the curve (AUC) of 63% was shown by the ROC-curve analysis for the optimal Anti-TG level before PET/CT, indicating that serum TG level is a poor predictor of a positive PET/CT outcome. An estimated 27 ng/ml was the ideal level for the Anti-TG, with a sensitivity of 58%, specificity of 67 %, and an overall accuracy of 61 %.

Conclusion: Patients with suspicion of de-differentiation after an initial diagnosis of differentiated thyroid cancer might benefit from PET/CT as a diagnostic tool. Nevertheless, our findings suggest that the optimal TG level for PET/CT to reliably detect a positive result is more than 25 ng/ml.