Abstract
Fluorodeoxyglucose positron emission tomography–computed tomography has become an essential imaging technique in modern oncology and plays an important role in the evaluation of Well-differentiated thyroid cancer. It combines positron emission tomography, which detects cellular metabolism, with computed tomography (PET-CT), which provides detailed anatomical localization.
The principle of FDG uptake is based on the increased glucose metabolism seen in malignant cells. In thyroid cancer, FDG uptake is closely related to the degree of tumor differentiation. Well-differentiated thyroid cancers generally retain the ability to absorb radioactive iodine because they preserve sodium-iodide symporter function. Consequently, these tumors may show relatively low FDG uptake. However, as the tumor becomes less differentiated and more aggressive, it gradually loses its iodine concentrating ability while its glucose metabolism increases.
Current 2025 ATA guidelines have made the role of FDG PET/CT in differentiated thyroid cancer more nuanced and clinically meaningful. It has placed the role of FDG PET in a wider clinical context while keeping previous indication. It is now considered earlier in evaluation of patients with recurrent or persistent disease, and its findings not only help in localization of disease but also provide meaningful insight regarding tumor biology and aggressiveness.
FDG PET/CT also provides prognostic information early identifying patients with high risk for rapid disease progression and disease specific mortality. Additionally, PET/CT also helps in evaluation of post treatment responses following systemic or local therapy of invasive disease. FDG PET/CT is complementary to WBS even in the presence of iodine avid metastatic lesion because FDG uptake can be seen at sites with or without iodine avidity.
Importantly, despite this expanded role, FDG-PET/CT is still not recommended for routine initial imaging. Overall evolution from 2015 to 2025 reflects a gradual but important transition from using FDG-PET/CT when radioiodine imaging fails to integrating it earlier as a part of more personalized, risk adapted approach to patient management.

Key words: Well differentiated, Thyroid cancer, ATA 2025, FDG, PET CT,