*2.2. Glucose Transporter-Targeting Probes*

[ 18F]FDG, mainly transported by glucose-transporter family-1 (GLUT1), is a wellknown radiopharmaceutical glucose analog used in clinical PET imaging [52]. Aggressive TCs with low radioiodine uptake generally show high levels of [18F]FDG uptake [53].

[ 18F]FDG-PET/CT has shown great sensitivity in patients who otherwise do not benefit from 131I treatment. This is because metastases without radioiodine uptake tend to have high glycolytic rates, causing enhanced [18F]FDG uptake [54]. Furthermore, [18F]FDG can help detect TC recurrence or metastases and predict radioiodine uptake [54,55]. More specifically, [18F]FDG maximum standard unit value (SUVmax) higher than 4.0 would predict poor radioiodine uptake [54]. Besides, most PDTC, ATC, and MTC cells do not concentrate 131I. Thus, in these entities, [18F]FDG PET/CT imaging is useful in initial diagnosis, subsequent disease grading, treatment, and follow-up after treatment. Even though numerous glucose analog tracers other than [18F]FDG were created [56], their head-to-head comparisons with the widely used [18F]FDG are lacking.
