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Objective. We retrospectively analyzed activated BAT visualization on FDG-PET/CT in patients with various conditions and TH levels to clarify the relationships between visualization of BAT on FDG-PET/CT and the effect of TH. Methods. Patients who underwent clinical FDG-PET/CT were reviewed and we categorized patients into 5 groups: (i) thyroid hormone withdrawal (THW) group; (ii) recombinant human thyrotropin (rhTSH) group; (iii) hypothyroidism group; (iv) hyperthyroidism group; and (v) BAT group. A total of sixty-two FDG-PET/CT imaging studies in fifty-nine patients were performed. To compare each group, gender; age; body weight; serum TSH, FT3, and FT4 levels; and outside temperature were evaluated. Results. No significant visualization of BAT was noted in any of the images in the THW, rhTSH, hypothyroidism, and hyperthyroidism groups. All patients in the BAT group were in a euthyroid state. When the BAT-negative and BAT-positive patient groups were compared, it was noted that the minimum and maximum temperature on the day of the PET study and maximum temperature of the one day before the PET study were significantly lower in BAT-positive group than in all those of other groups. Conclusions. Elevated TSH condition before RIT, hyperthyroidism, or hypothyroidism did not significantly impact BAT visualization of clinical FDG-PET/CT images.