Objectives: Several researchers have reported that type 2 idothyronine deiodinase 2 (D2) is considered to be important for the synergism of thyroid hormone (TH) and to be required to generate the T3 from T4 in the brown adipose tissue (BAT), which maintains the normal acute thermogenic function of BAT to occur. However, to date, details about the relationship between visualization of BAT on FDG-PET/CT and the effect of TH remain unknown. In this present of study, we retrospectively analyze BAT visualization on FDG-PET/CT in patients with various conditions and TH levels.Methods: In this retrospective investigation, we reviewed all patients who underwent clinical FDG-PET/CT studies from October 2010 to July 2015, and categorized patients for 5 groups: (i) Thyroid hormone withdrawal (THW) group; (ii) recombinant human thyrotropin (rh TSH) group; (iii) Hypothyroidism group; (iv) hyperthyroidism group; and (v) BAT group. 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, FT4 levels; and outside temperature were compared among groups.Results: No significant visualization of BAT was noted in any of the images included in the THW, rhTSH, hypothyroidism, and hyperthyroidism groups. Serum TSH level was significantly in the rhTSH group and significantly low in the hyperthyroidism group. Elevated serum TSH was noted in the THW and hypothyroidism group. All patients in the BAT group were in a euthyroid state. In comparison of BAT-negative and -positive patients, the minimum and maximum temperature on the day of the PET study and maximum temperature of the one day before of the PET study were significantly lower in BAT positive patients than those of other groups.Conculusion: In summary, we investigated the influence of TH on the visualization of activated BAT on FDG-PET/CT images of patients with various conditions. Although TH was considered to be associated with thermogenesis through UCP1 gene expression and D2 in BAT, elevated TSH status before RIT, hyperthyroidism, or hypothyroidism did not significantly impact BAT visualization and the interpretation of clinical FDG-PET/CT images.SNMMI 201