Brown adipose tissue (BAT) controls metabolic rate through thermogenesis. As its regulatory factors during the transition from hyperthyroidism to euthyroidism are not well established, our study investigated the relationships between supraclavicular brown adipose tissue (sBAT) activity and physiological/metabolic changes with changes in thyroid status.
Participants with newly diagnosed Graves’ disease were recruited. A thionamide anti-thyroid drug (ATD) such as carbimazole (CMZ) or thiamazole (TMZ) was prescribed in every case. All underwent energy expenditure (EE) measurement and supraclavicular infrared thermography (IRT) within a chamber calorimeter, as well as 18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography/magnetic resonance (PET/MR) imaging scanning, with clinical and biochemical parameters measured during hyperthyroidism and repeated in early euthyroidism. PET sBAT mean/maximum standardized uptake value (SUV mean/max), MR supraclavicular fat fraction (sFF) and mean temperature (Tscv) quantified sBAT activity.
Twenty-one (16 female/5 male) participants aged 39.5 ± 2.5 years completed the study. The average duration to attain euthyroidism was 28.6 ± 2.3 weeks. 8 participants were BAT-positive while 13 were BAT-negative. sFF increased with euthyroidism (72.3 ± 1.4% to 76.8 ± 1.4%; P<0.01), but no changes were observed in PET SUV mean and Tscv. Significant changes in serum free triiodothyronine (FT3) levels was related to BAT-status (interaction P value= 0.04). FT3 concentration at hyperthyroid state was positively associated with sBAT PET SUV mean (r=0.58, P=0.01) and resting metabolic rate (RMR) (P<0.01).
Hyperthyroidism does not consistently lead to detectable increase in BAT activity. FT3 reduction during transition to euthyroidism correlated with BAT activity.