Thyroid hormone status defines brown adipose tissue activity and browning of white adipose tissues in mice

The present study aimed to determine the effect of thyroid hormone dysfunction on brown adipose tissue activity and white adipose tissue browning in mice. Twenty randomized female C57BL/6NTac mice per treatment group housed at room temperature were rendered hypothyroid or hyperthyroid. In-vivo small animal ¹⁸F-FDG PET/MRI was performed to determine the effects of hypo- and hyperthyroidism on BAT mass and BAT activity. Ex-vivo¹⁴C-acetate loading assay and assessment of thermogenic gene and protein expression permitted analysis of oxidative and thermogenic capacities of WAT and BAT of eu-, hyper and hypothyroid mice. ¹⁸F-FDG PET/MRI revealed a lack of brown adipose tissue activity in hypothyroid mice, whereas hyperthyroid mice displayed increased BAT mass alongside enhanced ¹⁸F-FDG uptake. In white adipose tissue of both, hyper- and hypothyroid mice, we found a significant induction of thermogenic genes together with multilocular adipocytes expressing UCP1. Taken together, these results suggest that both the hyperthyroid and hypothyroid state stimulate WAT thermogenesis most likely as a consequence of enhanced adrenergic signaling or compensation for impaired BAT function, respectively.