Brown adipose tissue (BAT) plays a role as energetic buffer in rodents and recent data indicate the presence of active tissue in adult humans. 3,5-diiodo-L-thyronine (T2), a thyroid hormone derivative, enhances rat metabolic rate and prevents high fat diet-induced obesity. A deep investigation on the effect of T2 on BAT is lacking, thus we thought it interesting to examine the involvement of this thermogenic tissue in the metabolic effects of T2.

The studies were performed on three groups of rats: euthyroid, hypothyroid and hypothyroid rats receiving a chronic administration of T2 (25 mg/100g bw) for 1 week. Histological analysis of intrascapular BAT showed that in hypothyroid rats a trans-differentiation of the brown to white adipocyte phenotype takes place, as lipid droplets were enlarged and the number of unilocular cells within the tissue were enhanced. T2 administration reversed the effect induced by hypothyroidism and enhanced the uncoupling protein-1 immunoreactivity in multilocular cells. T2 also affects the BAT oxidative capacity being effective in enhancing cytochrome-oxidase activity when it was in vivo administered to hypothyroid rats. Interestingly, the stimulatory effect of T2 on oxidative capacity was also observed following the in vitro addition of T2 to BAT homogenate from hypothyroid rats. As a whole these data point toward the involvement of BAT in the metabolic effect exerted by T2.