Evidence from clinical and experimental data suggests that thyrotropin receptor (TSHR) signaling is involved in energy expenditure through its impact on white adipose tissue (WAT) and brown adipose tissue (BAT). TSHR expression increases during mesenchymal stem cell (MSC) differentiation into fat. We hypothesize that TSHR activation [TSHR*, elevated thyroid-stimulating hormone, thyroid-stimulating antibodies (TSAB), or activating mutation] influences MSC differentiation, which contributes to body composition changes seen in hypothyroidism or Graves' disease (GD). The role of TSHR activation on adipogenesis was first investigated using ex vivo samples. Neck fat (all euthyroid at surgery) was obtained from GD (n = 11, TSAB positive), toxic multinodular goiter (TMNG, TSAB negative) (n = 6), and control patients with benign euthyroid disease (n = 11, TSAB negative). The effect of TSHR activation was then analyzed using human primary abdominal subcutaneous preadipocytes (n = 16). Cells were cultured in complete medium (CM) or adipogenic medium [ADM, containing thiazolidinedione (TZD), PPAR. agonist, which is able to induce BAT formation] with or without TSHR activation (gain-of-function mutant) for 3 weeks. Adipogenesis was evaluated using oil red O (ORO), counting adipogenic foci, qPCR measurement of terminal differentiation marker (LPL). BAT [PGC-1 alpha, uncoupling protein 1 (UCP1), and ZIC1], pre-BAT (PRDM16), BRITE-(CITED1), or WAT (LEPTIN) markers were analyzed by semiquantitative PCR or qPCR. In ex vivo analysis, there were no differences in the expression of UCP1, PGC-1a, and ZIC1. BRITE marker CITED1 levels were highest in GD followed by TMNG and control (p for trend = 0.009). This was associated with higher WAT marker LEPTIN level in GD than the other two groups (p < 0.001). In primary cell culture, TSHR activation substantially enhanced adipogenesis with 1.4 +/- 0.07 (ORO), 8.6 +/- 1.8 (foci), and 5.5 +/- 1.6 (LPL) fold increases compared with controls. Surprisingly, TSHR activation in CM also significantly increased pre-BAT marker PRDM16; furthermore, TZD-ADM induced adipogenesis showed substantially increased BAT markers, PGC-1a and UCP1. Our study revealed that TSHR activation plays an important role in the adipogenesis process and BRITE/pre-BAT formation, which leads to WAT or BAT phenotype. It may contribute to weight loss as heat during hyperthyroidism and later transforms into WAT posttreatment of GD when patients gain excess weight.