Hyperthyroidism is associated with an increased body temperature. The molecular nature of this has not been clarified although it is thought that the reason is activation of brown adipose tissue. In the brown fat cells, thermogenesis would occur via uncoupling-protein-1 (UCP1). To test this hypothesis, Dittner et al. have examined thyroid thermogenesis induced by peripherally administered thyroxine in wild-type mice and UCP1 knockout mice. The authors found that thyroid thermogenesis was independent of the presence of UCP1.