Thyroid hormone (TH) plays a fundamental role in thermoregulation, yet the molecular mediators of its effects are not fully defined. Recently, skeletal muscle (SKM) uncoupling protein (UCP) 3 was shown to be an important mediator of the thermogenic effects of the widely abused sympathomimetic agents 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy) and methamphetamine. Expression of UCP3 is regulated by TH. Activation of UCP3 is indirectly regulated by norepinephrine (NE) and is dependent upon the availability of free fatty acids (FFAs). We hypothesized that UCP3 may be a molecular link between TH and hyperthermia, requiring increased levels of both NE and FFAs to accomplish the thermogenic effect. Here, we demonstrate that MDMA (40 mg/kg s.c.) significantly increases plasma FFA levels 30 min after treatment. Pharmacologically increasing NE levels through the inhibition of phenylethanolamine N-methyltransferase with Ϯ2,3-dichloro-␣-methylbenzylamine potentiated the hyperthermic effects of a 20 mg/kg dose of MDMA. Using Western blots and regression analysis, we further illustrated that chronic hyperthyroidism in rats potentiates the hyperthermic effects of MDMA and increases levels of SKM UCP3 protein in a linear fashion according to levels of circulating plasma TH. Conversely, chronic hypothyroidism results in a hypothermic response to MDMA that is directly proportionate to decreased UCP3 expression. Acute TH supplementation did not change the skeletal muscle UCP3 expression levels or temperature responses to MDMA. These findings suggest that, although MDMA-induced hyperthermia appears to result from increased NE and FFA levels, susceptibility is ultimately determined by TH regulation of UCP3-dependent thermogenesis.Hyperthermia results from a severe, unregulated rise in core body temperature induced by high ambient temperature, strenuous exercise, endocrinopathy, or drug exposure. To our knowledge, no drug treatment has been established through controlled trials as an efficacious therapy for conditions that involve severe hyperthermia, with the exception of malignant hyperthermia, which is effectively reversed with dantrolene (Blank and Boggs, 1993). The lack of therapeutic options for the management of hyperthermia probably results from an inadequate understanding of the basic molecular mechanisms of thermogenesis.The principal active thyroid hormone (TH) 3,5,3Ј-triiodo-Lthyronine is formed from the precursor 3,5,3Ј,5Ј-tetraiodothyronine (T 4 , thyroxine) by deiodinases present in various tissues (Bianco and Larsen, 2005). Although TH has been established as the primary endocrinologic regulator of body temperature (Silva, 2005) and facultative thermogenesis (FT) (for review see Lowell and Spiegelman, 2000), the mechanisms involved are complex and incompletely characterized. One family of genes regulated by TH (Gong et al., 1997) and believed to play a significant role in FT encodes for the mitochondrial uncoupling proteins (UCPs).UCPs "uncouple" free energy stored in the mitochondrial electrochemical p...