The effect of thyroid hormones on metabolism has long supported their potential as drugs to stimulate fat reduction, but the concomitant induction of a thyrotoxic state has greatly limited their use. Recent evidence suggests that 3,5-diiodo-L-thyronine (T2), a naturally occurring iodothyronine, stimulates metabolic rate via mechanisms involving the mitochondrial apparatus. We examined whether this effect would result in reduced energy storage. Here, we show that T2 administration to rats receiving a high-fat diet (HFD) reduces both adiposity and body weight gain without inducing thyrotoxicity. Rats receiving HFD + T2 showed (when compared with rats receiving HFD alone) a 13% lower body weight, a 42% higher liver fatty acid oxidation rate, appoximately 50% less fat mass, a complete disappearance of fat from the liver, and significant reductions in the serum triglyceride and cholesterol levels (-52% and -18%, respectively). Thyroid hormones and thyroid-stimulating hormone (TSH) serum levels were not influenced by T2 administration. The biochemical mechanism underlying the effects of T2 on liver metabolism involves the carnitine palmitoyl-transferase system and mitochondrial uncoupling. If the results hold true for humans, pharmacological administration of T2 might serve to counteract the problems associated with overweight, such as accumulation of lipids in liver and serum, without inducing thyrotoxicity. However, the results reported here do not exclude deleterious effects of T2 on a longer time scale as well as do not show that T2 acts in the same way in humans.