This study was designed to investigate the effect of hyperthyroidism and/or iron supplementation on cardiac oxidative stress parameters-the lipid peroxidation end product glutathione (GSH), glutathione peroxidase (GSH-Px), and superoxide dismutase (CuZnSOD) - in rats, In plasma, ferritin as an indicator of iron status and glutamate oxaloacetate transaminase (GOT) as an indicator of damage to the heart tissue were analyzed. Our findings show that hyperthyroidism increased lipooxidative damage as reflected by higher lipid peroxidation end product levels and elevated antioxidant defense parameters-GSH and GSH-Px. Iron supplementation per se does not affect oxidative stress parameters studied in the euthyroid state. Although iron increased lipid peroxidation in the hyperthyroid state, this effect was less than that seen in euthyroidism. Iron supplementation to hyperthyroid rats significantly lowered plasma ferritin levels, suggesting increased iron elimination with consequently reduced oxidative stress.