Background : Free radical-mediated oxidative stress has been implicated in the etiopathogenesis of several autoimmune disorders. Also, there is growing evidence supporting the role of reactive oxygen species in the pathogenesis of thyroid disorders. The aim of this study was to investigate the influence of hypothyroidism, hyperthyroidism, and their treatments on the metabolic state of oxidative stress, and antioxidant status markers. Methods : A total of 20 newly diagnosed patients with overt hypothyroidism due to Hashimoto's thyroiditis, 20 patients with overt hyperthyroidism due to Graves' disease, and 20 healthy subjects as the control group were enrolled in the study. Fasting blood samples (12 h), taken at the initiation, after the 30th and 60th day of therapy were analyzed for malondialdehyde, nitrite, vitamin E, vitamin A, β-carotene, ascorbate, and myeloperoxidase and superoxide dismutase activity. No patient presented additional risk factors for increased reactive oxygen species levels. Results : Malondialdehyde, nitrite, vitamin E, and myeloperoxidase activity increased in patients with hypothyroidism. After 2 months, the levels of nitrite and vitamin E were reduced to control levels by treatment. The patients with hyperthyroidism had increased levels of malondialdehyde and myeloperoxidase activity in comparison with the controls. Treatment with propylthiouracil attenuated these increments after 1 month. Conclusions : Our results reveal an increased generation of reactive oxygen species and impairment of the antioxidant system in patients with hyperthyroidism, and particularly in patients with hypothyroidism. These findings indicate that thyroid hormones have a strong impact on oxidative stress and the antioxidant system. Clin Chem Lab Med 2008;46:1004–10.