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INTRODUCTION Oxidative stress has been implicated in pathogenesis of thyroid-associated orbitopathy (TAO) in patients with Graves' disease (GD). This study assessed the effect of thyroid hormone abnormalities on selected antioxidant parameters of in patients with active TAO. MATERIAL AND METHODS The study group consisted of 56 patients with GD and active TAO treated with antithyroid medication. Depending on the thyroid hormone level, they were subdivided into two groups: group 1 - hyperthyroid patients (n = 34) and group 2 - euthyroid patients (n = 22). The total oxidant status expressed as the ferric reducing ability of plasma (FRAP) as well as selected enzymatic and non-enzymatic components of the antioxidant system, including the activity of superoxide dismutase (SOD), glutathione peroxidase (GPx) and paraoxonase-1 (PON-1), as well as the levels of vitamin C, uric acid and lipid peroxidation products: malondialdehyde (MDA) and conjugated dienes (CD) were assessed in all enrolled participants. RESULTS The FRAP values in group 1 were significantly higher than in controls. The FRAP values in group 2 were lower than in group 1 and higher than in controls. However, the differences were not significant. In group 1, the activity of SOD and GPx, as well as serum levels of uric acid, MDA and CD were significantly higher than in controls. At the same time serum PON-1 activity and vitamin C levels were significantly lower in group 1 than in controls. In group 2, the SOD activity as well as MDA and CD levels were non-significantly lower than in group 1 and non-significantly higher than in controls. The activity of GPx in euthyroid patients with TAO was significantly higher than in controls. CONCLUSIONS Hyperthyroidism is a significant contributor to oxidative stress in patients with active TAO, which manifests as upregulated lipid peroxidation and antioxidant system activation. Euthyroid state restoration leads to a relative reduction in activity and levels of most studied antioxidant parameters, which still remain above the normal values. The autoimmune inflammation of the orbital tissue seems a thyroid hormone status-independent modifier of oxidative stress.