Our study was designed to test and compare the levels of enzymatic antioxidants (ARS), endogenous peroxides (POX), non-enzymatic antioxidants (Antiox-cap) and anti-oxidized low-density lipoprotein (LDL) antibody titers (oLAb) in hyperthyroid and hypothyroid patients. We measured POX, ARS, Antiox-cap and oLAb in the plasma from 68 patients (34 patients with hyperthyroidism, thyroid-stimulating hormone (TSH) <0.04; and 34 patients with hypothyroidism, TSH >4.0) and 34 healthy euthyroid controls. POX were highest in hyperthyroid patients, but differences between hyperthyroid and hypothyroid patients and controls were not significant. ARS were significantly higher in patients compared to controls. Antiox-cap were significantly lower in patients compared to controls. oLAb were significantly higher in hypothyroid patients compared to controls and hyperthyroid patients. Our study shows that both hyperthyroidism and hypothyroidism are associated with enhanced oxidative stress involving enzymatic and non-enzymatic antioxidants. Higher POX in hyperthyroid patients might reflect the hypermetabolic state of these patients. oLAb, indicating progression of atherosclerosis, were highest in hypothyroid patients.