Since oxidative stress is related to autoimmune thyroid disease, we studied the production of 8-hydroxy-2'-deoxyguanosine (8-OHdG) and cytochrome c by culture of mononuclear cells from patients with Graves' disease and Hashimoto's thyroiditis. In patients with untreated Graves' disease, 8-OHdG and cytochrome c levels in culture supernatant of mononuclear cells were significantly higher than those of healthy control subjects, while the cytochrome c levels were significantly higher in patients with untreated Graves' disease and Hashimoto's thyroiditis than those of control subjects. Significant correlations between 8-OHdG and FT4, and cytochrome c were found. These results indicated that thyroid function has a potent influence on oxidative stress.