Introduction: In subclinical hypothyroidism (SH), serum lipid and lipoprotein concentrations are frequently changed. Compared with the healthy population, the levels of oxidized low-density lipoprotein cholesterol are higher, and the levels of high-density lipoprotein cholesterol are lower. In patients with SH, the mechanism of atherosclerosis may beattributed to the lipid abnormalities. There is evidence showing that oxidation plays an important role during the process of atherosclerosis. Inthis study, we evaluated the activity of paraoxonase (PON) and arylesterase (ARE) in patients with SH and investigated their relation with oxidative stress.