Changes in serum lipid and lipoprotein concentrations occur frequently in disorders of thyroid function. LDL-cholesterol (LDL-C) oxidation susceptibility is higher in these patients than in normal population. This study aims at assessing lipids, lipoproteins, apolipoproteins and serum paraoxonase 1 (PON1) activity in patients with thyroid dysfunction. Ninety-nine patients with thyroid dysfunction, (49 hypothyroid and 50 hyperthyroid) were compared with 2 separately age- and sexmatched control groups. A fasting blood sample was obtained and serum total cholesterol, triglycerides, apolipoproteins A-I and B, and PON1 activity were measured. In hyperthyroid patients, significantly lower PON1 activity (45±23 vs 67±37 IU/ml, p<0.001), triglycerides (112±53 vs 166±130 mg/dl, p<0.05), apolipoprotein A-I (137±26 vs 154±21 mg/dl, p<0.001) and apolipoprotein B (75±18 vs 86±25 mg/dl, p<0.05) were found. Hypothyroid patients had lower PON1 activity (46±21 vs 64±32 IU/ml, p<0.005) compared with controls, and higher total cholesterol (224±69 vs 185±41 mg/dl, p<0.001), LDL-C (133±59 vs 93±36 mg/dl, p<0.001), and apolipoprotein B (107±37 vs 84±23 mg/dl, p<0.001). The results show significant changes of lipid levels in thyroid dysfunction. In addition, a significant reduction in PON1 activity was observed in both hyper- and hypothyroid patients. Increased LDL-C oxidation in thyroid dysfunction observed in other studies, at least to some extent, can be attributed to reduced PON1 activity.