Serum paraoxonase activity before and after treatment of thyrotoxicosis

Antioxidant effects of paraoxonase, a high density lipoprotein (HDL)-associated enzyme that inhibits low density lipoprotein cholesterol (LDL-C) oxidation in human serum, have been reported. Patients with thyroid dysfunction are more susceptible to oxidative stress, and may show enhanced LDL-C oxidation. The purpose of this study was to evaluate serum paraoxonase activity in patients with hyperthyroidism before and after treatment with methimazole (MMI).

Twenty-four hyperthyroid patients (15 women and nine men, aged 43.0 +/- 12.9 years) and 23 age- and sex-matched healthy controls were studied. Serum paraoxonase activity, lipid, lipoprotein and apolipoprotein levels were measured in fasting samples. Patients were treated with MMI 20-30 mg daily for the first month, and 5-10 mg daily thereafter, and re-evaluated after 6-9 months of treatment.

Significantly lower serum paraoxonase activity was present in hyperthyroid patients before treatment compared with the controls (43.4 +/- 21.9 vs. 72.6 +/- 41.2 U/ml, P < 0.005). After a mean follow-up of 7.3 months, 15 patients became euthyroid (treated) and nine were still hyperthyroid. After follow-up, serum paraoxonase activity had increased to 62.2 +/- 37.4 U/ml in those who became euthyroid (P < 0.05 compared with baseline). In patients who were still hyperthyroid serum paraoxonase was unchanged from baseline, at 43.2 +/- 23.2 U/ml.

Serum paraoxonase is reduced in patients with hyperthyroidism and reverts to normal after euthyroidism is attained. Reduced serum paraoxonase activity in thyrotoxicosis might predispose lipids to oxidation.