The mechanism of atherosclerosis in patients with subclinical hypothyroidism (sHT), which has been partly attributed to lipid abnormalities, is still controversial. There is substantial evidence that ox-LDL plays an important role during the atherosclerosis process and paraoxonase-1 (PON-1) significantly inhibits generation of lipid peroxidation and thus plays a role in against atherosclerosis. The aim of the study was evaluate qualitative changes in lipoprotein metabolism, hs-CRP concentrations and PON1 activities with respect to common carotid artery intima-media thickness (CIMT) in 25 sHT (aged 48.96±8.42 yr) patients before and after 4 months of levothyroxine substitution therapy and 24 normolipidemic healthy individuals (aged 42.79±8.12 yr) comprised with the control group. There were no significant differences between controls and patients with sHT for age (P=0.05). At baseline, compared to controls, patients with sHT showed similar levels of total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides levels. PON1 activities, hs-CRP concentrations and mean CIMT were similar between sHT and control group. Levothyroxine treatment had no effect on serum PON-1 activities and hs-CRP concentrations but resulted a significant reduced mean CIMT in the subgroup of patients with TSH levels >10 mIU/L (P=0.017).
In multiple linear regression analysis, we found the decrement in mean-CIMT was directly related to the decrement of waist circumference (r=0.532, P=0.006). In conclusion, monitoring of PON-1 activities and hs-CRP concentrations did not offer additional arguments for treating patients with sHT. However, the fact that levotiroxine replacement therapy was able to reduced CIMT suggests that beneficial effects of levotiroxine treatment for decreasing the risk of atherosclerosis in the subgroup of patients with TSH levels >10 mIU/L.