The association between subclinical hypothyroidism and cardiovascular disease and the beneficial effect of levothyroxine replacement in subclinical hypothyroidism are still under debate. The present study was designed to determine whether subclinical hypothyroidism is associated with an increase in the intima-media thickness of the common carotid artery (C-IMT) and whether thyroid hormone replacement can reverse this change in the C-IMT. Patients with newly-diagnosed subclinical (n=36) and overt (n=40) hypothyroidism and healthy euthyroid individuals (n=32) participated in this study. All the patients were examined for clinical characteristics, and the serum lipid levels and the C-IMT were measured. Patients with subclinical hypothyroidism had a C-IMT measurement after 18 months of levothyroxine replacement. There were meaningful differences in total cholesterol and LDL-cholesterol levels between patients with subclinical hypothyroidism and euthyroidism. The subjects with subclinical and overt hypothyroidism had a greater C-IMT compared with euthyroid controls (0.66+/- 0.10 and 0.70+/- 0.11 vs. 0.57+/- 0.08 mm, respectively; P < 0.05). After 12 months of euthyroidism, 28 of 36 patients with subclinical hypothyroidism completed the follow-up study. Thyroid hormone replacement significantly decreased the C-IMT (0.67+/- 0.11 to 0.60+/- 0.10 mm; P = 0.021) and improved the lipid profile. Based on multiple regression analysis, the decrement in LDL-cholesterol was independently associated with the regression of the C-IMT. Subclinical hypothyroidism was closely related to an increased C-IMT. Thyroid hormone replacement resulted in regression of the increased C-IMT, which was attributed to the improvement in the lipid profile.