BACKGROUND: Common carotid artery intima-media thickness (IMT), which is increased in patients with overt hypothyroidism, is an independent risk factor of the atherosclerosis-related disease. This study was performed to compare serum lipid level and common carotid artery IMT among patients with overt hypothyroidism, subclinical hypothyroidism and euthyroidism. METHODS: Patients with newly-diagnosed subclinical (n=32) and overt (n=32) hypothyroidism were selected for this study. All of the patients and an age- and sex-matched euthyroidism cohort were checked for clinical characteristics and serum lipid levels. Common carotid artery IMT was also measured using high resolution B-mode ultrasonography. RESULTS: Statistically significant differences were observed between the total cholesterol levels of patients with subclinical hypothyroidism and those with euthyroidism. When common carotid artery IMT measured by ultrasonography, subclinical (0.67 +/- 0.11 mm) and overt (0.71 +/- 0.12 mm)) hypothyroidism showed significantly increased mean IMT compared to that of euthyroidism (0.58 +/- 0.07 mm, P < 0.05, respectively), but no differences were found between subclinical and overt hypothyroidism. CONCLUSION: We concluded that subclinical hypothyroidism is related to increased common carotid artery IMT as well as dyslipidemia. Therefore, we recommend that treatment principle of subclinical hypothyroidism be established through large-scale, prospective studies performed to determine the effect of thyroid hormone replacement on the reduction of common carotid artery IMT.