BACKGROUND: Thyroid hormones play an important role in regulating lipid and glucose metabolism. Thus this study was conducted to investigate the relationship between the thyroid hormone (FT4) or thyroid stimulating hormone (TSH) and the cardiovascular risk factors and metabolic syndrome in the individuals with subclinical thyroid dysfunction. METHODS: The female health examinee with normal range of FT4 were classified into three groups according to the level of TSH; euthyroid group (n=4,410), subclinical hypothyroidism group (n=438) and subclinical hyperthyroidism group (n=66). Age, blood pressure, BMI, fasting glucose, total cholesterol, HDL cholesterol, LDL cholesterol, triglyceride, lipoprotein(a), and high-sensitivity C-reactive protein (hsCRP) levels of serum specimens were compared among the groups and association of FT4 or TSH with these parameters. RESULTS: Fasting glucose was significantly higher in subclinical hyperthyroidism than in euthyroid and subclinical hypothyroidism groups (P=0.031), and total cholesterol was higher in subclinical hypothyroidism than in subclinical hyperthyroidism (P=0.011). But the other factors showed no difference among the groups. The level of TSH increased as triglyceride increased, while FT4 decreased as BMI or triglyceride increased. The FT4 also lowered when fasting glucose was above 126 mg/dL. TSH was not related with the metabolic syndrome, but the possibility of the syndrome was 1.3 times higher in the lowest quartile of the normal range of FT4 than in its highest quartile. CONCLUSIONS: For the interpretation of FT4, its reference interval needs to be divided into 4 quartiles, which can be used as one of the predicting factors of the metabolic syndrome.