Although the effects of hypothyroidism or hyperthyroidism on weight have clearly been demonstrated, data regarding the effects of relatively minor defects in thyroid function are limited. In the present study, the relationship between body mass index (BMI) and alterations in thyroid function within the normal range was investigated in obese patients. Three hundred fifty-seven euthyroid, obese patients (309 female and 48 male; mean age, 42 years) were included in the study. The patients were divided into 2 groups based on a BMI threshold value of 40 kg/m(2). Thyroid function, BMI, and the epidemiologic characteristics of the patients were retrospectively evaluated. No statistically significant difference was detected between the groups with respect to gender, place of birth, place of residence, smoking habits, and family history. Severely obese patients with a BMI >= 40 kg/m(2) were older and had a lower serum-free thyroxine level (fT4) than obese patients with a BMI < 40 kg/m(2) (P < 0.01). In patients with normal thyroid function, a negative correlation between fT4 and BMI existed. No statistically significant association was detected between BMI and epidemiologic characteristics in euthyroid, obese patients.