Leptin is considered to play a role in maintenance of energy balance and body weight by neuroendocrine mechanisms. The physiological mechanisms for thyroid hormone-induced alteration in serum leptin are not well known. In the present study, the relationship between thyroid hormones and leptin levels was investigated in patients with overt hypothyroidism and hyperthyroidism before and after successful treatment. Leptin levels were determined by radioimmunoassay and body mass index (BMI) was calculated for each subject. Serum leptin levels of 26 hypothyroid and 22 hyperthyroid patients were compared with those of 20 healthy volunteers who comprised the controls. Serum teptin levels of hypothyroid patients (28.4 &PLUSMN; 4.1 ng/ml) were found to be significantly higher than the controls (19.1 &PLUSMN; 3.2 ng/ml) (p< 0.01), whereas hyperthyroid patients had lower levels (10.7 &PLUSMN; 1.2 ng/ml) (p< 0.01). In hypothyroid patients, serum leptin levels were decreased significantly to 20.6 &PLUSMN; 2.1 ng/ml with thyroxin treatment (p< 0.05). However, in hyperthyroid group, serum leptin levels were increased to 12.4 &PLUSMN; 2.2 ng/ml by treatment (p> 0.05). BMI was not changed with the treatment in either group. The serum leptin levels were correlated with BMI and thyrotropin (TSH) in both hypothyroid and hyperthyroid patients. Serum leptin levels are affected in thyroid disorders and the correlation of leptin with TSH is independent of thyroid hormones.