In this study it was of interest to investigate the relationship between plasma leptin levels and thyroid function in recently diagnosed 27 Basedow-Graves patients and 22 multinodular goiter patients brought to euthyroid state with 1 mo of propylthiouracil (PTU) treatment. A group of 15 control subjects matched for age, gender, and body mass index were also studied. In Basedow-Graves patients there were significantly higher T4 and T3 levels accompanied by a marked fall in plasma thyroid-stimulating hormone (TSH). In multinodular goiter patients with PTU, the plasma T3 and T4 levels were significantly lower than in Basedow-Graves and TSH was markedly higher. However, TSH values in multinodular goiter patients were signifcantly lower than the control Plasma leptin levels were unchanged in all groups. These data suggest that leptin levels are not affected by thyroid dysfunction.