ABSTRACT: BACKGROUND: Leptin is the protein product of the ob gene, secreted by adipocytes. It has been suggested that it may plays an important role in regulating appetite and energy expenditure, but beside that, little is known about the physiological actions of leptin in humans. OBJECTIVE: To evaluate a possible influence of primary hypothyroidism on serum leptin levels. METHODS: Fifty-six newly diagnosed patients with primary hypothyroidism (40 females and 16 males) and 32 normal controls matched for age, ethnic status and body mass index (BMI) were studied. Body mass index (BMI; kg/m2), thyroid function(using enzyme-linked immunoflourescent assay) and serum levels of leptin, thyroid autoantibodies (measured by enzyme-linked immunosorbant assay) and lipid profile(measured by enzymatic colourimeteric assays) were assessed in all studied subjects. RESULTS: No significant difference in serum leptin levels was recorded between hypothyroid patients and controls (16.3±14.9; 14.8 ± 12.9, P> 0.05), but women in each group had significant higher leptin concentrations than men (patients: 19.6 ±16.3 vs. 8.3 ±5.0; controls: 19.0 ±14.4 vs. 7.7 ±4.1; P< 0.05). Serum levels of cholesterol (p<0.002), LDL-cholesterol (p<0.004) and atherogenic ratio (p<0.03) were generally higher in patients than controls. The serum leptin concentration correlated positively with BMI within both patients (r=0.32; p<0.016) and controls (r=0.28; p<0.024). However, no association was demonstrated between values of serum T3, T4, TSH, triglyceride, HDL-cholesterol and thyroid auto antibodies. CONCLUSION: Circulating thyroid hormones do not appear to play any significant effect on leptin levels in patients with primary hypothyroidism.