Leptin is an adipocyte-secreted protein, which functions as a metabolic, neuroendocrine hormone beside its role in regulation of food intake and energy balance. This study was done to assess the influence of hypo- and hyperthyroidism on plasma leptin levels. Also, the correlation between the change of T3 (triiodothyronine), T4 (thyroxine), TSH (thyroid stimulating hormone) and plasma leptin was done in those patients under medical treatment. Material and Methods: The current study was done on patients with newly discovered thyroid disorders (17 cases with hypothyroidism and 19 cases with hyperthyroidism) and 19 euthyroid subjects as a control group where thyroid hormones were measured and their relation to plasma leptin. Results: The mean leptin levels were higher in hypothyroid patients (19.5ng/ml ±1.9 in males and 21.9 ng/ml ±4.97 in females) and lower in hyperthyroid patients (8.2 ng/ml ±0.43 in males and 7.8ng/ml ±0.36 in females) compared with controls (12.4 ng/ml ±1.1 in male and 15.6 ng/ml ±3 in female). Leptin was significantly negatively correlated with T4 (r = - 0.65, P<0.0001), T3 (r = -0.57, P<0.005) and positively correlated with TSH (r =0.67, P<0.005) after adjusting for BMI. Also, plasma leptin was significantly positively correlated with body mass index (BMI) (r = 0.45, P<0.005). Conclusion: Plasma leptin was significantly elevated in hypothyroidism and decreased in hyperthyroidism. Its level was negatively correlated with T3 and T4 and positively correlated with TSH. So, T4, T3 and TSH are potent regulator of plasma leptin independent of BMI.