Aim: to reveal the effects of obesity on thyroid function in obese children and adolescents. Material and methods: we conducted a retrospective cohort study on a total of 183 participants. Patient group included pre-obese and obese children and adolescents (102 patient). While control group included normal individuals within the age-range that of patients' group (81 participants). Relation of TSH and fT4 with BMI was investigated. Result: There was a significant difference in the mean and median levels of TSH as obese children showed higher mean levels of TSH compared to the control group (95% C.I of the mean difference= 0.8 to 2.1). When data was stratified by gender, mean TSH were higher in males and female obese children (P <0.001 and 0.017 respectively). Moreover, mean TSH levels was compared between obese and non-obese children and it’s found to be higher in children younger than 10 years old (P < 0.001). Conclusion: Obesity is associated with an elevation of TSH serum levels with normal fT4 serum levels.