Objective: Thyroid functions in obese children and adolescentswere evaluated in order to determine subclinicaland clinical hypothyroidism.Materials and methods: In this study, 85 obese (Bodymass index >97th percentile) children, aged 2-14 years, aswell as 47 healthy controls were enrolled. Levels of serumfree triiodothyronine (fT3), free thyroxine (fT4) and thyroidstimulatinghormone (TSH) of the two groups were compared.Obese children with TSH level above 5.4 IU/mlwere also analyzed for thyroid autoantibodies and thyroidultrasounds were performed.Results: Obese children showed higher serum concentrationsof TSH and fT3 than the controls but no significantdifference in serum fT4 levels was found between the twogroups (P=0.001). One child had high auto antibodiesand 32 had high TSH levels. Of 28 children with TSH >5,4IU/ml, 25 children had normal thyroid ultrasound findingsand three had nodules or thyroiditis but no enlargementof the thyroid gland.Conclusion: TSH and fT3 levels were found to be higherin obese children compared with non-obese children withno difference of fT4 levels between two groups.Key words: Body mass index, obesity, thyroid functions