Background/Aims: The aim of this study was to analyze thyroid hormones in female adolescents with obesity and anorexia nervosa (AN) before and after normalization of weight. Methods: Thyroid-stimulating hormone (TSH), fT3, and fT4 were determined in 100 obese girls, 32 normal-weight girls and 20 girls with AN aged 14–18 years at baseline and 1 year later. Additionally, leptin, insulin, and the insulin resistance index HOMA were analyzed in the obese and normal-weight girls. Results: TSH and fT3 levels of girls with AN were significantly lower compared to TSH concentrations of normal-weight girls, while TSH and fT3 levels of the obese girls were significantly higher. The 21 obese females with weight loss >5% demonstrated a significant decrease in fT3 and TSH, while the 9 adolescents with AN and weight gain >5% showed a significant increase in fT3 and TSH. Insulin and HOMA were not significantly correlated to TSH, fT3 and fT4, while leptin was correlated to TSH and fT3 in both cross-sectional and longitudinal analysis. Conclusions: Thyroid function seems to be reversibly related to weight status with increased TSH and fT3 concentrations in obesity and decreased TSH and fT3 levels in AN. We hypothesize that leptin may be the link between weight status and TSH.