Abstract: The aim of this study was to determine whether iron supplementation in iron-deficient adolescent girls would improve thyroid function. A double-blind clinical trial was performed in a region in southern I.R. Iran. A total of 103 iron deficient participants were chosen. In all, 94 participants successfully completed this study. Participants were randomly assigned to one of two groups and treated with a 300 mg ferrous sulfate 5 times/week (n = 47) and placebo 5 times/week (n = 47) for 12 weeks. Blood samples were collected and assayed for hemoglobin, hematocrit, serum ferritin, iron, total iron binding capacity (TIBC), Thyroid stimulating hormone (TSH), total thyroxine (TT4), total triiodothyronine (TT3), free thyroid hormones (FT4 and FT3), triiodothyronine resin uptake (T3RU), reverse triiodothyronine (rT3), selenium and albumin concentrations. Statistical analysis was performed with parametric and non-parametric methods as appropriate. Data analysis revealed a significant increase in TT4, TT3, T3RU and a significant decrease in rT3 concentration in comparison to initial values in iron treated group (12%, p<0.001; 3.5%, p<0.001; 16%, p<0.05 and 47%, p<0.001, respectively). At 12 week there were significant differences between control and placebo in TT4, TT3, T3RU and rT3 concentrations (9.9 vs 8.4 μg dL-1, 145.2 vs 130.4 μg dL-1, 32.5 vs 28.4% and 23 vs 41 μg dL-1, respectively, all p<0.001). Alterations in FT3 and TSH concentration were not significant, but concentration of FT4 revealed a significant difference between the beginning and the end of the study in iron treated group (10.3 vs 11.4, p<0.001). Iron supplementation improves some indices of thyroid function in iron-deficient adolescent girls.