This study investigated blood lead (Pb-B) levels and Pb-B effects on thyroid functions in long-term low-level-lead-exposed male adolescents who work as auto repairers. Pb-B and ALAD index (logarithm of activated δ-aminolaevulinic acid dehydratase/nonactivated δ-aminolaevulinic acid dehydratase) were measured as indicators of exposure to lead. Thyroid function tests including free thyroxine (FT4), free triiodothyronine (FT3), and thyrotrophin (TSH) were conducted and thyroid ultrasounds were performed in 42 lead-exposed adolescents and 55 healthy control subjects. Mean Pb-B levels and ALAD index were found significantly higher in the study group than in the normal control group (7.3±2.92μg/dl vs. 2.08±1.24μg/dl, Symbol and 0.44±0.26 vs. 0.29±0.23, Symbol, respectively). FT4 levels were found significantly lower in the study group (1.02±0.18mI/mL and 1.12±0.14mIU/mL, Symbol). No subject in the control group had an abnormal FT4 level, but FT4 levels were found under normal limits in 11 subjects (26%) in the study group. FT3 and TSH levels in the study and control groups did not differ (Symbol). Thyroid volumes in the study and control groups did not exhibit any significant differences (Symbol). Pb-B was found to be negatively correlated to FT4 levels (Symbol, Symbol). This study revealed that long-term low-level lead exposure may lead to reduced FT4 level without significant changes in TSH and T3 levels in adolescents even at low Pb-B levels.