Objective To explore the therapeutic effect, safety and appropriate dosage of Lthyroxine on child hypothyroidism. Methods In the treatment group (L Group) Lthyroxine was used (n =40), while in the control group (T Group) dry thyroxine was used (n=35). The cl inical behavior, TSH, T4, T3 and bone ages were observed. Results Significant difference in remission in the 4th week of therapy was found between the 2 groups.The L Group was significantly higher than the T Group (χ 2=13.78, P<0.01). There was a significant difference in the 4th we ek in the convalescence of TSH and T4. The L Group was higher than the T Group (χ2=24.75，P<0.05). No hyperthyroidism was found in the L Group. Conclusions Lthyroxine therapy for child hypothyroidism resu lts in significant improvement in brain development and milder side effects than tranditional treatment. Lthyroxine may be considered for clinical use.