OBJECTIVE:To explore the effects of levothyroxine on the follicular helper T lymphocyte level of patients with Hashimoto's thyroiditis. METHODS:51 patients with Hashimoto's thyroiditis were selected as levothyroxine group,and another 30 healthy people were control group. Levothyroxine group was orally given 25-50 ug Levothyroxine sodium tablet,once a day,the dose was adjusted based on thyroid function,and the dose maintained when thyroid function turned normal,for continuous 90 d. Free triiodothyronine(FT3),free thyroxine(FT4),thyroid stimulating hormone(TSH)and the percentage of Th17 cells account-ed for peripheral blood CD4+ T cells(Th17/CD4+)in levothyroxine group before and after treatment were observed and compared with control group,the correlation of Th17/CD4+ and thyroid function. RESULTS:After treatment,Th17/CD4+ level in levothyrox-ine group were significantly lower than before and higher than control group,the differences were statistically significant(P<0.05). TSH in levothyroxine group were significantly lower than before,FT3 and FT4 were significantly higher than before,the differenc-es were statistically significant(P<0.05);compared with control group,the differences were not statistically significant(P>0.05). Th17/CD4+ showed positive correlation with thyroid function in levothyroxine group. CONCLUSIONS:Levothyroxine can signifi-cantly reduce the Th17/CD4+ and TSH,increase FT3 and FT4,and Th17/CD4+ is involved in the occurrence and progression of Hashimoto's thyroiditis.