Objective:To investigate correlation between neutrophil extracellular traps(NETs) formation and T cell subsets in mice with experimental autoimmune thyroiditis(EAT) and the impact of active vitamin D intervention.Methods:Six-week-old female BALB/c mice were randomly divided into Control group, EAT group and 1, 25 dihydroxy vitamin D 3[1, 25(OH) 2D 3] treatment group(VitD group; n=6/group). HE staining was used to observe thyroid pathology. Plasma thyroglobulin antibody(TGAb), thyroid peroxidase antibody(TPOAb), and 1, 25(OH) 2D 3 were measured by ELISA. Peripheral NETs formation, Th1, Th2, and Th17 cell ratio from spleen were measured by flow cytometry. Correlation between NETs formation rate and Th1, Th2, and Th17 cell ratio was analyzed. Results:Compared with Control group, mice in EAT group had significantly increased thyroid inflammation scores, thyroiditis morbidity, TPOAb, TGAb levels, NETs formation rate, Th2(CD4 + IL-4 + or CD4 + IL-13 + )and Th17 cell proportions( P were <0.001, 0.002, 0.007, <0.001, <0.001, 0.003, 0.001, and 0.002, respectively), and significant decreased 1, 25(OH) 2D 3, Th1 cell proportions, Th1/Th2(CD4 + IL-4 + ), Th1/Th2(CD4 + IL-13 + ), and Th1/Th17 ratios( P were 0.010, 0.018, 0.010, 0.005, and 0.007, respectively). Compared with the EAT group, the VitD group had lower thyroid inflammation scores, TPOAb, TGAb levels, NETs formation rate, Th2(CD4 + IL-4 + or CD4 + IL-13 + ) and Th17 cell proportions( P were 0.044, 0.007, <0.001, 0.001, 0.014, 0.008, and 0.001, respectively), and significant higher Th1 cell ratio, Th1/Th2(CD4 + IL-13 + ) and Th1/Th17 ratio( P were 0.011, 0.009, and 0.003, respectively). The Th1/Th2(CD4 + IL-4 + ) was not significantly increased in VitD group compared with EAT group( P=0.174). NETs formation rate was positively correlated with Th2(CD4 + IL-4 + or CD4 + IL-13 + ) and Th17 cell proportion( r were 0.65, 0.59, and 0.61; and P were 0.004, 0.010, and 0.007, respectively), but not with Th1 cell proportion( r=-0.47, P=0.051). Conclusion:EAT mice were more prone to NETs formation. Active vitamin D may relieve immune imbalance with increased Th2 and Th17 cell ratio and decreased Th1 cell ratio by reducing the formation of NETs in EAT mice. Vitamin D played the protective role in thyroid by reducing thyroid pathological damage and thyroid autoantibody levels, and relived overall lymphocyte imbalance.