Abstract: Objective To observe the effects of the aqueous extract isolated from Prunella vulgaris (PVAE) on thyroid inflammation and Th1/Th2 equilibrium in rats with experimental autoimmune thyroiditis (EAT). Methods Lewis rats were randomly divided into 5 groups:normal control group (C), EAT model control group (M), PVAE preventive treatment group (P), low dose of PVAE treatment group (TL) and high dose of PVAE treatment group (TH). Rats were challenged with porcine thyroglobulin (pTg) for immune sensitization except those of group C. Rats of Group P and group TH received 10 g/kg bodyweight per day of crude drugs by ig, and 2.5 g/(kg·d) in group TL, while those in control groups were given normal saline. The thyroid pathologies, serum autoantibody level and cytokine levels were examined. Results ①Compared with the group C, both the serum levels of autoantibodies and TSH were higher, while the level of TPOAb in rats of group P were lower than those in group M post treatment (P=0.037). ②The serum cytokine levels produced by T helper 1 and 2 were increased in rats with EAT, PVAE modulated the imbalance of Th1 and Th2 cytokine responses in EAT rats by reducing the levels of Th1 cytokines (IFN-γ and IL-12), but increasing Th2 cytokine (IL-4 and IL-10), while reducing the ratio of Th1/Th2. The serum interleukin-12 concentration and the ratio of IL-12/IL-10 were significantly lower in rats of group P than those in group M after the test (P<0.05). Serum IL-12 concentration was also significantly lower in rats of group TL than those in group M after test (P=0.018). Additionally the Serum IL-4 concentration was significantly higher in rats of group TH than those in group M after test (P=0.021); while the ratio of IFN-γ/IL-4 was lower (P=0.018). ③The correlation analysis showed that the levels of antibodies and T-helper cytokines were all significantly correlated with TSH (P<0.001). The level of TPOAb, IL-10 and TgAb may contribute in the modulation of the concentration of TSH (R²=0.868, F=191.389, P<0.001). Conclusion Our research suggesting PVAE as a promising drug for intervention of autoimmune thyroiditis, especially for the prevention treatment of PVAE.