Background: High mobility group box-1 (HMGB1), a non-histone protein, plays an important role in autoimmune diseases. However, the significance of HMGB1 in the pathogenesis of autoimmune thyroiditis has not been reported. The purpose of this study was to explore whether HMGB1 participates in the pathogenesis of autoimmune thyroiditis, and whether glycyrrhizin (GL), a direct inhibitor of HMGB1, attenuates the severity of thyroid inflammatory infiltration in a murine model of autoimmune thyroiditis. Methods: A total of 80 male NOD. H-2(h4) mice were randomly divided into a control or iodine supplement (NaI) group at four weeks of age, and the control group was fed with regular water, whereas the NaI group was supplied with 0.005% sodium iodine water. Another 24 male NOD. H-2(h4) mice were also randomized into three groups (eight mice per group) as follows: control, NaI, and GL treatment after iodine supplementation (NaI + GL). The NOD.H-2 (h4) mice were fed with 0.005% sodium iodide water for eight weeks to enhance autoimmune thyroiditis. After iodine treatment, the mice received intraperitoneal injections of GL for four weeks. The severity of lymphocytic infiltration in the thyroid gland was measured by histopathological studies. The serum levels of HMGB1, tumor necrosis factor alpha, interleukin (IL)-6, IL-1 beta, and thyroglobulin antibody titers were measured using an enzyme-linked immunosorbent assay. HMGB1 expression was measured by immunohistochemical staining and real-time polymerase chain reaction. TLR2, HMGB1, MyD88, and nuclear transcription factor kappa B were measured by Western blot. Results: The mRNA expression of HMGB1 was significantly higher at 8 and 16 weeks in the NaI group than it was in the control group. Serum levels of thyroglobulin antibodies, HMGB1, tumor necrosis factor alpha, IL-6, and IL-1 beta were significantly increased in the NaI group, but they were dramatically attenuated with GL injection. The prevalence of thyroiditis and the infiltration of lymphocytes were significantly decreased in the NaI + GL group. GL administration also significantly reduced the protein expression of TLR2, MyD88, HMGB1 and nuclear transcription factor jB in the thyroid gland and attenuated the severity of thyroiditis. Conclusion: HMGB1 may play a crucial role in autoimmune thyroiditis by causing inflammatory infiltration, thus increasing the severity of autoimmune thyroiditis. GL effectively attenuated thyroiditis in the iodine-induced NOD. H-2(h4) mice via a molecular mechanism related to the inhibition of TLR2-HMGB1 signaling.