〔Abstract〕 Objective To study the effects of Hashimoto′s thyroiditis(HT) on the content of serotonin(5-HT) and the distribution of serotonin 2 c receptor(5-HT2 CR) in hippocampus of early pregnant mice. Methods 70 NOD female mice were randomly divided into normal pregnancy group(CON group) and HT pregnancy group(HT group).The HT mouse model was established by 2 times induction of immunization with porcine thyroglobulin(pTg) plus Freund′s adjuvant. Then they were caged with male rats and ended at 4.5 d of pregnancy. Each group randomly selected 16 pregnant rats. Serum thyroid hormone and antibody levels were determined by electrochemiluminescence immunoassay(ECLIA) and enzyme-linked ELISA. Thyroid was stained by HE staining. The content of 5-HT in hippocampus of mice in each group was measured by ELISA. The content and distribution of 5-HT2 CR in hippocampus of mice in each group were observed by Western blot and immunohistochemistry. Results Compared with the CON group, the levels of thyroid peroxidase antibody(TPOAb) and anti-thyroglobulin antibody(TGAb) in the HT group increased(P<0.05), and there was no statistically significant difference in free triiodothyronine(FT3), free tetraiodothyronine(FT4) and thyrotrophin(TSH). HE staining showed that some thyroid follicles were destroyed and lymphocytes infiltrated in the thyroid gland of mice in HT group than those in CON mice. The expression of 5-HT in hippocampus of HT group was lower than that of CON group(P<0.01). The expression level of 5-HT2 CR of HT group was higher than that of CON group. The increase(P<0.001) was mainly distributed in the pyramidal cell layer of hippocampal CA3 region(P<0.01), and the CA1 and DG regions showed an increasing trend. Conclusion HT leads to the decrease of 5-HT expression and the increase of 5-HT2 CR expression in hippocampus of early pregnancy mice.