Objective:By establishing the reference intervals of free thyroxine (FT 4), thyroid stimulating hormone (TSH) and thyroid peroxidase antibody (TPOAb) of pregnant women in different pregnancy in Yangzhou, and analyzing the dynamic trends of each indicator, so as to provide a basis for timely and accurate diagnosis of thyroid disease during pregnancy, and promote prenatal and postnatal care. Methods:Clinical data of 3 726 healthy early (1 747 cases), middle (1 481 cases), and late (498 cases) pregnant women were collected from the Department of Perinatal and Health Care in Yangzhou Women and Children Hospital, the Affiliated Hospital of Yangzhou University Medical College from October 2017 to October 2018. At the same time, data of 407 non-pregnant women in the same period were collected as normal controls. The levels of serum FT 4, TSH and TPOAb in each stage of pregnancy were detected by Beckman automatic chemiluminescence analyzer. The reference interval was established by using the 95% reference value of the bilateral limit, and the differences of early, middle and late pregnancy were compared. Results:There were significant differences in FT 4 levels between early, middle and late stages of pregnancy ( H = 82.56, P < 0.01), with the early stage higher than the middle stage ( P < 0.01) and the middle stage higher than the late stage( P < 0.01). The difference of TSH levels was statistically significant ( H = 91.27, P < 0.01), in which the early stage was lower than the middle stage ( P < 0.01), and the middle stage was lower than the late stage ( P < 0.01). There was a statistically significant difference in TPOAb levels ( H = 30.36, P < 0.01). There was no significant difference between the early stage and the middle stage ( P > 0.05), and the early and middle stages were higher than the late stage ( P < 0.01). The reference intervals of thyroid function index in different pregnancies were, early pregnancy: FT 4 8.28 - 15.66 pmol/L, TSH 0.11 - 4.23 mU/L, TPOAb 0.10 - 16.46 U/ml; middle pregnancy: FT 4 7.38 - 14.36 pmol/L, TSH 0.13 - 4.67 mU/L, TPOAb 0.10 - 18.97 U/ml; and late pregnancy: FT 4 6.33 - 11.39 pmol/L, TSH 0.40 - 3.96 mU/L, TPOAb 0.10 - 6.17 U/ml. Conclusions:There are significant differences in serum thyroid function indicators in different pregnant women. Establishing reference intervals of thyroid function indicators in different stages of pregnancy have important clinical significance for diagnosis of thyroid disease and eugenics.