Objective To study the variation of serum thyroid hormones in each trimester of pregnancy and their relationship with thyroid peroxidase antibody(TPOAb).Methods Three-hundred and seventy-seven pregnant women in different trimesters were enrolled,the serum TSH,FT3,FT4,TT3,TT4,and TPOAb were determined by ICMA assay.Results Serun TSH in normal pregnant group was gradually increased as the pregnant trimester grew up ( all P＜0.05 ).The median of TSH in positive TPOAb group was higher than that of negative TPOAb group( P＜0.05 ).The prevalences of both clinical and subclinical hypothyroidism in positive TPOAb group were also higher than those in negative TPOAb group( all P＜0.05 ).The prevalences of subclinical hypothyroidism were 23.61％ and 18.04％,while those of clinical hypothyroidism were 0.27％ and 5.84％ by using TT4 and FT4 determination,respectively( P＜0.05 ).Conclusions More attention should be paid to the difference between FT4 and TT4 in hypothyroidism diagnosis during both second and third trimesters of pregnancy.Establishment of normal thyroid hormoues cut-off points in each trimester of pregnancy and screening for TPOAb is necessary.