Background The importance of diagnosis of thyroid dysfunction during pregnancy has been widely recognized.Our study was designed to compare two different detection reagents between Abbott and Roche and to establish the gestational-related reference intervals for thyroid function tests (TFT) in Chinese women and to assay the reference ranges with the American Thyroid Association recommended standard.Methods Serum samples were collected from 693 normal pregnant Chinese women and divided into five groups according to their gestational age:9-13,16-20,24-28,32-34 and 37-40 weeks.Thyroid stimulating hormone (TSH) and free thyroxine (FT4) levels were determined by two different detection reagents:Abbott Architect l 2000 and Roche Cobas Elecsys 600.The reference ranges of the TFT indexes were calculated according to the National Academy of Clinical Biochemistry (NACB).The 2.5th and 97.5th percentiles of each stage were calculated,and the results were analyzed by one-way analysis of variances,t-test,and Spearman correlation analysis.Results Thyroid hormone levels varied greatly among different gestational stages.TSH levels,as assessed via two different TSH ELISA kits showed consistent changing patt during pregnancy and displayed linear correlation (P ＜0.001).In 9-13 gestational weeks,TSH levels were significantly lower than that of other groups; and in 37-40 gestational weeks,it was higher than that of other groups (all P ＜0.001).TSH reference ranges determined by Roche detection reagent in each group were higher than those by Abbott detection reagent (P ＜0.01 respectively).FT4 levels were higher in 9-13 gestational weeks than that of other groups (P ＜0.001).FT4 levels determined by Roche reagent were higher than Abbott reagent in 9-13 weeks,(P ＜0.001),and lower in 24-28 and 37-40 weeks (P ＜0.001 and P=0.016,respectively).The TSH level was correlated with FT4 levels in 9-13 gestational weeks by detection reagents (for Abbott reagent,r=-0.319 for FT4 P ＜0.001; for Roche reagent,t=-0.352 for FT4,P ＜0.001).Conclusion Accurate evaluation of TFT in pregnant women should be based on the gestational-related reference intervals in Chinese population,and different detection reagents should also establish their own reference intervals.