Abstract： Background:Maternal thyroid dysfunction is common during pregnancy,and physiological changes during pregnancy can lead to the overdiagnosis of hyperthyroidism and misdiagnosis of hypothyroidism with nongestation-specific reference intervals.Our aim was to compare sequential with nonsequential methods for the evaluation of thyroid function in pregnant women.Methods:We tested pregnant women who underwent their trimester prenatal screening at our hospital from February 2011 to September 2012 for serum thyroid stimulating hormone (TSH) and free thyroxine (FT4) using the Abbott and Roche kits.There were 447 and 200 patients enrolled in the nonsequential and sequential groups,respectively.The central 95％ range between the 2.5th and the 97.5th percentiles was used as the reference interval for the thyroid function parameter.Results:The nonsequential group exhibited a significantly larger degree of dispersion in the TSH reference interval during the 2nd and 3rd trimesters as measured using both the Abbott and Roche kits (all P ＜ 0.05).The TSH reference intervals were significantly larger in the nonsequential group than in the sequential group during the 3rd trimester as measured with both the Abbott (4.95 vs.3.77 mU/L,P ＜ 0.001) and Roche kits (6.62 vs.5.01 mU/L,P =0.004).The nonsequential group had a significantly larger FT4 reference interval as measured with the Abbott kit during all trimesters (12.64 vs.5.82 pmol/L;7.96 vs.4.77 pmol/L;8.10 vs.4.77 pmol/L,respectively,all P ＜ 0.05),whereas a significantly larger FT4 reference interval was only observed during the 24 trimester with the Roche kit (7.76 vs.5.52 pmol/L,P =0.002).Conclusions:It was more reasonable to establish reference intervals for the evaluation of maternal thyroid function using the sequential method during each trimester of pregnancy.Moreover,the exclusion of pregnancy-related complications should be considered in the inclusion criteria for thyroid function tests.