Objective The purpose of this research is to calculate the prevalence of thyroid dysfunction during pregnancy and to discuss the relationship between gestational thyroid diseases and pregnancy outcomes.Methods There were 3 745 pregnant women who took antenatal care at Sun Yat-sen Memorial Hospital,Sun Yat-sen University were chosen as the research objects during March 2012 to February 2014.All of them were given close monitoring and tracking till the termination of pregnancy.These subjects took thyroid function testing (TT4,FT4,TSH,thyroid peroxidase antibody),if the result was abnormal,they were followed up periodically.Results The prevalence of overt hyperthyroidism is 1.90％,1.20％ for subclinical hyperthyroidism,1.17％ for hypothyroidism,6.78％ for subclinical hypothyroidism,and 3.82％ for isolated hypothyroxinemia.Compared with normal thyroid function group,the morbidity of placental abruption increased in overt hyperthyroidism group (5.8％ vs 1.0％,P＜ 0.01) ; the incidence of gestational diabetes mellitus was increased in subclinical hyperthyroidism group (37.5 ％ vs 16.6％,P＜0.01) ; and the morbidity of preterm birth increased in subclinical hypothyroidism group (14.3％ vs 7.7％,P =0.002).Conclusion Gestational thyroid diseases may increase the incidence of adverse pregnancy outcomes ; such as placental abruption,gestational diabetes mellitus,and premature birth.