Objective To investigate the development of thyroid function before 20 weeks of pregnancy refer-ence range and prevalence of thyroid dysfunction.Methods 1 706 healthy routine check early single births over the same period of non-pregnant women and maternal health check were chosen for the study.Thyroid function were meas-ured,characteristics of thyroid function changes were analyzed.Results TSH median values minimum and maximum eight weeks of pregnancy,respectively,19 +6 weeks of gestation,respectively 0.90mU/L,2.74mU/L,FT4 median value minimum and maximum respectively nine weeks pregnant, pregnant 19 +6 week, respectively 11.34mpmol/L, 8.75pmol/L,TSH with the extension of time to pregnancy gradually increase, while FT4 decreased.Compared with non-pregnant women,pregnant 8 to 12 +6 weeks pregnant median TSH decreased 28.48%,FSH level in 34 cases of pregnant women during pregnancy was lower than the reference range,accounting for 5.54%.FT4 than non-pregnant women the median increased 7.98%,39 cases of pregnant women during pregnancy than non-FT4 reference range,ac-counting for 6.35%.In section 2.5% and 97.5% of euthyroid reference range, non-pregnant females, 8 to 12 +6 weeks of pregnancy,13 to 16+6 weeks of pregnancy,17 to 19 +6 weeks of gestation TSH reference range was 0.16-5.90mU/L, 0.03-3.66mU/L, 0.06-3.75mU/L, 0.32-4.32mU/L, FT4 respectively 6.70-14.04pmol/L, 7.99-18.67pmol/L,6.19-16.23pmol/L,6.43-13.50pmol/L.In 1 706 cases of pregnant women before 20 weeks of pregnancy,41 cases of thyroid dysfunction exists,the total prevalence of 2.40%,including one case of hyperthyroid-ism,1 case of hypothyroidism,subclinical hypothyroidism in 33 cases,6 cases of low FT4 hyperlipidemia,most common subclinical hypothyroidism.Thyroid peroxidase autoantibodies ( TPOAb ) positive rate was 18.70%.Conclusion Pregnancy 20 weeks pregnant TSH and FT4 before presenting specific changes,TSH at 8 to 12 +6 weeks of pregnancy decreased gradually,gradually increased,while the FT4 in 8 to 12 +6 weeks of pregnancy increased gradually,gradually reduce;thyroid dysfunction prevalence is higher,to subclinical hypothyroidism is most common in early pregnancy,so we recommend routine thyroid function tests.