Objective To investigate the changes and clinical value of thyroid functional levels in pregnant woman. Methods The five thyroid functional indexes, triiodothyronine (TT3), thyroxin (TT4), thyrotropin (TSH), free triiodothyronine (FT3) and free thyroxine (FT4) in 747 early and midtrimester pregnancy women (experimental group) and 593 un-pregnancy reproductive age women (control group) were detected by using chemiluminescent analyzer. The association between TSH and human chorionic gonadotropin (HCG) was analyzed. Results The abnormal rate of thyroid functional indexes in experimental group was significantly higher than that in control group (P<0.05). The serum level of TSH in experimental group was significantly lower, and the serum levels of TT3 and TT4 were higher than that of in control group (P<0.05). However, there were no significantly difference in serum FT3 and FT4 between the two groups (P>0.05). The level of TSH was negatively related with HCG (r=-0.527, P<0.05). The serum level of HCG in the women with abnormally lower TSH was significantly higher than in that with normal TSH women (P<0.05). Conclusion It is necessary to enhance monitoring the thyroid function of pregnant woman due to the higher abnormal rate, and more attention should be paid on the effect of high density HCG to TSH.