Objective To discuss the levels of thyroid related hormones and incidence of hypothyroidism at different gestational ages and in different stages of gestation. Methods The 734 pregnant women without thyroid related medical history were selected for the study and 108 healthy non-pregnant women were selected as the control group. All these gravidas were divided into different groups by age (group A: <28 years, group B: 28-34 years, and group C: 35-43 years) and by the duration of pregnancy (group T1: 0-13+6 weeks, group T2: 14-27+6 weeks, and group T3: 28 weeks-delivery). The free triiodothyronire (FT3), free thyroxine (FT4), and thyroid stimulating hormore (TSH) levels were measured by chemiluminescent method. Results The differences of thyroid related hormone (FT3, FT4, and TSH) levels of

pregnant women in each stages of gestation and women in the control group were statistically significant (P<0.01). For T2 and T3 groups, the levels of FT3 and FT4 were lower than those of T1 group, while the level of TSH was higher than that of T1 group, and the incidence of hypothyroidism of FT3 and FT4 groups were obviously higher than that of T1 group. All these differences were statistically significant (P<0.05). But the differences of incidence of subclinical hypothyroidism among each stages of gestation were not statistically significant (P>0.05). For group C, the levels of FT3 and FT4 were lower than those of group A, while the level of TSH was higher than that of group A, and the differences were statistically significant (P<0.05). TSH increased with age. But the differences of incidence of hypothyroidism and subclinical hypothyroidism among gestational age groups were not statistically significant (P>0.05). Conclusion The thyroid related hormone levels are closely related to gestational ages and stages of gestation. Detection of gestational thyroid hormone levels plays an important role in screening and intervention of hypothyroidism.