Objective To evaluate the trimester specific reference ranges and dynamic changes of thyroid functional indicators during gestation of healthy pregnant women.Methods According to the selection criteria,139 cases were selected as the normal pregnant population to establish a self-sequential longitudinal reference range.We collected samples eight times in every case throughout the gestation(including the 8th,12th,16th,20th,28th,36th week of prenatal,the 3rd and the 6th month postpartum) and detected the indicators of thyroid funciton.Meanwhile,301 healthy non-pregnant women of childbearing age were selected as the control group.Results The median of TSH was lowest in the first trimester,and then was increased in the second and third trimesters.The reference ranges of TSH were 0.20-4.01 mIU/L,0.43-3.97 mIU/L,and 0.57-3.96 mIU/L in the above trimesters.The free thyroxine (FT4) level from the 12th week of gestation to the 6th month of postpartum was gradually decreased and was lower than the non-pregnant level.The total thyroxine (TT4) was significantly increased at the 12th week of gestation and thereafter it was stable and returned to normal after childbirth.The free triiodothyronine(FT3) was gradually declined from the 12th week to the 28th week and returned to normal postpartum.Total triiodothyronine(TT3) was significantly increased at the 12th week and thereafter was stable,and returned to normal after childbirth.There was a significant negative correlation between TSH and FT4 in the first trimester(P＜0.01).There was no correlation between TSH and TT4 during the whole gestation.There were positive correlations between FT4 and TT4,FT3 and TT3 since the 12th week of gestation (P ＜ 0.01).Conclusions The gestational specific reference range of TSH based on the selfsequential longitudinal study was narrow,however,the upper limit was still higher than that recommended in foreign guidelines.