Objective  To observe dynamic changes in thyroid functional indicators among healthy women during 1st and 2nd trimester of pregnancy in Ma’anshan city, Anhui province, and to establish a normal reference range of cutoff value for the diagnosis of thyroid dysfunction.

  Methods  From May 2013 to September 2014, a total of 2 378 pregnant women attending the Maternal and Child Health Care Center of Ma’anshan City for antenatal care were recruited in a cohort. Demographic information of the women were collected before 14 gestational weeks and serum samples were collected during the 1st and 2nd trimester of pregnancy to evaluate thyroid function-related indicators.

  Results  The established reference ranges for thyroid stimulating hormone (TSH) were 0.02 – 4.94 mIU/L and 0.79 – 6.22 mIU/L and those for free thyroxine (FT₄) were 13.20 – 23.85 pmol/L and 9.18 – 15.24 pmol/L, respectively, during the 1st and 2nd trimester of pregnancy. The median value of total thyroxine (TT₄) and total triiodothyronine (TT₃) were gradually increased with the increment of gestational weeks between the 6th to 13th gestational week and maintained at stable level between 24th and 28th gestational week. A significantly positive correlation was observed between TSH in the 1st trimester and that in the 2nd trimester of pregnancy (r = 0.591, P < 0.001). Similar positive associations were found among FT₄, TT₃ and TT₄ during the two semesters of pregnancy (r = 0.447, 0.537, 0.532, respectively; P < 0.001 for all).

  Conclusion  There are significant differences as well as associations in thyroid functional indicators (TSH, FT₄, TT₄ and TT₃) among the women during 1st and 2nd trimester of pregnancy. The establishment of region-specific gestation-based reference ranges of thyroid hormones will be helpful to screening, diagnosis and treatment of thyroid diseases among pregnant women.