Reference ranges for thyroid hormones among women during 1st and 2nd trimester of pregnancy: a population-based cohort study in Ma'anshan city

  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.