Journal of Korean Endocrine Society 1994;9(3):183-189.
Published online November 6, 2019.
Changes of Thyroid Function According to the Stages of Normal Pregnancy.
W B Kim, B H Yoon, J H Chung, S I Lee, M S Kim, T G Oh, B Y Cho, H K Lee, C S Koh
It is well known that normal pregnancy is accompanied by a rise in serum concentrations of thyroxine-binding globulin(TBG) and human chorionic gonadotropin (hCG). Alterations of biochemical parameters of thyroid function are recognized during gestation and sensitive tests to evaluate the alterations easily are required. Therefore, a cross-sectional study was undertaken in 140 healthy pregnant women to evaluate the efficacy of free T_4 measured by 2-step RIA compared to other thyroid function tests and to confirm the changes of thyroid function according to the stages of normal pregnancy.The sensitivities of free T_4 index, free T_4(by 2-step RIA), T_3 and TSH were realtively high(99.3%, 93.6%, 92.9%, 83.6%, respectively) compared to those of T_4 and T_3 bead upgake(49.3%, 21.4%) during all stages of pregnancy. There were positive correlations between free T_4 index and free T_4 or total T_4(r=0.68, r=0.72; p<0.001). The values of free T_4 index sharply decreased from 3.22+-0.10(meam +-SEM) during 6th-12th week to an plateau after 16th-20th week of gestation(p<0.01). The serum concentrations free T_4 and T_3 bead uptake also significantly decreased from 1.65+-0.05 ng/dl, 24.7+- 0.7% during 6th-12th week to an plateau after 16th-20th week of gestation, respectively(p<0.001), No differences were found in the changes of serum concentrations of T_3, T_4 and TSH according to the stages of pregnancy.In conclusion, it is adequate to measure some tests including free T_4 index and free T_4 to evaluate thyroid function during pregnancy. The thyroid physiology and changes of thyroid function according to the stages of pregnancy should be considered in the interpretation of thyroid function status during pregnancy.
Key Words: Free thyroxine, Free thyroxine index, Pregnancy

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