A population-based study of nearly 6 million Chinese women finds that both low and high levels of the hormone thyrotropin at preconception may be associated with an increased risk of adverse pregnancy outcomes, such as preterm birth, small for gestational age, and perinatal infant death.
Thyrotropin regulates the production of thyroid hormones and their levels during gestation can have a significant effect on pregnancy outcomes, write the authors of this study published online April 13 in JAMA Network Open.
“In this study, both low and high maternal thyrotropin levels were associated with a significantly increased risk of adverse pregnancy outcomes,” wrote authors who were led by Haixia Guan, MD, PhD, of Guangdong Academy of Medical Sciences, China.
This was a population-based cohort study of 5,840,894 women (mean age 26.30 years) from the National Free Pre-pregnancy Checkups Project in China. Participants underwent a thyrotropin examination six months in advance of conceiving. The optimal preconception thyrotropin levels may be between 0.37 mIU/L and 2.50 mIU/L. For patient in this study, thyrotropin levels measured as less than 0.10 mIU/L, 0.10 to 0.36 mIU/L, 0.37 to 2.49 mIU/L, 2.50 to 4.87 mIU/L, 4.88 to 9.99 mIU/L, and 10.00 mIU/L or greater with 1.60 being the median level, which is significantly lower than that of women who conceived later than 6 months.
Of all women included in this study, 223,181 (3.82%) had a subnormal thyrotropin level (<0.37 mIU/L), and 147,087 (2.52%) had a supranormal thyrotropin level (≥4.88 mIU/L). Thyrotropin levels between (0.37-4.87 mIU/L) was applied to 94% of women in in the study. 74% of the women had thyrotropin levels between 0.37 and 2.49 mIU/L and 1 160 286 (19.86%) with thyrotropin levels between 2.50 and 4.87 mIU/L.
In terms of adverse pregnancy outcomes, 6.56% had a relative risk of preterm birth (PTB); small for gestational age affected 7.21% of the women; birth defect applied to 0.02%; and perinatal infant death occurred in 0.33% of cases. Maternal preconception thyrotropin levels were associated with higher risk of preterm births.
“In this cohort study, both low and high maternal thyrotropin levels were associated with a significantly increased risk of adverse pregnancy outcomes. Results suggest that the optimal preconception thyrotropin levels may be between 0.37 mIU/L and 2.50 mIU/L to prevent adverse pregnancy outcomes,” the authors wrote.
Normal maternal thyroid function is essential for achieving optimal pregnancy outcomes, especially during the early gestation period, but 0.61% of all women experience thyroid dysfunction during pregnancy.
“Although screening for thyroid dysfunction during pregnancy is recommended by medical guidelines, there are, to our knowledge, limited studies on preconception screening. Furthermore, controversies remain on whether to perform universal screening or targeted screening. Based on current evidence, the optimal time for screening is believed to be earlier rather than later,” the authors wrote.
This study included several limitations including the fact that thyroid autoantibodies—which are associated with a higher risk of premature delivery—were not tested in the study participants.
Ying Yang, MD, PhD; Tonglei Guo, MS; Jinrong Fu, MBBS5; et al. “Preconception Thyrotropin Levels and Risk of Adverse Pregnancy Outcome,” JAMA Open Network. April 13, 2021. doi:10.1001/jamanetworkopen.2021.5723