Timing of mild maternal hypothyroidism ‘pivotal’ for predicting pregnancy outcomes

Key takeaways:

• Mild maternal hypothyroidism in late pregnancy was tied to risk for preterm birth, preeclampsia and fetal death.
• The impact of isolated maternal hypothyroxinemia on pregnancy outcomes varied by trimester.

Third-trimester subclinical hypothyroidism and first- and third-trimester isolated maternal hypothyroxinemia were tied to various adverse pregnancy outcomes, highlighting the importance of trimester-specific thyroid function evaluations.

“Since the majority of studies in this field have focused on the role of thyroid function during early pregnancy, data regarding the impact of maternal thyroid function during late pregnancy on pregnancy outcomes are lacking,” Xueying Liu, MD, from the International Peace Maternity and Child Health Hospital at the School of Medicine at Shanghai Jiao Tong University, China, and colleagues wrote in the American Journal of Obstetrics and Gynecology.

Liu and colleagues conducted a large prospective study using data from 34,860 pregnant women who underwent first-trimester prenatal screening at the International Peace Maternity and Child Health Hospital from 2013 to 2016. All participants had first- and third-trimester thyrotropin and free thyroxine concentrations available for evaluation. Researchers assessed the impact of maternal subclinical hypothyroidism and isolated maternal hypothyroxinemia in the first and third trimesters on obstetric and perinatal outcomes.

Overall, 884 women had first-trimester subclinical hypothyroidism and 846 had isolated maternal hypothyroxinemia.

Compared with euthyroid women, those with first-trimester subclinical hypothyroidism had a lower risk for gestational diabetes (adjusted OR = 0.64; 95% CI, 0.5-0.82). Conversely, women with third-trimester subclinical hypothyroidism were at greater risk for preterm birth (aOR = 1.56; 95% CI, 1.1-2.2), preeclampsia (aOR = 2.23; 95% CI, 1.44-3.45) and fetal death (aOR = 7; 95% CI, 2.07-23.66).

First-trimester isolated maternal hypothyroxinemia was associated with increased risks for preeclampsia (aOR = 2.14; 95% CI, 1.53-3.02), gestational diabetes (aOR = 1.45; 95% CI, 1.21-1.73), large for gestational age infants (aOR = 1.64; 95% CI, 1.41-1.91), macrosomia (aOR = 1.85; 95% CI, 1.49-2.31) and cesarean delivery (aOR = 1.35; 95% CI, 1.06-1.74) compared with euthyroid women. Isolated maternal hypothyroxinemia during the third trimester was also associated with increased risks for preeclampsia (aOR = 2.85; 95% CI, 1.97-4.12), large for gestational age infants (aOR = 1.49; 95% CI, 1.23-1.81) and macrosomia (aOR = 1.6; 95% CI, 1.2-2.13) compared with euthyroid women.

“These results suggest the timing of mild hypothyroidism in pregnancy may be pivotal in determining its effects on adverse pregnancy outcomes and underscore the importance of trimester-specific evaluations of thyroid function, guiding potential preventive measures and clinical management strategies to optimize maternal and fetal health,” the researchers wrote.