Mild hypothyroidism, including subclinical hypothyroidism and isolated maternal hypothyroxinemia, is fairly common in pregnant women, but its impact on pregnancy outcomes is less clear, especially mild hypothyroidism in late pregnancy.

To evaluate the impact of subclinical hypothyroidism and isolated maternal hypothyroxinemia in the first and third trimesters, respectively, on obstetric and perinatal outcomes.

This large prospective study was conducted at the International Peace Maternity and Child Health Hospital in Shanghai; 52,027 pregnant women who underwent the first-trimester antenatal screening at International Peace Maternity and Child Health Hospital were consecutively enrolled from January 2013 to December 2016. To evaluate the impact of maternal subclinical hypothyroidism and isolated maternal hypothyroxinemia in the first trimester on pregnancy outcomes, participants were divided into 3 groups according to thyroid function in the first trimester: first-trimester euthyroidism group (n=33,130), first-trimester subclinical hypothyroidism group (n=884), and first-trimester isolated maternal hypothyroxinemia group (n=846). Then, to evaluate the impact of maternal subclinical hypothyroidism and isolated maternal hypothyroxinemia in the third trimester on pregnancy outcomes, the first-trimester euthyroidism group was subdivided into 3 groups according to thyroid function in the third trimester: third-trimester euthyroidism group (n=30,776), third-trimester subclinical hypothyroidism group (n=562), and third-trimester isolated maternal hypothyroxinemia group (n=578). Obstetric and perinatal outcomes, including preterm birth, preeclampsia, gestational hypertension, gestational diabetes mellitus, large for gestational age, small for gestational age, macrosomia, cesarean delivery, and fetal demise were measured and compared between those in either subclinical hypothyroidism/isolated maternal hypothyroxinemia group and euthyroid group. Binary logistic regression was used to assess the association of subclinical hypothyroidism or isolated maternal hypothyroxinemia with these outcomes.

Thirty-four thousand eight hundred sixty pregnant women who had first (weeks 8–14) and third trimester (weeks 30–35) thyrotropin and free thyroxine concentrations available were included in the final analysis. Maternal subclinical hypothyroidism in the first trimester was linked to a lower risk of gestational diabetes mellitus (adjusted odds ratio 0.64, 95% confidence interval 0.50–0.82) compared with the euthyroid group. However, third-trimester subclinical hypothyroidism is associated with heightened rates of preterm birth (adjusted odds ratio 1.56, 95% confidence interval 1.10–2.20), preeclampsia (adjusted odds ratio 2.23, 95% confidence interval 1.44–3.45), and fetal demise (adjusted odds ratio 7.00, 95% confidence interval 2.07–23.66) compared with the euthyroid group. Isolated maternal hypothyroxinemia in the first trimester increased risks of preeclampsia (adjusted odds ratio 2.14, 95% confidence interval 1.53–3.02), gestational diabetes mellitus (adjusted odds ratio 1.45, 95% confidence interval 1.21–1.73), large for gestational age (adjusted odds ratio 1.64, 95% confidence interval 1.41–1.91), macrosomia (adjusted odds ratio 1.85, 95% confidence interval 1.49–2.31), and cesarean delivery (adjusted odds ratio 1.35, 95% confidence interval 1.06–1.74), while isolated maternal hypothyroxinemia in the third trimester increased risks of preeclampsia (adjusted odds ratio 2.85, 95% confidence interval 1.97–4.12), large for gestational age (adjusted odds ratio 1.49, 95% confidence interval 1.23–1.81), and macrosomia (adjusted odds ratio 1.60, 95% confidence interval 1.20–2.13) compared with the euthyroid group.

This study indicates that while first-trimester subclinical hypothyroidism did not elevate the risk for adverse pregnancy outcomes, third-trimester subclinical hypothyroidism was linked to several adverse pregnancy outcomes. Isolated maternal hypothyroxinemia in the first and third trimesters was associated with adverse pregnancy outcomes, yet the impact varied by trimester. 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.

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