Retrospective Analysis of the Effects of Maternal Thyroid Dysfunction on Obstetrical Complications and Outcomes in a Cohort of 17,219 Pregnant Women

Background: Abnormal concentrations of maternal thyroid hormones are risk factors for certain obstetrical complications. However, the influence induced by different types of maternal thyroid dysfunction on obstetrical complications and outcomes remains controversial. This study aimed to systematically evaluate the prevalence of distinct thyroid dysfunction subtypes in pregnant women and their specific associations with adverse obstetric outcomes, thereby clarifying clinical management priorities. Methods: In a retrospective cohort study, a total of 17,219 pregnant women underwent a thyroid function test, including thyroid stimulating hormone (TSH) and free tetraiodothyronine (fT4). All participants were divided into seven groups based on their blood test results, and their pregnancy outcomes were followed up. The isolated hypothyroxinemia group was divided into two cohorts, depending on whether the patients received levothyroxine. Complications during pregnancy and the outcomes were observed and analyzed in both cohorts. Results: A total of 2621 (15.22%) women were identified with an abnormal thyroid function, including 1150 with subclinical hypothyroidism, 562 with gestational transient thyrotoxicosis, 419 with subclinical hyperthyroidism, 336 with isolated hypothyroxinemia, 78 with hyperthyroidism, and 76 with hypothyroidism. After adjusting for maternal characteristics, no significant associations were found between specific hyperthyroidism groups and the risk of pregnancy complications. However, mothers with overt hypothyroidism had nearly a 3-fold increased risk of developing postpartum hemorrhage (odds ratio (OR): 2.76; 95% confidence interval (95% CI): 1.19–6.38; p = 0.018). Subclinical hypothyroidism was associated with an increased risk of premature membrane rupture (OR: 1.44; 95% CI: 1.25–1.64; p < 0.001) and therapeutic abortion related to fetal anomalies (OR: 2.05; 95% CI: 1.13–3.74; p = 0.019). Additionally, both subclinical hypothyroidism, overt hypothyroidism, and isolated hypothyroxinemia were linked to more than a 2-fold increase in the risk of preeclampsia. Mothers with subclinical hypothyroidism exhibited a lower risk for gestational diabetes mellitus (OR: 0.67; 95% CI: 0.57–0.79; p < 0.001), while those with isolated hypothyroxinemia had approximately a 1.5-fold increased risk for gestational diabetes mellitus (OR: 1.41; 95% CI: 1.11–1.80; p = 0.005). There were no significant differences in outcomes between those receiving levothyroxine treatment in the isolated hypothyroxinemia group and those who did not. Conclusions: Our results showed a high incidence of thyroid dysfunction in pregnant women, with subclinical hypothyroidism being the most common, followed by gestational transient thyrotoxicosis. In general, pregnant women with hypothyroidism presented with a high risk of complications during pregnancy. Isolated hypothyroxinemia in pregnant women is concerning, and levothyroxine treatment did not improve pregnancy outcomes and obstetrical complications.