Comparing the pregnancy risks of hyperthyroidism and hypothyroidism: A study of an American population database with more than 200,000 cases

Purpose Both hypothyroidism and hyperthyroidism have been linked with unfavourable pregnancy outcomes. Our goal was to better understand the implications of these disease processes on pregnancy and compare outcomes in women with hyperthyroidism versus hypothyroidism. Methods We performed a retrospective population-based cohort study using the Health Care Cost and Utilization Project-Nationwide Inpatient Sample. We included all deliveries with maternal hyperthyroidism or hypothyroidism between 2004–2014. We compared pregnancy, delivery, and neonatal outcomes between participants with hyperthyroidism and hypothyroidism. Multivariate logistic regression analysis was used to calculate adjusted-odds-ratios (aOR) and 95% confidence internals (95%CI). Results We studied 16,984 pregnancies with hyperthyroidism, and 184,655 with hypothyroidism. Participants with hyperthyroidism were less likely to have gestational diabetes mellitus (aOR:0.75;95%CI:0.70–0.79), operative vaginal delivery (aOR:0.90;95%CI:0.84–0.97), and cesarean section (aOR:0.89;95%CI:0.86–0.93) when compared to participants with hypothyroidism. They were more likely to have preterm premature rupture of membranes (aOR:1.30;95%CI:1.15–1.47), preterm delivery (aOR:1.37;95%CI:1.30–1.44), abruptio placenta (aOR:1.43;95%CI:1.26–1.63), wound complications (aOR:1.30;95%CI:1.06–1.59) and blood transfusions (aOR:1.35;95%CI:1.19–1.54). Infants born to hyperthyroid mothers were likelier to be small for gestational age (aOR:1.47;95%CI:1.35–1.59) compared to infants born to hypothyroid mothers. There were no differences for gestational hypertensive disorders, chorioamnionitis, hysterectomy, postpartum hemorrhage, maternal death, and congenital anomalies (p > 0.05). Conclusions Hyperthyroidism was associated with increased risk of pregnancy outcomes compared to hypothyroidism, with a few exceptions, particularly rates of gestational diabetes and caesarean section which were decreased in this group. Our study is novel as it compares the risks of hyperthyroidism versus hypothyroidism during pregnancy, thereby providing a new perspective to the existing literature.