Pregnancy and Perinatal Outcome Among Hypothyroid Mothers: A Population-Based Cohort Study

Background: Maternal hypothyroidism has been associated with adverse pregnancy outcomes. A large nationwide register-based cohort with data on medication purchases was established to study the associations between maternal hypothyroidism, levothyroxine (LT4) use, and pregnancy and perinatal complications. Methods: The data included all singleton births between 2004 and 2013 (N = 571,785) in Finland. Hypothyroid mothers (n = 16,364) were identified in the Finnish Medical Birth Register. Of these women, 95.8% used LT4 medication, and 37.5% had consistent LT4 use during pregnancy. Hypothyroid mothers were compared to mothers without thyroid disease (N = 550,860) using logistic regression. The main outcome measures were pregnancy and perinatal complications. Results: Maternal hypothyroidism was associated with several pregnancy and perinatal complications, including gestational diabetes mellitus (odds ratio [OR] = 1.19 [confidence interval (CI) 1.13-1.25]), gestational hypertension (OR = 1.20 [CI 1.10-1.30]), severe preeclampsia (OR = 1.38 [CI 1.15-1.65]), cesarean section (OR = 1.22 [CI 1.17-1.27]), preterm births (OR = 1.25 [CI 1.16-1.34]), large-for-gestational age newborns (OR = 1.30 [CI 1.19-1.42]), major congenital anomalies (OR = 1.14 [CI 1.06-1.22]), and neonatal intensive care unit admission (OR = 1.23 [CI 1.17-1.29]). However, among mothers with consistent LT4 purchases, only the associations between gestational diabetes mellitus (OR = 1.12 [CI 1.03-1.22]), cesarean section (OR = 1.13 [CI 1.06-1.21]), neonatal intensive care unit admission (OR = 1.09 [CI 1.01-1.29]), and large-for-gestational age newborns (OR = 1.26 [CI 1.10-1.45]) and maternal hypothyroidism remained. Conclusions: Maternal hypothyroidism is associated with several pregnancy and perinatal complications, but consistent LT4 use may reduce many of the risks.