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

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.

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.

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.

Maternal hypothyroidism is associated with several pregnancy and perinatal complications, but consistent LT4 use may reduce many of the risks.