Maternal Thyroid Function, Use of Antithyroid Drugs in Early Pregnancy, and Birth Defects

Context: Antithyroid drug (ATD) therapy in early pregnancy is associated with birth defects, but more data are needed to substantiate the risk associated with different types of ATD. Furthermore, the role of abnormal maternal thyroid function per se remains unclarified. Objective: To evaluate the risk of birth defects associated with the use of ATD in an extended nationwide cohort and the role of abnormal maternal thyroid function in birth cohorts including stored maternal blood samples from early pregnancy. Participants: Danish pregnant women and their live-born children, including 1,243,353 children from a Nationwide Register-Based Cohort (NRBC), 1997 to 2016; 8830 children from the Danish National Birth Cohort (DNBC), 1997 to 2003; and 14,483 children from the North Denmark Region Pregnancy Cohort (NDRPC), 2011 to 2015. Main Outcome Measures: Birth defects diagnosed before 2 years of age. Results: In the NRBC, altogether 2718 (0.2%) children had been exposed to ATD in early pregnancy. The overall frequency of birth defects was 6.7% (95% CI, 6.7% to 6.8%) in nonexposed children and higher after exposure to methimazole/carbimazole (9.6%; 95% CI, 8.2% to 11.2%) and propylthiouracil (8.3%; 95% CI, 6.7% to 10.3%). On the other hand, the frequency of maternal thyroid dysfunction in early pregnancy was similar in the random cohort and in cases of birth defect in the DNBC (12.4 vs 12.6%, P = 0.8) and the NDRPC (15.1 vs 15.4%, P = 0.8). Conclusions: Results corroborate an increased risk of birth defects associated with the use of ATD in early pregnancy and suggest that abnormal maternal thyroid function is not a major risk factor for birth defects.