Maternal antithyroid drug (ATD) use during pregnancy has been associated with an increased risk of birth defects in offspring. Uncertainty remains on the size of this risk and how it compares to untreated hyperthyroidism due to methodological limitations of previous studies.Methods: Systematic review of MEDLINE and EMBASE identifying observational studies examining ATD use during pregnancy and risk of birth defects by 28 August 2020. Data were extracted on study characteristics, effect estimates and comparator groups. Adjusted effect estimates were pooled using a random-effects generic inverse variance method and absolute risk calculated.Results: Seven cohort studies and 1 case-control study involving 6 212 322 pregnancies and 388 976 birth defects were identified reporting regression effect estimates. Compared to an unexposed population comparison, the association between ATD use during pregnancy and birth defects in offspring was: adjusted risk ratio (aRR) 1.16 95% confidence interval (CI) 1.08-1.25 for propylthiouracil (PTU); aRR 1.28 95%CI 1.06-1.54 for methimazole/carbimazole (MMI/CMZ); aRR 1.51, 95%CI

Background: Maternal antithyroid drug (ATD) use during the first trimester of pregnancy has been associated with an increased risk of birth defects in offspring. Uncertainty remains on the size of this risk and how it compares to untreated hyperthyroidism due to methodological limitations of previous studies. Methods: Systematic review of MEDLINE and EMBASE identifying observational studies examining ATD use during the first trimester of pregnancy and risk of birth defects. Data were extracted on study characteristics, adjusted effect estimates and comparator groups. Effect estimates were pooled using a random effects generic inverse variance method of analysis and absolute risk calculated. Results: Seven cohort studies and one case control study (involving 6212322 pregnancies and 388976 birth defects) were identified. Compared to unexposed women without hyperthyroidism, the association between ATD first trimester use and birth defects in offspring was: adjusted risk ratio [aRR] 1.16 95% CI 1.08 to 1.25 for propylthyoruacil (PTU); aRR 1.28 95% CI 1.06 to 1.54 for methimazole or carbimazole (MMICMZ); aRR 1.51, 95% CI 1.16 to 1.97 for both (MMICMZ) and PTU; and aRR 1.15 95%CI 1.02 to 1.29 for untreated hyperthyroidism. The risk of major birth defects per 1000 live births was: 9.6 for PTU; 16.8 for MMICMZ; 30.6 for both MMICMZ and PTU; and 9.0 for untreated hyperthyroidism. Conclusions: When appropriately analysed this risk of birth defects associated with ATD use in the first trimester of pregnancy is attenuated. Although still elevated, the risk of birth defects is smallest with PTU compared to use of MMICMZ and may be similar to that of untreated hyperthyroidism.