Hyperthyroidism in Danish Pregnant Women During a 20-Year Period

Context Hyperthyroidism in pregnancy is a clinical concern, and surveillance of any change in the occurrence of hyperthyroidism in pregnancy is important, especially when a mandatory iodine fortification (IF) program is implemented such as in Denmark in the year 2000. Objective To investigate any change in the occurrence of hyperthyroidism and the use of antithyroid drugs (ATDs) in Danish pregnant women during a 20-year period before and after the implementation of IF. Methods A nationwide register-based cohort (1997-2016) and 2 birth cohorts with biochemical data (the Danish National Birth Cohort, 1997-2003, and the North Denmark Region Pregnancy Cohort, 2011-2015) were used to study maternal use of ATDs in pregnancy and frequency of early pregnancy biochemical hyperthyroidism during a 20-year period prior to and after the implementation of mandatory IF. Results In the nationwide cohort, the adjusted odds ratio (aOR) for treatment with ATDs was 1.51 (95% CI, 1.30-1.74) after mandatory IF (2001-2004) compared with baseline (1997-1999). The increase was more pronounced in the previously moderately iodine-deficient West Denmark (aOR 1.67; 95% CI, 1.36-2.04) than the mildly deficient East Denmark (aOR 1.30; 95% CI, 1.06-1.60) and returned to baseline levels at the end of follow-up in both regions. No time-related difference in early pregnancy biochemical hyperthyroidism was observed. Conclusion The use of ATDs in Danish pregnant women increased following the implementation of IF and then leveled out. Results comply with observations in the general Danish population and suggest that IF influences the occurrence of autoimmune hyperthyroidism in younger individuals.

Automated summary

This study investigated the changes in the occurrence of hyperthyroidism and the use of antithyroid drugs (ATDs) in Danish pregnant women before and after the implementation of iodine fortification (IF). The results showed that after IF, the proportion of pregnant women using ATDs increased and then leveled out at the end of the follow-up period, indicating the influence of IF on the occurrence of autoimmune hyperthyroidism in younger individuals.