Hypothyroidism in Danish Pregnant Women Across Decades

Iodine fortification programs have been implemented worldwide, and concerns are raised about an increase in autoimmune hypothyroidism following the increase in population iodine intake.

To study the consecutive occurrence of hypothyroidism in Danish pregnant women across decades starting before the nationwide iodine fortification of salt, implemented in 2000.

Nationwide register-based study of pregnant women in Denmark, 1997-2016, who carried a singleton, live-born pregnancy (n = 1 189 134). The geographical residence was used as a proxy of iodine intake, and the cohort was stratified by East Denmark (n = 549 410), with previously mild iodine deficiency, and West Denmark (n = 639 724), with previously moderate iodine deficiency. Information on maternal hypothyroidism was assessed from redeemed prescriptions of levothyroxine and reported as incidence and prevalence in pregnancy.

In 1997, 3 years prior to iodine fortification, the incidence of maternal hypothyroidism in pregnancy in the East was 0.55/1000 pregnancies [95% confidence interval (CI): 0.30-0.93 and 0.48/1000 pregnancies (95% CI: 0.27-0.77)] in West Denmark. The incidence increased in both cohorts, and at the end of follow-up (2016), it was 9.1 (95% CI: 8.0-10.3) in East Denmark and 5.1 (95% CI: 4.3-6.0)/1000 pregnancies in West Denmark. Similarly, the prevalence of hypothyroidism in pregnancy had increased (1997-1999: East 3.8 and West 3.1/1000 pregnancies; 2013-2016: East 24.5 and West 15.8/1000 pregnancies).

When evaluated in a large nationwide cohort across decades, the occurrence of hypothyroidism in Danish pregnant women increased. Regional differences were found and encourage further studies on the underlying mechanisms.