Hypothyroidism in Danish pregnant women across decades.

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

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

Methods: 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 (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.

Results: In 1997, three years prior to iodine fortification, the incidence of maternal hypothyroidism in pregnancy in East was 0.55/1,000 pregnancies (95% confidence interval (CI): 0.30-0.93) and 0.48/1,000 pregnancies (95% CI: 0.27-0.77) in West Denmark. The incidence increased in both cohorts and at the end of follow-up (the year 2016), it was 9.1 (95% CI: 8.0-10.3) in East and 5.1 (95% CI: 4.3-6.0)/1,000 pregnancies in West Denmark. Similarly, the prevalence of hypothyroidism in pregnancy had increased (1997-1999: East 3.8 and West 3.1/1,000 pregnancies; 2013-2016: East 24.5 and West 15.8/1,000 pregnancies).

Conclusions: 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.