Maternal hypothyroidism and the risk of preeclampsia: a Danish national and regional study.

Background: Maternal hypothyroidism in pregnancy has been proposed to increase the risk of preeclampsia, but uncertainties persist regarding the underlying causal mechanisms. Thus, it remains unclear if an increased risk of preeclampsia in hypothyroid pregnant women is caused by the lack of thyroid hormones or by the autoimmunity per se.

Methods: We conducted a retrospective study of two pregnancy cohorts in the Danish population. The nationwide cohort (n = 1,014,775) was register-based and included all singleton pregnancies in Denmark from 1999-2015. The regional cohort (n = 14,573) included the biochemical measurement of thyroid stimulating hormone (TSH), thyroid peroxidase antibodies (TPO-Ab), and thyroglobulin antibodies (Tg-Ab) (ADVIA Centaur XPT, Siemens Healthineers) among pregnant women in The North Denmark Region from 2011-2015 who had a blood sample drawn in early pregnancy as part of routine prenatal screening for chromosomal anomalies. The associations between diagnosed and biochemically assessed hypothyroidism and a diagnosis of preeclampsia were evaluated using logistic regression (adjusted odds ratio (aOR) with 95% confidence interval (CI)) adjusting for potential confounders, such as maternal age, diabetes, and parity.

Results: In the nationwide cohort, 2.2% of pregnant women with no history of hypothyroidism (reference group (ref.)) were diagnosed with preeclampsia, whereas the prevalence was 3.0% among pregnant women with hypothyroidism (aOR 1.3 (95% CI: 1.2-1.4)) and 4.2% among women with newly diagnosed hypothyroidism in the pregnancy (aOR 1.6 (95% CI: 1.3-2.0)). In the regional cohort, 2.3% of women with early pregnancy TSH < 2.5 mIU/L (ref.) were diagnosed with preeclampsia. Among women with TSH ≥ 6 mIU/L, the prevalence was 6.2% (aOR 2.4 (95% CI: 1.1-5.3)). Considering thyroid autoimmunity, preeclampsia was diagnosed in 2.2% of women positive for TPO-Ab (> 60 U/mL) or Tg-Ab (> 33 U/mL) in early pregnancy (aOR 0.86 (95% CI: 0.6-1.2)).

Conclusions: In two large cohorts of Danish pregnant women, maternal hypothyroidism was consistently associated with a higher risk of preeclampsia. Biochemical assessment of maternal thyroid function revealed that the severity of hypothyroidism was important. Furthermore, results did not support an association between thyroid autoimmunity per se and preeclampsia.