Thyroid disease in pregnancy and the risk of venous thromboembolism: a Danish nationwide cohort study.

OBJECTIVES: Thyroid hormones have widespread regulatory effects, including an effect on haemostasis. It has been hypothesised that abnormal levels of thyroid hormones can influence the risk of developing thrombosis, and the association is yet to be investigated during pregnancy and the postpartum period. METHODS: A retrospective cohort study utilising nationwide health registers on diagnoses and medications was performed examining pregnancies in Denmark from 1999 to 2015 exposed to maternal hyperthyroidism or hypothyroidism. Cases of venous thromboembolism (VTE) during pregnancy and up to one year after the pregnancy had ended were identified. Multivariate logistic regression was used to evaluate the association between maternal thyroid disease and venous thromboses, reported as adjusted odds ratio (aOR) with a 95% confidence interval (CI). RESULTS: Among 1,149,871 pregnancies studied, 12,633 (1.1%) were exposed to maternal hyperthyroidism and 17,940 (1.6%) to maternal hypothyroidism. Overall, VTE was diagnosed more often among women with hyperthyroidism (0.41%; aOR: 1.39, 95% CI: 1.06-1.83) and hypothyroidism (0.44%; aOR: 1.43, 95% CI: 1.14-1.79) than among non-exposed women (0.28%). However, when adjusting for maternal pre-pregnancy body mass index, a higher risk of thrombosis was found among women with hyperthyroidism (aOR: 1.47, 95% CI: 1.08-2.01), whereas the association with hypothyroidism levelled out (aOR: 1.25, 95% CI: 0.96-1.62). CONCLUSION: In a large, nationwide cohort, an association between maternal thyroid disease and VTE diagnosed in relation to pregnancy was found. Results point towards maternal hyperthyroidism as the predominant risk factor for thrombosis and draw attention to confounding factors in the association with hypothyroidism.