Background: Maternal hypothyroidism has been associated with adverse pregnancy outcomes. We established a large nationwide register-based cohort with data on medication purchases to study the associations between maternal hypothyroidism, levothyroxine use and pregnancy and perinatal complications. Methods:Our data included all singleton births between 2004 and 2013 (N=571,785) in Finland.Hypothyroid mothers (N=16,364) were identified in the Finnish Medical Birth Register. Of these women 95.8% used levothyroxine medication and 37.5% had consistent levothyroxine use during pregnancy.Hypothyroid mothers were compared with mothers without thyroid disease (N=550,860) using logistic regression. Main outcome measures were pregnancy and perinatal complications.

Hyperthyroidism is known to be associated with foetal and maternal complications. 1 Autoimmune Graves' disease is the most frequent cause of hyperthyroidism in women of childbearing age. 1,2 The reported prevalence of Graves' disease in pregnancy is approximately 0.5%-1.0%, although the prevalence varies through the published literature. 3 In individuals with Graves' disease,

<b><i>Objective:</i></b> Thyroid dysfunction affects up to 5–7% of all pregnancies. The rates of thyroid hormone use in nonpregnant population have substantially increased in recent years. The aim of this study was to assess possible changes in the use of levothyroxine substitution and antithyroid drugs over time in pregnant women. <b><i>Methods:</i></b> The study data consisted of all singleton pregnancies (<i>N</i> = 736,873) between 2004 and 2016 in Finland collected from the Finnish Medical Birth Register. The Prescription Register and Special Refund Entitlement Register provided information on levothyroxine and antithyroid drug purchases. The annual rates of levothyroxine and antithyroid drug prescription redemptions were explored to estimate changes in exposure rates to thyroid medication from 2004 to 2016. Joinpoint regression analyses were performed to explore interannual variability in levothyroxine and antithyroid drug treatment. <b><i>Results:</i></b> There was more than a five-fold increase in levothyroxine use during the study period; in 2004, 1.1% of pregnant women had levothyroxine treatment, and by 2016, the prevalence increased to 6.2%. In addition, we observed a slight increase in antithyroid medication during pregnancy, but antithyroid drug use during pregnancy overall was very rare. In 2004, 0.05% of pregnant women used antithyroid drugs, and by 2016, this percentage had increased to 0.14%. <b><i>Conclusions:</i></b> Our study shows that the rate of levothyroxine use in pregnancy has markedly increased. This suggests that tracing and screening relevant patients and awareness of thyroid disorders on pregnancy and their significance for the pregnancy outcome have increased and the threshold to treat thyroid disorders has declined.