Antithyroid drug treatment in pregnancy: a first report from the Danish PRETHYR multicenter study

CONTEXT: Hyperthyroidism caused by Graves' disease (GD) should be treated in pregnancy to prevent maternal and fetal complications, however, side effects observed with antithyroid drugs (ATDs) may complicate current clinical management. OBJECTIVE: To investigate current treatment practices for hyperthyroidism during pregnancy in Denmark and the characteristics of women receiving ATDs. METHODS: The Danish multicenter study PRETHYR (Pregnancy Investigations on Thyroid Disease) included women with GD and women treated with ATDs in pregnancy. Maternal characteristics, pre-conception measurements of thyroid stimulating hormone (TSH), total triiodothyronine (TT3), and TSH-receptor antibodies (TRAb), and information on the clinical management and treatment before and during pregnancy, were obtained through patient questionnaires and review of the medical record. RESULTS: Of the 121 women included, 97.5% had GD. Among the 102 women with known thyroid disease preconceptionally and no prior definitive treatment, 58.8% (n = 60) received ATDs in pregnancy. These women were more often managed by endocrinologist at conception, had lower preconception TSH and higher TT3 and TRAb compared to women not treated with ATDs. Treatment was predominantly confined to the first half of the first trimester and generally discontinued by the second or third trimester. Prior to conception, 59.1% of women treated with methimazole (MMI) were switched to propylthiouracil (PTU), and 10.7% were switched from PTU to MMI. CONCLUSIONS: This is the first report from a multicenter study on pregnant women with GD and women treated with ATDs during pregnancy. The results provide insight into the current clinical treatment practices of hyperthyroidism in Denmark.