Antithyroid Drug Side Effects in the Population and in Pregnancy.

OBJECTIVE: Methimazole (MMI) and propylthiouracil (PTU) are both associated with birth defects and may also rarely be associated with agranulocytosis and liver failure. The frequency of these side effects when antithyroid drugs (ATDs) are used in the population in general or in pregnancy remains to be elucidated.

DESIGN: All individuals registered as the parent of a live-born child in Denmark, 1973–2008, were identified (n = 2 299 952) and studied from 1995 through 2010 for the use of ATDs. Outcomes were agranulocytosis, liver failure, and birth defects in their offspring. To evaluate the frequency of these side effects associated with the use of ATDs in pregnancy, all live-born pregnancies (n = 830 680), 1996–2008, were identified in a subanalysis.

RESULTS: In the population studied, 28 998 individuals redeemed prescriptions of ATDs (exposure in 2115 pregnancies), which was associated with 45 cases of agranulocytosis (one in pregnancy) and 10 cases of liver failure (one in pregnancy). This corresponded to 41 and 11 cases of agranulocytosis and liver failure per 5 million inhabitants during a 10-year period (agranulocytosis: 0.16% of ATDs exposed [MMI: 0.11% vs PTU: 0.27%, P = .02]; liver failure: 0.03% of ATDs exposed [MMI: 0.03% vs PTU: 0.05%, P = .4]). The majority (83%) developed the side effect within 3 months of ATD treatment and 25% during hyperthyroidism relapse. The use of ATDs in pregnancy was associated with birth defects in 3.4% of exposed children (44 cases per 5 million inhabitants per 10 y), and the frequency of birth defects after ATD exposure was 75 times higher than both maternal agranulocytosis and liver failure in pregnancy.

CONCLUSIONS: In the Danish population in general, ATDs associated birth defects and agranulocytosis had similar frequencies and were more common than liver failure, whereas for the use of ATDs in pregnancy, birth defects were dominant. The burden of side effects to the use of ATDs can be reduced by restricting the use of ATDs in early pregnancy.