Antithyroid Drugs Increase Risk of Congenital Malformations: Study

A new study by South Korean researchers raises serious concerns abut the potential pregnancy side effects of antithyroid drugs (ATDs), such as Tapazole and propylthiouracil, indicating that use during the first trimester of pregnancy could result in birth defects and health complications for the unborn child. 

Antithyroid drugs were linked to a nearly 20% increase in children born with congenital malformations, according to the study, published this month in the Annals of Internal Medicine. The drugs are often given for the treatment of Graves disease, which can put mother and child at risk.

Researchers conducted a nationwide cohort study of nearly three million live births from 2008 to 2014, using data from the Korean National Health Insurance database. The study looked for maternal prescriptions for antithyroid drugs like Tapazole by mothers in the first trimester.

According to the findings, 12,891 pregnancies were exposed to the drugs during those first few weeks of pregnancy. Among those who were given antithyroid drugs, 7.27% gave birth to children with congenital malformations. That compares with just 5.94% among children born to pregnant women who were not given the drugs, resulting in an increased risk of about 19%.

In cases where women were given Tapazole, also known as MMI, or methimazole, there was an increase of 17.05 cases per 1,000 births. Propylthiouracil was linked to an increase of 8.81 cases of malformation per 1,000 live births, and when pregnant women took both, researchers found an increase of 16.53 cases per 1,000 births.

Researchers also found a dose-response relationship with Tapazole, with a cumulative high dose resulting in more malformations than when pregnant women were given lower doses.

“Exposure to ATDs during the first trimester was associated with increased risk of congenital malformations, particularly for pregnancies in which women received prescriptions for MMI or both ATDs,” the researchers determined.

This latest study appears to confirm the findings of a Danish study published in October 2013, which found that the two drugs could increase the risk of birth defects by 50% to 75%. Those researchers found an increased risk of respiratory problems, neck defects, and facial deformities.

That study also found that Tapazole, or MMI, was linked to a higher risk of defects, when the two drugs were compared.