Methimazole, a Prescription Thyroid Medicine, Linked to Birth Defects

Methimazole, a medication prescribed to treat thyroid problems such as Grave’s disease, has been linked to birth defects for a second time, according to researchers in Japan.

According to the U.S. National Institutes of Health (NIH), a study led by Naoka Arata, PhD, of Tokyo’s National Center for Child Health and Development, suggests that women taking methimazole have a 75% higher risk of having an infant with birth defects when compared to the general population.

While studying a group of pregnant women who had already been diagnose with Grave’s disease, along with another group of women who did not have the disease, researchers determined that the women with the disease who were prescribed methimazole were more likely to have infants who develop:

The women in the study were in the first trimester of pregnancy (prior to 13 weeks gestation), but Dr. Arata suggests that all pregnant women should switch medications immediately. Another thyroid medication, propylthiouracil (PTU) has been recommended, although it comes with its own risks. The research also studied PTU’s effects on pregnant women, which showed a heightened increase of having infants with birth defects, as well as maternal liver problems. However, while methimazole heightened the risk of birth defects by 75%, PTU heightened the risk by 50%.

When Dr. Arata was asked about all women switching to PTU, she indicated that it was hard to tell if they should switch medications before conception or after, given its association with maternal liver problems.

“But as soon as they’ve conceived, we should advise them to change from methimazole to PTU,” she said.

Although he was not involve in the study, another physician, Dr. Bryan Haugen, head of endocrinology at Denver’s University of Colorado, suggests that all women who are pregnant or who are trying to conceive should be thoroughly educated regarding the risks of birth defects and thyroid medication.

“The current recommendation is to have patients on PTU in the first trimester, and then to switch them to methimazole in the second and third trimesters because methimazole has some questions around developmental abnormalities in the fetus,” said Dr. Haugen. “But PTU has potential risks to the liver of the mother,” he continued.

Meanwhile, the final analysis of the study is still underway with additional women, and is expected to finalize by 2015, once a total of 1,000 women have been studied.

During pregnancy, estrogen and human chorionic gonadotropin (hCG) can cause an increase in the thyroid hormone level. While a slightly enlarged thyroid is common in most pregnancies, if it’s excessively large it can indicate a medical problem. Uncontrolled thyroid problems during pregnancy can lead to a host of problems during pregnancy, including:

  • Low birth weight
  • Congestive heart failure
  • Preeclampsia
  • Miscarriage
  • Pre-term birth

Pregnant women with thyroid disorders are caught in a difficult decision. Physicians must assess the risks of medications compared to the risks of thyroid diseases and should always inform women of the potential dangers of both so that they can make an informed choice. Future studies are being planned in order to assess a better treatment plan for pregnant women with thyroid problems.