Are Hyperthyroid Drugs Linked to Birth Defects?

Two of the most commonly used drugs for treating hyperthyroidism (overactivity of the thyroid gland) have been linked to birth defects when used in early pregnancy, although researchers report that they have differing effects.

Analysis of Danish national data showed infants born to mothers taking propylthiouracil (PTU) early in pregnancy had approximately a 50 percent higher risk of birth defects, and those taking methimazole (MMI) had a 75 percent greater risk than children born to women in the general population not taking the drug. This was reported by Peter Laurberg, MD of Aalborg University Hospital in Denmark at a meeting of the American Thyroid Association.

It was reported that the two drugs were linked to very different birth defects:  MMI was associated with more musculoskeletal problems; and PTU was linked to face and neck defects.

Dr. Laurberg warned, “We should restrict the use of anti-thyroid drugs in early pregnancy,” adding that it would “be nice to have some anti-thyroid drugs with fewer side effects.”

Laurberg and his colleagues reviewed the Danish national data and reported that fetuses that were exposed to either drugs early in pregnancy were compared with children born to mothers who received thyroid drugs but NOT during pregnancy, as well as those who never had thyroid drugs at any time.

The analysis from 1996 to 2008 included 849,416 births from Danish Civil Registration System, as well as Danish National Hospital Registry bank on birth defects, which were registered prior to the child’s second year of life.

A total of 564 infants had been exposed to PTU; 1,097 were exposed to MMI; and 159 had been exposed to MMI but were then switched to PTU.  3,546 of the women had used thyroid drugs, but not during pregnancy; 811,730 never used thyroid drugs in their lifetime.

It was determined that the risk of birth defects was significant among newborns who were exposed to both drugs in utero compared with the general population, with an excess of 2-4 percent of exposed children having birth defects.

Laurberg reported that MMI appeared to be worse that PTU, with a higher risk of birth defects than PTU.

(The researchers added that there were no significant risks for infants born to mothers who had used thyroid drugs but not during pregnancy).

Of interest, the researchers found that the birth defects differed by drug.  MMI showed more musculoskeletal risks, thought to largely be due to abdominal wall defects, Laurberg stated. In addition, these infants also had higher rates of skin, digestive, eye and urinary defects, followed by respiratory and circulatory defects. Face and neck defects did not appear to be common in these children.

However, PTU was linked to a higher risk of face and neck defects, followed by urinary defects. There were no significantly increased risks of any other defects, including respiratory, circulatory, digestive or skin defects. There were no cases of eye or musculoskeletal problems.

There has been research suggesting that switching from MMI to PTU early in pregnancy could help diminish birth defects. However, it should be noted that among the 149 patients in the cohort who had received this therapeutic strategy, there still was a higher risk of birth defects in the infants compared with the general population.

Laurberg strongly recommended that clinicians restrict the use of any thyroid drug in early pregnancy. He stated that if thyroid treatment is absolutely indicated, healthcare providers should probably use PTU.

Laurberg went as far as recommending that fertile women be advised to stop using their thyroid drug within one week after the first day of missing a period if pregnancy is possible. He also recommended if women are planning on becoming pregnant, they should switch to PTU.

Hyperthyroidism is a condition that develops when the thyroid gland makes too much thyroid hormone.  It is a very common condition that affects millions of people. It is estimated that about 1 in 500 women have hyperthyroidism during pregnancy. In some women, it is pre-existing, in others, the condition will develop during the course of pregnancy. Thyroid hormones help control body temperature, heart rate, growth and how you gain or lose weight. All of these symptoms can be bothersome, however, are they really worth risking your baby’s health? The increased risks of defects that could affect your child are worse than any of the symptoms associated with hyperthyroidism, especially since the thyroid drugs can be resumed after the baby is born.  The decision is yours when it comes to what drugs you take and you should weigh the risks and benefits.

For more information, contact one of our Gacovino Lake attorneys at 1-800-246-HURT (4878).

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