Hyperthyroid Medications Associated With Birth Defects

hyperthyroud_medications_birth_defectsBirth defects are a potential side effect of taking hyperthyroid medications such as propylthiouracil and methimazole. In fact, babies born to women taking either of these medications in the early months of pregnancy experience 50-75 percent increased risks for various birth defects, according to emerging research.

Studies Tying Hyperthyroid Medication and Birth Defects

The two popular drugs prescribed for the treatment of hyperthyroidism have been tied to birth defects when taken by women during the early stages of pregnancy; however, the side effects were different for each medication, according to the researchers. Peter Laurberg, MD, of Aalborg University Hospital in Denmark, and colleagues reported that a review of Danish national data revealed that:

  • Women who took propylthiouracil (PTU) in the early months of pregnancy to treat their hyperthyroidism experienced a 50 percent increased risk of their babies being born with birth defects associated with the neck and face.
  • Babies born to women who took methimazole (MMI) experienced a 75 percent increased risk of birth defects tied to the musculoskeletal system when compared to children born to women in the general population who were not taking the medication,

“We should restrict the use of anti-thyroid drugs in early pregnancy,” Laurberg said.

Generally, the research team discovered that birth defect risks were significant in newborns who were exposed to either drug in comparisons to the regular population, with more than 2-4 percent of the exposed children diagnosed with birth defects. MMI was associated with greater birth defect risks, according to Laurberg.

Birth Defects Tied to Methimazole and Propylthiouracil Use:

Methimazole and popylthiouracil are Category D pregnancy risk drugs, which means that positive evidence of risk exists when these medications are used during pregnancy.


Methimazole is often sold under the brand name Tapazole

  • Musculoskeletal risks
  • Abdominal wall defects
  • Integumentary defects
  • Digestive defects
  • Eye defects
  • Urinary defects
  • Respiratory defects
  • Circulatory defects


  • Facial defects
  • Neck defects
  • Urinary defects


According to the Mayo Clinic, yyperthyroidism—or overactive thyroid—occurs when the thyroid gland produces too much of the hormone, thyroxine. Hyperthyroidism can significantly increase the body’s metabolism and can lead to serious side effects including sudden weight loss, rapid or irregular heartbeat, sweating, nervousness, and/or irritability. The condition can also cause increased appetite, tremor, bowel changes, goiter (enlarged thyroid gland), fatigue, muscle weakness, sleep issues, thinning skin, and thinning or brittle hair.

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