Two Antithyroid Drugs Associated with Birth Defects

Sout Korean researchers report that women using methimazole and propylthiouracil in the first trimester of pregnancy incur significantly increased risk of having babies with congenital birth defects. The risk is increased by 31% for methimazole (brand name Tapazole) and 16% for propylthiouracil (brand name Propyl-Thyracil or PTU).(1)

Antithyroid drugs are hormones that are used to reduce the production of the thyroid hormone. They are used treat an over-active thyroid gland, to bring metabolism into balance, and as a pretreatment for the use of radioactive iodine to treat other conditions.

The South Korean study directly contradicts the advice about these drugs currently published on sites like WebMD.(2)

If you’re pregnant and have thyroid issues, you may want to print the first citation below and take it to you doctor.


  1. Gi Hyeon Seo, Tae Hyuk Kim, Jae Hoon Chung. Antithyroid Drugs and Congenital Malformations. Annals of Internal Medicine, 2018; DOI: 10.7326/M17-1398
  2. American College of Physicians. “First trimester exposure to antithyroid drugs associated with birth defects.” ScienceDaily. ScienceDaily, 22 January 2018. <>.

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