THE FULL ARTICLE TITLE
Andersen SL et al (2019) Maternal thyroid function, use of antithyroid drugs in early pregnancy and birth defects. J Clin Endocrinology Epub 2019 Aug 13. PMID: 31408173
SUMMARY OF THE STUDY
The authors looked at how often children were diagnosed with birth defects before reaching 2 years of age. The study looked at the frequency in over one million children born between 1997 and 2016. The researchers compared how often the birth defects occurred in children who were exposed to MMI, or PTU or not exposed at all to these antithyroid drugs. The researchers found that birth defects were seen in only 6.7% of children who were not exposed to ATDs, but were higher for those children exposed to MMI, at 9.6%, and 8.3% for those exposed to PTU. The researchers also checked specifically only the kinds of birth defects previous studies have found were especially common in children of mothers who take antithyroid drugs. Of the children who were not exposed to antithyroid drugs only 3.1% had these kinds of birth defects, but of the children who were exposed to MMI, 6.4% had these kinds of birth defects and only 4.4% were seen in PTU exposed children.
In the children who were exposed to PTU, the birth defects were found only in the face, neck and urinary system, while children who were exposed to MMI, the birth defects involved many organs: some had aplasia cutis (lack of skin in the scalp), esophageal or choanal atresia (back of the nasal passage is blocked) and omphalocele (abdominal wall defect with abdominal organs misplaced outside the abdomen). In children of women who switched from MMI to PTU during the first trimester, 5% had this specific type of birth defects as compared to 3.1% in the unexposed children.
When the authors looked at whether a mother’s hyperthyroidism in general seemed to cause birth defects, they found no evidence that it did. However, they did find that that in the women who specifically had overt hypothyroidism (low thyroid hormone levels), there were more birth defects.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study supports the current ATA recommendations that PTU should be used in women with hyperthyroidism during the first trimester of pregnancy, because it shows that children who were exposed to MMI in the first trimester had more frequent and more severe birth defects. Switching from MMI to PTU in early pregnancy has also been found to lower the risk of birth defects as compared to staying on MMI (although it may be best to avoid MMI use completely in the first trimester). Hyperthyroidism alone was not associated with birth defects, but overt hypothyroidism should be avoided because it increases the risk of birth defects.
— Susana Ebner MD