Mildly Low Thyroid Function in Pregnancy Not a Threat: Study

There's no benefit to treating pregnant women who have mildly low thyroid function, researchers report.

Very low thyroid function during pregnancy is associated with impaired fetal brain development and increased risk of preterm birth and miscarriage. Some studies have suggested that even mildly low thyroid function (so-called subclinical hypothyroidism) during pregnancy could also pose a threat to a newborn.

This new study of more than 97,000 pregnant women across the United States found no evidence of that. Researchers saw no differences in brain development between children born to mothers with low thyroid function who did or did not receive medication during pregnancy.

There were also no differences between the groups in rates of preterm birth, stillbirth, miscarriage and gestational diabetes, according to the study, conducted by a U.S. National Institutes of Health (NIH) research network.

"Our results do not support routine thyroid screening in pregnancy since treatment did not improve maternal or infant outcomes," study author Dr. Uma Reddy said in an NIH news release.

Reddy is with the U.S. National Institute of Child Health and Human Development's pregnancy and perinatology branch.

Thyroid hormones help regulate energy levels and other vital bodily functions. For the study, Reddy and her colleagues tested thyroid hormone levels of more than 97,000 women before the 20th week of pregnancy.

The researchers randomly assigned about 500 women with mildly low thyroid function to receive either treatment with the drug levothyroxine (Synthroid) or a placebo.

After birth, all of their children underwent IQ testing and developmental testing each year until 5 years of age.

The new findings confirm the results of an earlier study that included about 22,000 women and followed their children until age 3, the researchers said.

The study results were published March 1 in the New England Journal of Medicine.