Should You Test for an Underactive Thyroid?

It's been linked to miscarriage, preterm birth and developmental delays. Why aren't more OBs screening expectant moms?


Located in your neck, the thyroid gland produces hormones that affect metabolism, breathing, heart and nervous system functions, body temperature and more. During your pregnancy, your baby is entirely dependent on receiving thyroid hormone—which plays a large role in fetal brain and nervous system development—from you for the first trimester.

"In essence, pregnancy is a stress test for the thyroid," says endocrinologist Alex Stagnaro-Green, M.D., the lead author of guidelines on thyroid management during pregnancy from the American Thyroid Association (ATA). "The body has to produce about 50 percent more thyroid hormone during pregnancy."

Often, the thyroid is up to the task. However, a study of half a million pregnant women in 2012 found that 15 percent of pregnant women had heightened levels of thyroid stimulating hormone, or TSH.

Higher-than-normal TSH levels are an indicator of an underactive thyroid, referred to clinically as hypothyroidism. This finding comes as research mounts suggesting that too-little thyroid hormone increases the chances of miscarriage, preterm delivery and mental retardation in children—causing some experts to question whether all pregnant women should have their thyroid function tested. Pregnant women with low thyroid hormone can be safely treated by taking a daily thyroid hormone supplement.

Are you at risk?

Experts agree that certain women should have their thyroid function tested early in pregnancy. They include women with a personal history of thyroid disease; a strong family history of thyroid disease; an autoimmune disease such as type I diabetes or lupus; previous radiation to the neck or symptoms of an under- or overactive thyroid.

"Extreme fatigue, weight gain and feeling cold all the time are the main symptoms to watch for," says maternal-fetal medicine specialist Sarah J. Kilpatrick, M.D., PH.D., former chairwoman of the American College of Obstetricians and Gynecologists' Committee on Obstetric Practice. She and the college maintain that if none of this applies to you, there is no reason to check your thyroid.

The controversy over universal testing

Because of the potentially devastating consequences of too-little thyroid hormone in pregnancy, the ATA suggests that all pregnant women older than 30 (the risk of hypothyroidism increases with age) receive thyroid screening as well as those with a history of miscarriage, preterm delivery or infertility. But depending on the study, testing only "at-risk" women misses 30 percent to 80 percent of women with thyroid problems.

Testing everyone is problematic, too, because research is mixed over whether treating women with milder cases of hypothyroidism is beneficial or a waste of time and money.

To complicate facts further, a 2012 study in the New England Journal of Medicine tested the IQs in 3-year-olds and found no difference among those whose mothers had hypothyroidism—mild or severe—and took medication and those who had the condition but were not treated. However, the treatment, begun mostly after the first trimester, may have come too late to be effective.

Multiple large studies are under way to test the potential harms of mild and severe hypothyroidism in pregnancy and whether thyroid medication helps, but results aren't due for several years. "Thyroid function in pregnancy is a very rapidly evolving field," says Stagnaro-Green of the ATA. "A woman should discuss it with her OB or internist. Our guidelines were developed to optimize the health of the mother during pregnancy and the healthy development of the fetus."

Read more about prenatal testing