Untreated Low Thyroid Function During Pregnancy Can Cause Trouble

With commentary by Spyros Mezitis, MD, PhD, an endocrinology consultant and clinical investigator for Lenox Hill Hospital, New York City

Treating pregnant women with low thyroid function severe enough to produce symptoms does help them avoid ill effects such as high blood pressure, according to a new study that evaluated thousands of women. Women not treated have more adverse effects, the researchers found. The researchers also found the threshold at which to treat women for this hypothyroidism, however, may be higher than some experts currently believe, the researchers say.

hypothyroidism and pregnancyThe researchers, from the University of Texas Southwestern Medical Center, evaluated the pregnancy outcomes of more than 26,000 women, publishing their results online July 8 in the American Journal of Obstetrics & Gynecology.

Women had been pregnant in the years 2000 to 2003. In all, 47 were confirmed to have hypothyroidism with symptoms, called overt hypothyroidism, and were treated. Another 182 women had an abnormal result indicating hypothyroidism on the first blood test but did not have the typical follow up test to confirm it and were not treated. Those untreated women who had thyroid stimulating hormone (TSH) levels above 4.5  on the initial test had higher rates of pregnancy-related high blood pressure than women with normal thyroid test results, lending further support to the value of treating low thyroid function during pregnancy, the researchers say.

Experts do agree that untreated hypothyroidism with symptoms is linked with an increased risk of problems, both for the mother and the child. In addition to high blood pressure, these include preterm birth, miscarriage and even fetal death. However, screening of all pregnant women is not recommended by many organizations, including the American College of Obstetricians and Gynecologists (ACOG) and the Endocrine Society.

In the most recent ACOG guideline, updated in 2015, it says the universal screening is not recommended during pregnancy ''because identification and treatment of maternal subclinical hypothyroidism has not been shown to result in improved neurocognitive function in offspring." The ACOG does support tests of thyroid function in pregnant women with a personal history of thyroid disease or symptoms of it. Among the symptoms are fatigue, hair loss, cold intolerance and dry skin.

However, based on the study results, the researchers do seem to be recommending universal screening of pregnant women for thyroid function, says Spyros Mezitis, MD, PhD, an endocrinology consultant and clinical investigator for Lenox Hill Hospital, New York. He reviewed the findings but did not participate in the study. "Their bottom line finding, is that depending on the initial TSH (thyroid stimulating hormone) value, they can tell what the prenatal outcomes would be."

The increased risk of high blood pressure problems was only seen, the researchers say, in women with initial TSH levels above 4.5 mU/L. Others put the threshold for treating lower, at 3.0, Dr. Mezitis says. The higher the TSH results, the more severe the hypothyroidism, he explains. 

In the study, ''the women who were unconfirmed [for hypothyroidism with symptoms] when compared to normal thyroid function women had higher blood pressures during pregnancy," the authors found, but only in those with initial TSH levels above 4.5.

The American Thyroid Association recommends different reference ranges for TSH, depending on the trimester. In the first trimester it is 0.1 to 2.5 mIU/L; 2nd trimester is 0.2 to 3.0; and the 3rd trimester is .3 to 3.0.

This study alone is not enough to change the guidelines that don't recommend universal screening, Mezitis says. "You need several similar studies," he says. The blood test for thyroid function, he says, is relatively inexpensive, about $70 or higher. 

"If there is a thyroid issue and in these cases they are mostly asymptomatic, when you pick it up wth a thyroid test and intervene with one pill a day treatments, you are going to prevent these [bad] outcomes," he says. According to ACOG guidelines, pregnant women who have hypothyroidism and notice symptoms should receive thyroid hormone replacement beginning with levothyroxine.

Women should get the thyroid function test ''right when they learn they are pregnant,'' Dr. Mezitis says. The test is a simple blood test that can help detect whether the thyroid gland is underactive or overactive. Ideally, a pregnant woman who is found to have symptomatic hypothyroidism will be referred to an endocrinologist, he says, and started on thyroid hormone replacement. Her prenatal care should be monitored by both her obstetrician and her endocrinologist, he says.

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