As many as one in six pregnant women tested were positive for gestational hypothyroidism, or underactive thyroid during pregnancy, under new clinical guidelines for detecting the disorder, according to a study published in The Journal of Clinical Endocrinology & Metabolism (JCEM).

The findings, based on a review of laboratory tests of 502,036 pregnant women in the United States, found that 15.5% of patients, or about one in six, were positive when tested. The study, by scientists at Quest Diagnostics, is the first large-scale study to analyze the prevalence of hypothyroidism during pregnancy based on clinical guidelines issued by the American Thyroid Association (ATA) in 2011, and strongly suggests far more women are at risk for the disorder than suggested by prior studies.

A growing body of research associates gestational hypothyroidism with a range of medical complications for women and their babies, including miscarriage, pregnancy-induced hypertension, gestational diabetes, low-birth weight and abnormal fetal brain development which may lead to a lower IQ.

The disorder can escape detection without the aid of widely available and inexpensive laboratory tests to identify heightened blood levels of thyroid stimulating hormone (TSH). The ATA’s guidelines are the first from a medical association to recommend specific reference ranges for TSH blood levels based on recent data showing an association between low TSH levels, once believed to be harmless, and adverse health outcomes. The condition can be treated with FDA-approved oral therapies.

“Our findings should re-invigorate the medical community’s long standing debate about the best approach to clinical assessment and management of thyroid function during pregnancy,” said co-investigator and board-certified pediatric endocrinologist Jon M. Nakamoto, M.D., Ph.D., medical director for Quest Diagnostics and associate clinical professor at Rady Children’s Hospital and the University of California, San Diego.

“With the growing awareness of risks that even subclinical hypothyroidism can pose for the mother and fetus, it’s important that practitioners recognize the true prevalence of this condition.”

The study, titled ‘National Status of Testing for Hypothyroidism During Pregnancy and Postpartum,’ assessed results of 502,036 pregnant women who received lab testing through Quest Diagnostics between 2005 and 2008. Of these women, only about one in four, or 23% (117,892), were tested for TSH level.

Among the pregnant women who were tested for TSH level, 18,291, or 15.5%, had heightened levels of TSH consistent with hypothyroidism as defined by the ATA guidelines. By comparison, older, but widely cited studies suggest a percentage rate of gestational hypothyroidism in the low single digits.

The investigators also estimated that as many as 483,000 pregnant women could have had undetected gestational hypothyroidism in the United States in 2006, based on that year’s estimated 4 million live births and the study’s findings of a 15.5% prevalence rate.

Other key findings from the present study include:

• Women ages 35 through 40 years were 1.8 times more likely to develop gestational hypothyroidism as those ages 18 through 24 years. Yet, the study also found that fewer than 30% of women over the age of 30 received a TSH blood test, despite medical guidelines recommending they be tested. The findings suggest a possible gap in care for at-risk older women.
• Nearly one in five Asian women (19.3%) tested had TSH results consistent with hypothyroidism, the highest of any other ethnic group. Asian women were almost five times as likely to have gestational hypothyroidism as African-American women, who had the lowest prevalence of the disorder (6.7%), according to an odds-ratio estimate.
• Women weighing over 275 pounds were 2.5 times as likely to develop gestational hypothyroidism as those weighing between 100 and 124 pounds.

The study also found that only one in five women, or 20.7%, with gestational hypothyroidism were tested within six months after giving birth. Of these women, 11.5% tested positive for hypothyroidism. Symptoms of postpartum hypothyroidism can range from fatigue and depression to irritability and weight gain.

“Based on our study, it is likely that many women in the United States who have hypothyroidism during pregnancy also have, but are not treated, for hypothyroidism following their pregnancy,” said Harvey W. Kaufman, M.D., senior medical director, Quest Diagnostics. “We hope that our study is a call to action for much needed additional research into gestational and postpartum hypothyroidism and the potential of readily available oral therapies to improve outcomes for women and their infants.”

The present study is expected to appear in the March 2012 issue of JCEM and was published online in December 2011.

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