TUESDAY, June 22 (HealthDay News) -- Testing in the upper half of what is currently considered a normal range for thyroid function can nonetheless be problematic for pregnant women, who face an increased risk for miscarriage as a result, new research suggests.

The study is the first to link what has previously been considered normal thyroid function to miscarriage risk, according to co-investigator Dr. Alex Stagnaro-Green, a senior associate dean for education at the George Washington University School of Medicine and Health Sciences in Washington, D.C.

The newly observed risk does not involve the presence of thyroid peroxidase antibodies, something that prior research has linked to a threat for miscarriage. An underactive thyroid has also been associated with risk of miscarriage, the study authors noted in a news release from the Endocrine Society.

"There has been an ongoing discussion as to whether or not the normal range for thyroid function tests is too broad," Stagnaro-Green said in the news release. "This study provides clear evidence that the normal range for thyroid function tests during pregnancy needs to be redefined."

The findings are scheduled for presentation Tuesday at the Endocrine Society Annual Meeting and Expo in San Diego.

The researchers' concerns center on the use of the thyroid stimulating hormone blood test. Since 2007, pregnant women reading above the upper limit of 2.5 milli-international units per liter (mIU/L) in the first trimester and 3 mIU/L in the second and third trimesters have been deemed to have an "underactive thyroid," and thus a higher risk for miscarriage.

However, the authors pointed out that many doctors do not actually follow this guideline, instead relying on the non-pregnant "normal range" marker of 4.5 or 5 mIU/L as an upper limit.

Stagnaro-Green and colleagues sought to get a handle on what range is, in fact, safe for pregnant women, by tracking the first-trimester thyroid tests and pregnancy outcomes of more than 4,100 women attending two community hospitals in Italy.

The team found that having a high-end normal-range thyroid reading (above 2.5, but below 5) did not increase the risk for having a pre-term delivery, however, miscarriage rates among women with a high-normal range were "significantly higher" (about 6 percent versus 3.6 percent) than for women with the recommended below-2.5 thyroid reading for pregnant women.

"This leads us to think that all pregnant women should be screened for thyroid function and any abnormalities treated," Stagnaro-Green concluded in the news release.

More information

For more on thyroid disease in pregnancy, visit the U.S. National Endocrine and Metabolic Diseases Information Service.