Pregnant women with thyroid function test results in the upper half of the normal range have an increased chance of miscarriage, even when they lack thyroid-harming antibodies, according to a new study. The results, which the authors say show the need to change the normal range for thyroid function tests during pregnancy, will be presented Tuesday at The Endocrine Society’s 92nd Annual Meeting in San Diego.
Past studies have reported an increased miscarriage rate in women with an underactive thyroid and in women with normal thyroid function tests but with a positive thyroid peroxidase antibody test, indicating the presence of antibodies directed against the thyroid.
However, this study is the first evidence demonstrating that the upper half of the accepted normal range on thyroid function tests is associated with miscarriage in antibody-negative women, said Alex Stagnaro-Green, MD, co-investigator and senior associate dean for education, George Washington University School of Medicine and Health Sciences, Washington, D.C.
“There has been an ongoing discussion as to whether or not the normal range for thyroid function tests is too broad,” Stagnaro-Green said. “This study provides clear evidence that the normal range for thyroid function tests during pregnancy needs to be redefined.”
Thyroid function is measured by the thyroid stimulating hormone (TSH) blood test, and anything above the upper limit is an underactive thyroid, called hypothyroidism. Since 2007 the recommended upper limit of the normal range of the TSH test for a pregnant woman has been 2.5 milli-international units per liter (mIU/L) in the first trimester and 3 mIU/L in the second and third trimesters. This recommendation was made by an Endocrine Society international guideline committee, of which Stagnaro-Green was a member. However, he said doctors have not universally accepted this guideline, and most still use 4.5 or 5 mIU/L for an upper limit, the same as for nonpregnant women.
To see if TSH on the high end of normal affects pregnancy outcomes, the researchers determined the pregnancy loss and preterm delivery rates of women who had TSH values between 2.5 and 5 mIU/L in their first trimester of pregnancy. They conducted the study at two community hospitals in Italy among pregnant outpatients who had a TSH level at or below 5 mIU/L during their first trimester. After excluding patients who had positive thyroid antibody tests, the investigators studied 4,123 women. They divided subjects into two groups based on TSH level (mIU/L): group A: below 2.5, excluding those who had an overactive thyroid, and group B: between 2.5 and 5.
There was no difference in the rate of preterm delivery between the two groups. However, the rate of miscarriage was significantly higher in group B (higher normal TSH) than group A (lower normal TSH): 6.1 percent versus 3.6 percent, respectively.
“This leads us to think that all pregnant women should be screened for thyroid function and any abnormalities treated,” Stagnaro-Green said.
None of the antibody-negative women in the current study received thyroid hormone treatment. However, previous research shows that such treatment lowers the miscarriage rate in antibody-positive women, he said.
