Issue: February 2007
February 01, 2007
3 min read

Targeted screening for thyroid dysfunction missed one-third of pregnant women

Leaving maternal thyroid disease undiagnosed ‘no longer acceptable.’

Issue: February 2007
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Universal screening for hypothyroidism is not recommended for all pregnant women, but one-third with this condition were missed when only high-risk groups were targeted for testing.

Subclinical hypothyroidism in early pregnancy can cause a number of serious conditions: impaired neuropsychological development, premature birth, breech delivery and increased fetal mortality. The ongoing debate is whether all pregnant women should be screened for hypothyroidism in early pregnancy.

Currently, universal screening for hypothyroidism is not recommended for all pregnant women, according to guidelines recommended by an expert panel sponsored by the American Thyroid Association, American Association of Clinical Endocrinologists and The Endocrine Society.

The guidelines recommend screening only for high-risk women, such as women with a personal or family history of thyroid disorders, type 1 diabetes or any features that suggest a thyroid disorder.

Bijay Vaidya, PhD, MRCP, of the Department of Endocrinology at Royal Devon, Exeter Hospital, United Kingdom, and colleagues set out to evaluate whether the risk for thyroid dysfunction was only in high-risk pregnant women.

“We found that one-third of women were missed, and the question is whether that is acceptable,” Vaidya told Endocrine Today.

Identifying thyroid dysfunction

The study, published in the Journal of Clinical Endocrinology and Metabolism, was composed of 1,560 pregnant women (mean age 27; mean gestation nine weeks). The women were tested for thyroid function and 85% (n=1,327) for thyroperoxidase antibodies. They were also asked about their personal and family thyroid history at their first antenatal visit.

Eighty-nine women reported a history of thyroid disorder; 42, hypothyroidism; 25, hyperthyroidism; six, goiter or thyroid nodule; and 16, unspecified.

According to Vaidya, 413 (26.5%) of women were considered high-risk (pregnant women with a history of thyroid or other autoimmune disorders or a family history of thyroid disorders) compared with nearly 75% considered low-risk. High-risk women had more than a sixfold increased risk for subclinical or overt hypothyroidism in early pregnancy.

Forty women (2.6%) had raised thyroid-stimulating hormone (>4.2 mIU/L) and 70% of these women were in the high-risk group. However, 30% of the women with high TSH were in the low-risk group. A fully suppressed serum TSH (<0.03 mIU/L) was found in 1.9% of the women.

“If the criteria for the high-risk group is extended to include other possible risk factors, such as age older than 35 years, current smoking and a history of miscarriage, six of 40 women with raised TSH (15%) would still belong to the low-risk group,” the researchers wrote.

Raised TSH was attributed to a personal history of thyroid disease (P<.0001), other autoimmune disorders (P=.016), thyroperoxidase antibodies (P<.0001) and family history of thyroid disorders (P<.0001). Older age, smoking, previous pregnancy and miscarriage were not associated with raised TSH. Women of South Asian ethnicity were at an increased risk (P=.04).

There were no significant differences in maternal age, ethnicity, smoking status or number of pregnancies, miscarriages and stillbirths between the women with raised TSH in the high-risk group compared with the low-risk group.

Vaidya also said all hypothyroid women in the reproductive stage should ensure adequate thyroxine replacement before a planned pregnancy because nearly one-quarter of women on T4 replacement had raised TSH at their first antenatal visit. Proper T4 levels are essential for development of the fetus and increased doses may be necessary for these women.

High-risk screening

By testing only high-risk pregnant women, one-third of pregnant women with hypothyroidism would be missed. “Our study shows that, without universal screening, a significant number of such pregnant women – 30% – with thyroid dysfunction will not be picked up,” the researchers wrote.

“Is it enough to diagnose approximately two-thirds of pregnant women with thyroid disease?” Gregory Brent, MD, of the VA Greater Los Angeles Healthcare System, Departments of Medicine and Physiology, David Geffen School of Medicine at the University of California, asked in an accompanying editorial. “The women in the low-risk group had less severe hypothyroidism and were less likely to have a TSH level greater than 10 mIU/L than those in the high-risk group, but even mild elevations in maternal serum TSH are associated with adverse outcomes in pregnancy.”

Overt hypothyroidism, or raised serum TSH concentration and reduced free T4 concentration, is found in about 0.2% of pregnancies. Subclinical hypothyroidism, or elevated TSH and normal serum free T4 concentration, is found in about 2.3% of pregnancies. Mild or subclinical hyperthyroidism, or suppressed serum TSH alone, is found in about 1.7% of pregnancies, according to Brent.

He discussed case finding as an alternative to universal thyroid screening; this was the method Vaidya and colleagues used to identify high-risk women in their study.

Thyroid function testing varies among practices and clinicians. Some support a universal screening recommendation whereas others prefer screening on a case-by-case basis. A study of clinical practices comprising 85% of all deliveries in Maine found that nearly 50% of physicians regularly test at least 95% of pregnant women for thyroid disease in the first trimester. Also, obstetricians are more likely to screen for thyroid dysfunction (56%) vs. family practitioners (8%).

“When the potential adverse outcomes are so significant and the tools to diagnose and intervene are easily acceptable, however, leaving maternal thyroid disease undiagnosed, even in one-third of pregnant women, is no longer acceptable,” Brent wrote. –by Katie Kalvaitis

For more information:
  • Brent G. Diagnosing thyroid dysfunction in pregnant women: Is case finding enough? J Clin Endocrinol Metab. 2007;92:39-41.
  • Vaidya B, Anthony S, Bilous M, et al. Detection of thyroid dysfunction in early pregnancy: universal screening or targeted high-risk case finding? J Clin Endocrinol Metab. 2007;92:203-207.
  • Haddow JE, McClain MR, Palomake GE, et al. 2006 screening for thyroid disorders during pregnancy: Results of a survey in Maine. Am J Obstet Gynecol. 2006;194:471-474.