In 2011, the American Thyroid Association (ATA) recommended more stringent thyroid stimulation hormone (TSH) testing criteria. More women will now be defined as having hypothyroidism during pregnancy. A study by scientists at Quest Diagnostics was conducted on a national sample of 502,036 pregnant women in the United States to determine the prevalence of gestational hypothyroidism based on the ATA’s revised guidelines. Published in the March 2012 Journal of Clinical Endocrinology & Metabolism, the analysis found that as many as 15.5% of pregnant women tested have the condition. These findings sharply contrast the widely cited hypothyroidism rates of 2.0% to 3.0% based on older, higher TSH cutoff levels.
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Assessing the Rationale
The ATA revised its clinical guidelines in response to many retrospective studies that have demonstrated an association between hypothyroidism and increased risk of miscarriage, gestational hypertension, and gestational diabetes as well as low birth weight and pre-term birth. Prospective clinical studies suggest even mild hypothyroidism negatively affects fetal brain development.
“Whether TSH screening should be routine for all pregnant women is controversial, given a lack of data demonstrating if universal screening and treatment will lead to significantly improved outcomes.”
Much of the data on gestational hypothyroidism has shown that both subclinical and overt hypothyroidism can have harmful health effects. For this reason, the ATA’s 2011 guidelines recommend assay- and trimester-specific TSH cutoffs that are significantly lower than previously recognized for detecting subclinical hypothyroidism in pregnancy. If assay-specific reference intervals are not available, the ATA recommends TSH upper thresholds of approximately 2.5 mIU/L during the first trimester and 3.0 mIU/L during the latter two trimesters.
Gaps in Testing Women Postpartum
The Quest Diagnostics study also found gaps in testing women postpartum. The study found that about one in five women with abnormal TSH levels during pregnancy received thyroid testing in the 6 months postpartum despite widespread agreement that gestational hypothyroidism is a risk factor for postpartum thyroid dysfunction.
Data are lacking on whether or not universal screening and treatment will lead to significantly improved outcomes. Nevertheless, the higher incidence of gestational hypothyroidism under the new ATA guidelines and the low overall testing rate (23%) in the Quest Diagnostics study suggest that more women should be tested.
Wider Testing Warranted
Considering that TSH is now a routine part of many annual medical visits, wider testing of pregnant women hardly seems excessive. Concerns about overtesting must be considered against the condition’s potential harmful health effects and the availability of oral therapies to treat the condition.
The ATA also expanded on prior recommendations about which women should be tested. It now includes those with:
A family or personal history of thyroid disease.
Any symptoms suggestive of thyroid disease.
Infertility, previous miscarriage, or preterm birth.
It also includes women who are 30 years or older or morbidly obese. Physicians should counsel at-risk women about thyroid screening prior to and during pregnancy.
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