Subclinical Hypothyroidism in Pregnancy: to Screen or Not?

Lara C. Pullen, PhD

June 06, 2014

Targeted thyroid testing during pregnancy is ineffective for the identification of subclinical hypothyroidism. A comparison of targeted thyroid tested and untested women revealed that the prevalence of trimester-specific elevated thyrotropin (TSH) and overt hypothyroidism was equal in the 2 groups. A new study calculates that 63% of pregnant women would need to undergo targeted thyroid testing to substantially improve targeted screening.

Michaela Granfors, MD, from Uppsala University in Sweden, and colleagues present the results of their retrospective cohort study in an article published online June 4 in Obstetrics & Gynecology. They analyzed a population-based sample of pregnant women treated at a clinical practice to determine the prevalence of trimester-specific elevated TSH and overt hypothyroidism.

The investigators found that the proportion of patients with elevated trimester-specific TSH was 12.6% in the targeted thyroid testing group and 12.1% in the untested group (P = .8; odds ratio, 1.04; 95% confidence interval, 0.79 - 1.37).

"The prevalence of women with trimester-specific elevated TSH and especially of those with subclinical hypothyroidism was much higher in our study than commonly reported. However, similar and even higher prevalence rates of subclinical hypothyroidism have been described in a few studies that also applied trimester-specific upper reference levels for TSH as recommended by the International Guidelines," the authors write. "Whether the high prevalence of elevated trimester-specific TSH in our study reflects a more affected population (despite the fact that Sweden has been considered to be an iodine-sufficient country), or unrealistically low trimester-specific reference levels for TSH in our setting, remains unclear."

Hypothyroidism screening guidelines are implemented to improve offspring and pregnancy outcomes through treatment. The investigators found that most clinics find it challenging to implement such targeted screening guidelines. "Although conducted in Sweden, data suggest that the results from our study might be generalized also to other settings. In a survey from Wisconsin, International Guidelines reached only a minority (11.5%) of obstetric providers," the authors write.

Current implementation of targeted screening guidelines in a clinical practice missed more than 80% of pregnant women with elevated TSH. This finding led the authors to suggest that universal thyroid testing may be the most reasonable approach, especially if ongoing studies support the treatment of pregnant women with elevated TSH. Their suggestion is not necessarily well-received, however.

"It is anticipated that the controversy regarding routine screening and treatment of pregnant women with subclinical hypothyroidism eventually will be settled as new evidence emerges from ongoing treatment trials. As Granfors et al suggest, if there is evidence of benefit to mothers or their children, routine screening during pregnancy seems assured. However, if ongoing treatment trials yield negative results, as in the Controlled Antenatal Thyroid Screening study, the screening debate would seem to hinge on the additional women with overt hypothyroidism who would be identified through routine pregnancy screening," Brian M. Casey, MD, from the University of Texas Southwestern Medical Center at Dallas in Texas, writes in an accompanying editorial.

"We're still trying to figure this all out," he told Medscape Medical News.

The authors and Dr. Casey have disclosed no relevant financial relationships.

Obstet Gynecol. Published online June 4, 2014. Abstract

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