Study evaluates screening and treatment for subclinical thyroid disease during pregnancy

Observational studies over the past 30 years suggest that subclinical thyroid disease during pregnancy may be associated with adverse outcomes, including a lower-than-normal IQ in offspring. The results of these studies led several professional organizations to recommend routine prenatal screening for and treatment of subclinical hypothyroidism in pregnant women.

New research, however, indicates that universal screening for and subsequent treatment of subclinical hypothyroidism does not result in improved health outcomes for mothers or babies. The research was conducted through the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units (MFMU) Network and has been published this month in the New England Journal of Medicine.

The research team concluded that, compared to no treatment, treatment for subclinical hypothyroidism or hypothroxinemia during pregnancy did not result in significantly better cognitive outcomes in children through age five.

"The results of our study, the largest and most rigorous on this issue, do not support screening for subclinical hypothyroidism or hypothroxinemia during pregnancy," said Dwight Rouse, MD, one of the authors on the paper and the principal investigator for the MFMU at Brown University/Women & Infants Hospital of Rhode Island. "Our results do not apply to women with actual hypothyroidism during pregnancy - such women should be treated during pregnancy, as treatment benefits them and their babies."

The MFMU conducted two multi-center, randomized, placebo-controlled studies at its 15 centers, including at Women & Infants, a Care New England hospital. They screened women with singleton pregnancies before 20 weeks gestation for subclinical hypothyroidism, characterized by a mildly high thyroid-stimulating hormone (TSH) level and a normal thyroxine (T4) level, and for hypothyroxinemia, characterized by low maternal free thyroid hormone (fT4) concentrations with TSH in the normal range.

In separate trials, women were randomly assigned to receive levothyroxine, a commonly used medication to treat hypothyroidism, or placebo. Thyroid function was assessed monthly throughout the pregnancy, and children underwent developmental and behavioral testing for five years.

The research team found that treatment for subclinical hypothryoidism or hypothyroxinemia did not improve cognitive outcomes in children through five years and, moreover, did not improve obstetric or immediate neonatal outcomes.

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