The link between maternal thyroid parameters and adverse fetal and neonatal outcomes has been established in the literature. In a recent study, researchers in Rotterdam, the Netherlands, suggest that maternal high-normal free thyroxine levels in early pregnancy are linked to lower birth weight and increased risk for small size for gestational age at birth.
Generation RThey utilized data from women with a delivery date from April 2002 to January 2006 in the Generation R Study, a population-based cohort from early fetal life and beyond.
Maternal thyroid hormone parameters during early pregnancy and birth weight: The Generation R Study
M Medici, S Timmermans, W Visser, SMPF de Muinck Keizer-Schrama, VWW Jaddoe, A Hofman, H Hooijkaas, YB de Rijke, H Tiemeier, JJ Bongers-Schokking, TJ Visser, RP Peeters and EAP Steegers
Serum thyroid-stimulating hormone, free T4 and thyroid peroxidase antibody levels were measured early in 4,464 pregnant women at a mean of 13.3 weeks, researchers wrote. Cord serum TSH and free T4 levels were also measured in 2,724 newborns at a mean of 39.9 weeks. Small size for gestational age at birth (SGA) was defined as a gestational age-adjusted birth weight below the 2.5th percentile. Low birth weight (LBW) was defined as birth weight of less than 2,500 g.
According to data, higher maternal free T4 levels were associated with lower birth weight, and an increased risk for SGA newborns in mothers with normal-range free T4 and TSH levels. Further data indicate birth weight was linked to cord TSH and free T4 levels.
Accompanying editorialIn an accompanying editorial, , MD, PhD, of the division of epidemiology in biostatistics and prevention research at the Eunice Kennedy Shriver National Institute of Child Health and Human Development in Rockville, Md., wrote that lowering the free T4 reference intervals in pregnant women or screening all pregnant women for free T4 cannot be recommended.
Is there enough evidence of poor fetal growth to merit narrowing free T4 reference ranges during pregnancy?
“We are accumulating evidence on defining normal TSH levels during pregnancy, but even there we have many unanswered questions, especially regarding the treatment of subclinical hypothyroidism in preventing adverse pregnancy and neonatal outcomes”, Männistö wrote.
However, Männistö added that Medici and colleagues’ recent study is a step in the right direction. She suggests randomized and observational studies beginning before pregnancy, as well as longitudinal serum samples during pregnancy.
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