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Prenatal exposure to air pollution may influence newborn thyroid function

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September 14, 2018

Exposure to high levels of air pollution in pregnancy may have subtle influences on the development of the fetal thyroid gland and its subsequent function, according to findings published in JAMA Network Open.

Carrie Breton

“Most clinical concerns have focused on thyroid hormone deficiencies in pregnancy, which have been associated with various adverse effects, including reduced fetal growth and neurodevelopment,” Carrie Breton, ScD, associate professor and director of the MADRES Center at the Keck School of Medicine, University of Southern California, told Endocrine Today. “However, thyroid hormones are tightly regulated and there may be an optimal range above which high levels may also be detrimental.”

Breton and colleagues analyzed data from 2,050 children aged 5 to 7 years between 2002 and 2004 participating in the Children’s Health Study in California (50.5% boys; 58.6% Hispanic). Included children had available birth record data, information for at least one air pollutant across pregnancy, and retrospective newborn heel-stick blood-spot free thyroxine concentrations. Researchers assessed prenatal exposure to ambient nitrogen dioxide or ozone pollution and traffic-related air pollutant exposures, estimated monthly based on participants’ geocoded residence, obtained using birth certificate and residential history questionnaire data. Ambient air quality monitoring data from the U.S. Environmental Protection Agency’s Air Quality System was used for analysis, and federal reference method or federal equivalent method monitors were used to measure particulate matter (PM2.5) and PM with a diameter less than 10 µm (PM10). Within the cohort, 13.4% reported moving during the pregnancy period or during the birth month, which researchers accounted for in exposure assessment.

The mean newborn total T4 measure for the cohort was 16.2 µg/dL. In individual air pollutant models, researchers found that a 2-standard deviation increase in prenatal PM2.5 exposure and PM10 exposure was associated with a 1.2-µg/dL (95% CI, 0.5-1.8) and 1.5-µg/dL (95% CI, 0.9-2.1) higher total T4 level, respectively, in newborn screenings. Prenatal exposure to ozone and traffic-related air pollution were not associated with total T4 concentrations, according to the researchers.

“Further research should evaluate the many indicators of thyroid function simultaneously, and distinguish between the total and bound T4,” Breton said. “Evaluating maternal thyroid levels, in conjunction with infant levels, would help our understanding of which are truly the most affected by pollution.” – by Regina Schaffer

For more information:

Carrie Breton, ScD, can be reached at the MADRES Center, Division of Environmental Health, Keck School of Medicine, 2001 N. Soto St., Suite 102K, Los Angeles, CA 90032; email: breton@usc.edu.

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Disclosure: Breton reports no relevant financial disclosures.

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Perspective

This retrospective study of 2050 children (58.6% Hispanic, singleton births, 50.5% male, enrolled at age 5-7 years) evaluated sensitive windows of exposure to ambient and traffic-related air pollution during pregnancy and its association with altered newborn thyroid function. Levels of newborn heel-stick blood spot total T4 were assessed, and monthly prenatal air pollutant exposures, including those particles with diameters of less than 2.5μm (PM2.5) and less than 10μm (PM10) were estimated and standardized, based on geocoded residence across 13 Southern California communities. Using different models adjusted for a large number of maternal, environmental and newborn hypothesized confounders, and interactions between individual and multi-pollutants, newborn T4 concentrations were positively and significantly correlated with birth weight. Exposure to PM2.5 and PM10 was associated with a higher newborn T4 for PM2.5 beginning at the end of the first trimester and for PM10 throughout most of the pregnancy.

This study has several limitations, including no control group and no free T4 and/or thyroid-binding globulins levels available. The clinical significance of this finding is currently unknown. An inverse, U-shaped relationship between maternal thyroid hormone levels in pregnancy and offspring outcomes, such as birth weight and cognition, has been observed. Previous observations in this population have implicated higher exposure to air pollutants with rapid increases in BMI, measures of adiposity and type 2 diabetes. Additional studies are needed to assess the health consequences of PM-associated differences in newborn total T4 and whether high levels of total T4 early in life may have detrimental metabolic consequences in this high-risk population.

Lorena Alarcon-Casas Wright, MD, FACE

Division of Metabolism, Endocrinology and Nutrition,
University of Washington School of Medicine,
Diabetes and Endocrinology Care Center/Roosevelt, Harborview Medical Center

Disclosure: Wright reports no relevant financial disclosures.