Exposure to environmental chemicals may alter thyroid function for reproductive-aged women
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Key takeaways:
- Women exposed to higher concentrations of bisphenol A have lower levels of free and total T3.
- Exposure to higher amounts of all chemicals in the study was also linked to lower free T3.
Exposure to some types of endocrine-disrupting chemicals may lead to alterations in thyroid function for reproductive-aged women, according to a study published in Toxics.
In findings from date on a group of women who participated in the Environment and Reproductive Health (EARTH) study, women with higher levels of urinary bisphenol A had lower concentrations of free triiodothyronine and total T3. Additionally, women with higher levels of urinary triclosan had increased free T3 levels.
“Exposures to chemicals that can be found in everyday products were related to changes in thyroid function among reproductive-aged women,” Lidia Mínguez-Alarcón, PhD, MPH, assistant professor of medicine at Brigham and Women’s Hospital and Harvard Medical School, told Healio. “Thyroid diseases are predictors of many other important health diseases, so women should pay attention to the products they use every day as it can affect their thyroid and overall health.”
Mínguez-Alarcón and colleagues analyzed data from 339 women aged 18 to 45 years who participated in the EARTH study, a prospective cohort designed to study environmental and dietary determinants of fertility among couples seeking fertility care (median age, 34 years; 83% white). Women who were enrolled from 2009 to 2015 and provided spot urine and blood samples were included. Serum thyroid-stimulating hormone, free and total thyroxine, free and total T3, thyroperoxidase antibodies and thyroglobulin antibodies were obtained from blood samples. Exposure to chemicals was measured in urine samples.
Of the participants, 11% were positive for thyroperoxidase antibodies and 10% were positive for thyroglobulin antibodies. Median serum thyroid function concentrations were within normal ranges. Median urinary concentrations for bisphenol A, methylparaben and triclosan were similar to what has been reported in the general U.S. population, but concentrations for benzophenone-3 were higher among the study population.
In linear single-exposure models, higher levels of bisphenol A were associated with higher TSH and lower free and total T3 and free and total T4. In multi-exposure mixture models, each 1 log unit increase in urinary bisphenol A was associated with a decrease in free T3 (mean difference, –0.088 pmol/L; 95% CI, –0.151 to –0.025) and total T3 (mean difference, –0.066 nmol/L; 95% CI, –0.112 to –0.02). Increased urinary methylparaben concentration was associated with TSH, free T3 and free T4, but only in single-exposure models and not in mixture models. Higher concentrations of urinary triclosan were associated with lower levels of free T3 (mean difference, –0.041 pmol/L; 95% CI, –0.067 to –0.014) in mixture models. In single-exposure models, triclosan was also negatively associated with TSH and total T3.
The findings in additive models were similar to those in linear models. No evidence of nonlinear associations was found.
When the overall mixture of all chemicals was analyzed, serum free T3 were higher among women in the 75th percentile of exposure compared with those in the 25th percentile (mean difference, –0.19 pmol/L; 95% CI, –0.35 to –0.03) in the linear mixture model.
“Additional longitudinal studies among other women are needed to confirm these findings,” Mínguez-Alarcón said.
Perspective
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This study demonstrated that exposure to bisphenol A and to a mixture of environmental phenols was associated with decreased thyroid function among 339 women attending an infertility clinic. The study used rigorous methodology, including state-of-the art statistical methods for assessing effects of chemical mixtures, looking for both linear and nonlinear effects, and rigorously assessing thyroid function and autoimmunity.
The main limitations of the study are its small sample size (339), cross-sectional nature, and highly selected population. These data add to a growing but inconsistent body of literature. Differences in findings across studies evaluating effects of phenol exposures on thyroid function may relate to differences in study populations, exposure measurement techniques and statistical methodology. It is important to note that all of the women in this study were euthyroid, and observed alterations in thyroid function associated with the phenol exposures were relatively subtle.
Modestly decreased thyroid function would be of highest public health concern in pregnancy and early infancy, when thyroid hormone is critical for fetal and infant brain development. Larger studies, particularly targeting vulnerable populations, are needed to better understand risks.
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