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Perfluorinated chemicals linked to mild hypothyroidism in women
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Higher serum concentrations of perfluorinated chemicals are associated with an increase in total triiodothyronine, total thyroxine and free thyroxine among women, according to researchers from Taiwan.
“Our study is the first to link perfluorinated chemical (PFC) levels in the blood with changes in thyroid function using a nationally representative survey of American adults,” researcher Chien-Yu Lin, MD, PhD, of En Chu Kong Hospital in Taiwan, said in a press release.
Researchers included data for 1,181 patients aged older than 20 years from the National Health and Nutrition Examination Survey in 2007-2008 and 2009-2010 to determine the relationship between serum PFCs and thyroid function.
Chien-Yu Lin
Of 672 men and 509 women included in the study, men demonstrated a higher average concentration of PFC compounds compared with women. Furthermore, older age was associated with a greater concentration of perfluorooctanoate (PFOA), perfluorooctane sulfonate (PFOS) and perfluorohexane sulfonate (PFHxS). Mexican Americans displayed lower mean serum concentrations of PFOA, PFOS, PFHxS and perfluorononanoic acid (PFNA) compared with Hispanic whites and non-Hispanic blacks, according to researchers.
Data indicate the mean concentrations of PCFs were 4.15 ng/mL (95% CI, 4.02-4.29) for PFOA, 14.2 ng/mL (95% CI, 13.59-14.86) for PFOS, 1.54 ng/mL (95% CI, 1.48-1.59) for PFNA and 2 ng/mL (95% CI, 1.89-2.11) for PFHxS.
Based on these correlating concentrations of four PFCs, researchers decided to weigh data for sampling strategies and determined that a 1-unit increase in natural log-serum PFO was associated with an increase in serum T3 concentrations (6.628 ng/dL; 95% CI, 0.545-12.712) in women. A 1-unit increase in natural log-PFHxS also was associated with an increase in total T4 (0.26 mcg/mL; 95% CI, 0.108-0.413) and total T3 (4.074 ng/dL; 95% CI, 2.32-5.916) in women, suggesting risk for mild hypothyroidism. However, natural log-free T4 decreased in men (0.016 ng/dL; 95% CI, –0.029 to −0.003), they wrote.
“Although some PFCs such as PFOS have been phased out of production by major manufacturers, these endocrine-disrupting chemicals remain a concern because they linger in the body for extended periods,” Lin said. “Too little information is available about the possible long-term effects these chemicals could have on human health.”
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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R. Thomas Zoeller, MS, PhD
The study by Wen et al to be published in the Journal of Clinical Endocrinology and Metabolism explores the statistical relationship between several perfluorinated chemicals (PFCs) and measures of thyroid function in the human population, as revealed in the NHANES dataset. Several features of this work are important. First, it is important to recognize that PFCs are employed in the production of many products that people use — from fabrics and furniture to some food wrappings. Second, these chemicals are very stable in the environment and are persistent.
Therefore, it is important to study the potential relationship between the ubiquitous exposure to these chemicals and endocrine effects such as altering thyroid function or thyroid hormone action. In this case, the reader may be somewhat perplexed by the findings. Serum total T4 and total T3 was higher in women with elevated PFCs but serum free T4 was lower in men with elevated PFCs. However, PFCs were not associated with serum TSH in men or women.
So, what does this mean and why is it important? Animal studies have demonstrated clearly that some chemicals — PFCs among them — appear to disconnect serum T4 from TSH. We do not know how this happens, but we need to be careful about ignoring data that currently don’t make sense to us, especially in the case of the thyroid because the regulation of thyroid hormone delivery to target cells and activation of the receptor is highly complex, requiring specific transporters, metabolic enzymes and transcriptional machinery. We are also finding in animal studies that various environmental chemicals can have very unexpected effects on thyroid hormone regulation and action.
Finally, the United States’ EPA’s Endocrine Disruptor Screening Program requires only that thyroid histology be performed — blood levels of hormones are not required, nor are endpoints of thyroid hormone action. This NHANES dataset provides a very small glimpse into the potential impacts of environmental chemicals on thyroid hormones and we know very little about how this may contribute to thyroid disease.
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