Adolescents aged 12 to 21 years are two to three times more sensitive than the general population to common environmental contaminants that can disrupt thyroid function and therefore should have the latter checked, according to US researchers.
The thyroid-blocking effects of exposure to three common environmental contaminants — perchlorate, thiocyanate, and nitrate — also appear to be different in boys and girls, say Jenica McMullen, New York University School of Medicine, New York, and colleagues in their report published online April 20 in the Journal of Clinical Endocrinology and Metabolism.
Perchlorate, thiocyanate, and nitrate all inhibit the function of sodium-iodide symporter (NIS) in thyroid follicular cells at levels currently found in the environment, they point out. NIS function is key to thyroid-hormone synthesis, and disruption is a leading cause of hypothyroidism.
In adolescents, disruptions of normal thyroid function can profoundly affect every organ system, including cognitive and cardiac function, bone strength, and metabolism. Clinically, this can present as declining growth rates and changes in academic performance, including poor attention, the researchers say.
Perchlorate, which occurs naturally, is used in rocket propellant and explosive manufacture and can migrate into water, milk, and water-rich vegetables such as celery, zucchini, radish, tomato, and green cabbage. Thiocyanate exposure comes primarily from cigarette smoke, although it can also be found in dairy products as well as radishes, kale, and other leafy greens. Nitrate is a preservative commonly used in fertilizer that can also be found in drinking water and vegetables.
"The effects that we're talking about here are not amenable to treatment. Prevention of the exposures is all we can do at this point," explained senior author Leonardo Trasande, MD, MPP, associate professor of pediatrics, environmental medicine and population health at New York University School of Medicine, in an interview.
"The good news is that there are safe and simple steps that can be taken to limit exposure to environmental chemicals known to disrupt thyroid function," he told Medscape Medical News. "Clinicians can advise patients about these steps to limit exposures."
Among his recommendations are maintaining a healthy diet that includes a variety of iodine-rich vegetables — such as kelp, cranberries, Greek yogurt, and navy beans — avoiding processed foods and consuming organic produce where possible. Appropriate supplementation with iodine can also reduce the impact of exposure to environmental chemicals. Most salt in the United States is iodized, Dr Trasande said, noting that sea salt is not.
And referring to recent initial research that has implicated chemicals in the home with papillary thyroid cancer, he also advised that air be recycled every other day.
Screen for Both TSH and Serum Free Thyroxine
Performing a cross-sectional analysis of data from 3151 youth participating in the 2009–2012 National Health and Nutrition Examination Surveys (NHANES), the researchers show that an 8% decrease in serum free thyroxine (FT4) was seen for every log-unit increase in perchlorate exposure in adolescent girls, compared with 4% in the general population.
In adolescent boys, however, a 9% decrease in serum FT4 was associated with each log-unit increase in thiocyanate exposure, compared with a 3% decrease in the general population.
"This study underscores the importance of age and sex stratification in evaluating exposure safety levels of different NIS inhibitors. The stronger effects in adolescents may indicate that they are a generally more sensitive population, or it may be due to fewer complicating health issues as compared with adults," they state.
However, depressions in FT4 were not associated with characteristic elevations in thyroid-stimulating hormone (TSH), "which has traditionally served as the screening test for thyroid function," Dr McMullen and colleagues point out.
For this reason, clinicians should probably be screening both TSH and FT4 in these patients, they suggest.
And even when FT4 and TSH values are in the normal range, there could be a risk to cognitive development, said Dr Trasande, pointing to findings from studies of subclinical hypothyroidism in pregnancy.
"We were concerned about the earliest exposures, but since we weren't able to examine children younger than 12, we were able to identify adolescence as a window of vulnerability."
To date, the researchers have examined chemicals with relatively short half-lives, so theoretically at least, there's potential for quick recovery after exposure, Dr Trasande said, but he noted that this is based on physiological principles rather than clinical evidence.
More research is needed, but longitudinal studies such as the Environmental Influences on Child Health Outcomes (ECHO) program, a 7-year initiative launched by the National Institutes of Health, will help increase understanding of the effects of environmental exposures on child health and development, he concluded.
This study was supported by NYU School of Medicine. The authors report no relevant financial relationships.
J Clin Endocrinol Metab. Published online April 20, 2017. Abstract
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