Much like the hand, body, and facial signals used by an orchestra conductor to ensure a great performance by the symphony, hormones are the signals that ensure healthy structure and function of the human body. If the conductor mistimes those signals — or comes on too fortissimo or lento — he can't go back and redo the performance from where the mistake occurred. Similarly, our butterfly-shaped gland in the neck, the thyroid, plays a crucial role in maintaining multiple organ systems.
Take the brain, for example. We've known about congenital hypothyroidism for decades; we screen newborns for this condition because it's eminently treatable. Without supplemental thyroid hormone, children with this condition suffer severe intellectual disability.
We now appreciate that the fetal thyroid gland doesn't become fully functional until the middle of the second trimester, and that the baby relies on mom's thyroid hormone until then. Even subclinical hypothyroidism, in which women have normal thyroid hormone levels but elevations in TSH, can produce cognitive deficits and even attention-deficit/hyperactivity disorder or autism.
The mistakes are costly. Besides the lost economic productivity, neurodevelopmental disorders require lifelong care in addition to intensive behavioral and educational therapy. One estimate of the lifetime societal cost of autism is as high as $3.6 million.
Causes of Thyroid Disruption in the Home
Pesticides (used in agriculture) and flame retardants (used in electronics and furniture) can disrupt thyroid hormone function in utero and induce cognitive deficits. Multiple separate studies from across the world have reproduced the same associations — independent of other environmental exposures as well as social and other determinants of health that could have otherwise explained the observed effects.
And it's not just cognitive testing that is impaired. In brains of children exposed to higher levels of organophosphate pesticides in utero, the frontal and parietal cortices were smaller on MRI, matching the neuropsychological deficits. The same flame retardants known to decrease cognitive potential in young children have also been associated with papillary thyroid cancer.
Perfluoroalkylsubstances (PFAS) — nonstick chemicals used in cooking and oil- and water-resistant clothing — have been associated with reductions in thyroid hormone in adults. The effects have ranged from subtle changes in total thyroxine to clinical hypothyroidism that requires medication.
Phthalates used in personal care products, cosmetics, and food packaging, and bisphenols used in aluminum can linings and thermal paper receipts antagonize thyroid hormone. One study found levels of phthalates in urine in young girls to be associated with lower free thyroxine at age 3. Given the crucial role of thyroid hormone in the skeletal system, this relationship may explain observations of lower bone mineral density in postmenopausal women. Bisphenols can also interfere with thyroid hormone synthesis, transport, and metabolism.
Perchlorate, a chemical used in the production of rocket fuel, missiles, fireworks, flares, and explosives, is also used to prevent static cling on plastic and paper packages. It interferes with the uptake of iodine needed for thyroid hormone production, as does thiocyanate, a contaminant found in cigarette smoke, and nitrate, which is used in fertilizers.
But Aren't Small Exposures Okay?
If so many chemicals disrupt the hypothalamic-pituitary-adrenal (HPA) axis, why didn't we hear about it in medical school? We used to think that "the dose makes the poison," meaning that the degree of exposure is as important as the nature of the substance. This saying is attributed to the 16th century Swiss philosopher Paracelsus. But science has shown hundreds of examples in which this paradigm has failed. It's now clear that low levels of exposure can have significant effects when they occur at the wrong points in brain development, with the biggest effects at lowest levels. The notion of epigenetics wasn't around in Paracelsus' time, either.
Ultimately, we will need better regulation to get rid of chemical hazards and screen chemicals for their safety. After a decade of wrangling and industry resistance, the EPA finally banned one of the known thyroid-disrupting pesticides, chlorpyrifos, from food. The policy delay was costly and unnecessary; studies going back to the early 2000s had identified serious concerns about disrupting thyroid hormone and brain development in children.
We recently estimated that organophosphate pesticides like chlorpyrifos accounted for an estimated $594 billion in societal costs, including added healthcare and education, between 2001 and 2016. Although the Toxic Substances Control Act now requires testing of chemicals for safety before they are introduced into the market, there remain serious loopholes in safety requirements for food additives that also disrupt thyroid hormone.
So, What Are We to Do?
There are safe and simple steps to avoid exposure to these thyroid-disrupting chemicals. We've described these before but they are well worth repeating:
Open your windows and use a wet mop
Replace nonstick cookware with stainless steel or cast iron
Avoid canned foods
Reduce your plastic footprint
Also, advise patients to consume a healthy diet with enough iodine. In 2007, the World Health Organization reported that 2 billion people worldwide have insufficient iodine intake. Iodine is critical for thyroid function. Seaweed is one of the best sources. Seafood and dairy products are other good sources, as are cranberries and strawberries.
In the meantime, manufacturers are changing their practices for the better. Organic food continues to climb in market share, bringing the price point down to a comparable range to conventional agriculture. Labels are now required on furniture to document flame retardant use. Major supermarket chains have removed PFAS-containing buffet-style food packaging from their shelves. Manufacturing practices are increasingly changing to reduce the use of plastic and contact with phthalates in the food supply.
As much as there is cause for concern, we've come very far — and through greater awareness and education of healthcare providers, we can build on that momentum.
Leonardo Trasande, MD, MPP, is the Jim G. Hendrick, MD, professor of pediatrics and directs the NYU Center for the Investigation of Environmental Hazards. His recent book, Sicker, Fatter, Poorer: The Urgent Threat of Hormone-Disrupting Chemicals on Our Health and Future...and What We Can Do About It, describes the health effects of synthetic chemicals that disrupt hormones.
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Cite this: Thyroid-Disrupting Chemicals in the Home and How to Avoid Them - Medscape - Sep 14, 2021.