The Kitty Hyperthyroidism Epidemic and Common Household Chemicals

When my first radio program and blog articles focused on the potential harm to cats from flame retardants on household furniture the topic was fairly unknown to the general public.  Now, just a few years later, far too many cat owners have become familiar with the problem.  That is because feline hyperthyroidism (otherwise known as ‘kitty wasting disease’) has become an epidemic, affecting millions of cats.  And, over that time, groundbreaking research and several scientific replication studies have been conducted and continue to point to the common household chemical class known as PBDEs as a front-runner culprit.

But cats are not the only ones vulnerable to adverse outcomes from PBDEs.  Humans (and especially infants and toddlers) are also at risk for developing health problems associated with these toxic chemicals.  Some solutions for minimizing those risks appear at the end of this blog post.

The Mystery of the Wasting House-Cats

Forty years ago, feline hyperthyroidism was virtually nonexistent. Now it’s an epidemic — and some scientists think a class of everyday chemicals might be to blame.

New York Times, Emily Anthes

The mystery behind feline hyperthyroidism:  Although definitive answers remain elusive, scientists are narrowing in on one possible explanation: A steady drumbeat of research links the strange feline disease to a common class of flame retardants that have blanketed the insides of our homes for decades. But even as the findings may answer one epidemiological question, they raise another in its place. If household chemicals are wreaking havoc on the hormones of cats, what are they doing to us?


Backstory: Thyroid abnormalities in cats…were rare until the late 1970s. But once the outbreak started, it spread fast. From 1979 to 1983, the vets at the Animal Medical Center saw three cases a month on average; by 1993, they were seeing more than 20. The disease hopscotched across the United States and then the world, striking cats in Canada, Europe, Japan, Australia and New Zealand…until today where millions of cats are now suffering from hyperthyroidism, one of the most mysterious diseases in veterinary medicine.


The search for answers:  A number of researchers…became epidemiological detectives, searching for dietary, environmental and lifestyle factors that distinguished the hyperthyroid cats from healthy ones, and they turned up many leads. Among the many behaviors that appeared to put cats at risk: spending time indoors, using cat litter, eating canned food, eating fish-flavored canned food, eating liver-and-giblet-flavored canned food, drinking puddle water, sleeping on the floor, sleeping on bedding treated with flea-control products and living in a home with a gas fireplace.

It was a long and eclectic list, and from the 1980s to the early 2000s, scientists proposed a wide range of potential culprits, including chemicals used in canning and a toxic mystery substance in cat litter. Eventually, researchers homed in on another possibility: a class of flame retardants known as polybrominated diphenyl ethers (PBDEs). Beginning in the 1970s, large quantities of the chemicals were routinely added to many household goods, including couch cushions, carpet padding and electronics. PBDEs can be itinerant compounds; they leach from our sofas and TVs and latch onto particles of house dust, coating our floors and furniture. They drift into soil, water and air and slip into the bodies of animals, collecting in everything from the eggs of peregrine falcons to the blubber of beluga whales.

PBDEs also happen to have a chemical structure that resembles thyroid hormones and may mimic or compete with these hormones in the body, binding to their receptors and interfering with their transport and metabolism. By the mid-2000s, it was clear that they could alter thyroid function in rodents, birds and fish, and the United States and the European Union have now largely phased the chemicals out. (They remain ubiquitous, however; PBDEs take years to degrade, and many people still own products manufactured before they were taken off the market.)

As the health risks of PBDEs became clear, two scientists at the Environmental Protection Agency — Linda Birnbaum, a toxicologist, and Janice Dye, a veterinarian — began to wonder whether the chemicals might also be responsible for the rise of hyperthyroidism in cats. “How do cats behave?” says Birnbaum, who now directs the National Institute of Environmental Health Sciences and its National Toxicology Program. “They crawl on the floor. They sit on the couch. They lick their paws all the time. So anything in the dust, they’re going to end up ingesting.” If PBDEs were to blame, it would explain why the disease didn’t appear until the late 1970s, why it first emerged in the United States — where use of the chemicals was especially heavy — and why indoor cats seemed to be at particular risk.

Birnbaum and Dye started a small pilot study to scour the blood of 23 cats, including 11 with hyperthyroidism, for traces of PBDEs…

The cats had PBDE levels 20 to 100 times as high as those typically observed in American adults. Birnbaum and Dye, who reported their results in a 2007 paper, also found relatively large quantities of PBDEs in several types of cat food, particularly seafood-flavored canned foods.

Several years later, a group in Illinois discovered that pet cats had higher PBDE levels than feral ones and that hyperthyroid cats tended to live in homes that were particularly saturated with the flame retardants. In 2015, a Swedish team found that hyperthyroid cats had significantly higher levels of three types of PBDEs in their blood than healthy cats did. Last year, researchers in California reported a similar result: Total PBDE levels were higher in cats with hyperthyroidism than those without.


Alternative explanations:  The findings are tantalizing but not definitive. Cats’ lengthening life spans may explain some of the increased incidence of the disease, and it’s possible that high PBDE levels are a result of hyperthyroidism, rather than a cause; the compounds, which are stored in fat, may be released into the bloodstream when cats lose weight. Even if flame retardants do contribute to the disease, they may not be the sole cause. Researchers at the California Department of Toxic Substances Control recently identified more than 70 different compounds that seem to be present in especially high concentrations in hyperthyroid cats.


Shared Risks…Connections among human, animal and environmental health: Environmental toxicants are equal-opportunity hazards; mercury, asbestos, pesticides and other compounds can cause health problems in humans and animals alike…Sick animals can be sentinels, warning of looming threats to human health. For household chemicals, cats and dogs, which tend to spend nearly all their time in the home and happily hoover up whatever detritus falls on the floor, may be particularly useful sentinels…

Could hyperthyroid cats be modern-day canaries in the coalmine? We know that flame retardants accumulate in our own bodies; scientists find PBDEs in nearly every person they test, including newborns. “It’s almost 100 percent detection,” says Heather Stapleton, an environmental chemist and exposure scientist at Duke University. The compounds turn up in human blood, breast milk and tissue and can persist for years in fat.

Over the course of decades, human PBDE levels skyrocketed, increasing 100-fold from the 1970s to the early 2000s. (These levels now appear to be declining, most likely as a result of the phasing out of the chemicals.) The rate of human thyroid cancer more than doubled over the same time period. These parallel trends may be more than coincidence: Multiple studies have shown that men and women with high concentrations of PBDEs in their bodies tend to have altered levels of thyroid hormones circulating in their bloodstreams. Last year, researchers reported that thyroid problems were more common among American women with elevated levels of PBDEs in their blood. And at a conference this spring, Stapleton and her colleagues presented findings suggesting that long-term exposure to PBDEs may be a risk factor for papillary thyroid cancer; according to the unpublished data, living in a home with high levels of one type of PBDE in the dust more than doubled the odds of having the disease.

Thyroid hormones also play a crucial role in brain development; a deficiency of these hormones, known as hypothyroidism, may cause neurological abnormalities. If PBDEs cause unusual fluctuations in hormone levels in early life, they may do lasting damage. Scientists have found that those who are exposed to high concentrations of PBDEs in utero or during early childhood score lower on tests of motor skills and cognition. These findings are particularly worrisome given that young children — who are not uncatlike in their behavior, ingesting up to 200 milligrams of dust a day — tend to have higher body burdens of PBDEs than adults.


Connecting the dots:  Peter Rabinowitz, who created the online Canary Database to index papers on animal outbreaks that may be relevant to human health, thinks scientists and clinicians could be more strategic about connecting the dots between species.


“We’re finding that there was really some utility in asking about both people and animals when looking at a new hazard.” He suggests that we consider linking the health records of pets and their owners.

“I remain convinced that paying more attention to what the animals are trying to tell us is a really good idea.”

-Dr. Peter Rabinowitz, Director University of Washington’s Center for One Health Research

Solutions:  The best solution for reducing exposure risks of PBDEs to humans and pets is to remove all furniture made between 1970 and 2010 from your home…sofas, chairs, bedding, anything that could potentially contain these flame retardant chemicals.  The same is true for any electronics that may still be working.  If your budget prohibits buying new furniture replacements you can search local newspapers and online retail sites for previously-owned sofas and chairs that were made after 2010. Pre-owned furniture can frequently be purchased for only a fraction of the original cost.  If you are financially able to swing it, purchasing vintage furniture (made prior to 1970) is also a good option.  For those people who are currently unable to replace their furniture containing PBDE flame retardants, the best option is to minimize pet and human exposure to these chemicals through (1) placing heavy fabric furniture covers over potentially problematic furniture, (2) keeping windows open and fresh air circulating during all times that the weather permits, and (3) while wearing a dust mask and removing pets and children from the room, carefully vacuum furniture, floors and all surfaces that have collected dust on a regular basis.