Solving the Mystery of Feline Hyperthyroidism and its Human Health Implications

Photo Credit: Flicker CC, Alice1523, catThe unraveling of the cause of a feline disease mystery is providing significant insight into human health matters. The condition known as hyperthyroidism has become an epidemic amongst aged cats in this country. Approximately ten percent of senior kitties are afflicted and suffer from the consequences of production of an excess of thyroid hormone. Typical symptoms include vomiting, diarrhea, increased thirst, heart-related abnormalities, and profound weight loss. Most hyperthyroid kitties can be successfully treated, but all of the therapeutic options are rather arduous and tend to be expensive.

It was while I was in veterinary school in the early eighties that the first cases of feline hyperthyroidism were being recognized. At the time, veterinarians described it as a new disease. Since then, hundreds of thousands if not millions of cats have been diagnosed. And, the incidence of this disease doesn’t appear to be slowing down one little bit.

Since feline hyperthyroidism was first recognized, the million-dollar question has been, what in the world causes it? What is it that changed back in the seventies? Was it a new cat litter or commercial cat food additive? Perhaps it was a byproduct of using chemical flea control products. Until now, the answer to this question has remained elusive.


Recent research has demonstrated that a likely cause of feline hyperthyroidism is a common class of flame retardants known as polybrominated diphenyl ethers (PBDEs). In the 1970’s, large quantities of PBDEs began to be routinely added to many household goods such as upholstery, carpet padding, and furniture cushions. It’s now known that PBDEs readily leach out of such products and latch onto house dust particles, ultimately becoming ubiquitous within the environment. They are found in the soil, water, and air, and it’s now proven that they have no difficulty entering into living breathing bodies. From the eggs of peregrine falcons to the blubber of beluga whales, PBDEs have been documented in a variety of species.

PBDEs and hyperthyroidism

The chemical structure of PBDEs and thyroid hormone resemble one another. It’s theorized that PBDEs may interfere with normal thyroid hormone metabolism, storage, and transport. There’s sound evidence that PBDEs alter thyroid function in rodents, fish, and birds, and it’s believed that kitty thyroid glands are also susceptible, particularly given the typical lounging around lifestyle of housecats and their proclivity for self-grooming.

Linda Birnbaum, a toxicologist, and Janice Dye, a veterinarian, both scientists at the Environmental Protection Agency, evaluated blood samples from 23 cats, including 11 with hyperthyroidism. They found that PBDE levels in cats were 20-100 times higher than those observed in people. They also found relatively large quantities of PBDEs in several types of cat foods, particularly seafood-flavored canned foods.

An Illinois study demonstrated that pet cats had higher PBDE levels than feral kitties, and a Swedish research team documented that PBDE concentrations in blood samples from hyperthyroid cats were significantly higher than in their healthy counterparts.

In spite of all of this evidence, some researchers have been reluctant to put all of their eggs into the PBDE basket. They argue that increased PBDE levels may be the result rather than the cause of feline hyperthyroidism. Because PBDEs are stored in body fat they are released into the bloodstream when cats lose weight, a symptom all hyperthyroid cats have in common. And, even if flame retardants do contribute to hyperthyroidism, they may not be the sole cause.

Cats as modern-day canaries

Much like coal miners used caged canaries to alert them to the presence of toxic gases, our hyperthyroid cats may serve as modern-day canaries. EPA toxicologist Birnbaum stated, “Our household pets are exposed to many of the same kinds of chemicals that we are. I think if we see a health problem in our animals, especially one that has arisen very recently- genetics doesn’t change that quickly- I think it’s kind of raising the canary-in-the-coal-mine issue.”

Since the 1970’s when the use of PBDEs began to increase, the rate of human thyroid cancer has more than doubled. And, research has shown that babies exposed to high concentrations of PBDEs in utero or during early childhood have lower cognition and motor skill test scores.

Peter Rabinowitz, director of the University of Washington’s Center for One Health Research, has created the online Canary Database as a means of indexing papers on animal outbreaks that may be relevant to human health. He encourages scientists and doctors to be more strategic about connecting the dots between the species. “I remain convinced that paying more attention to what the animals are trying to tell us is a really good idea,” says Rabinowitz. “There are still many disease outbreaks in animals that remain sort of unexplored or unexplained.”

PBDEs are now being phased out of household products within the United States and Europe. Unfortunately, because these chemicals degrade ever so slowly, their impact will continue to play out for quite some time.

Have you had experience with a hyperthyroid kitty? If so, do you think that PBDE exposure may have played a role?

Best wishes,

Nancy Kay, DVM

Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
Become a Fan of Speaking for Spot on Facebook

Please visit to read excerpts from Speaking for Spot and Your Dog’s Best Health.   There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at,, local bookstores, and your favorite online book seller.

Be Sociable, Share!

6 Comments on “Solving the Mystery of Feline Hyperthyroidism and its Human Health Implications

  1. At one time I had 6 hyperthyroid cats all on methimazole and all in their teens. They all started out as ferals and came inside, and one outside one had it so I brought him inside to give him the pills. I cover all my furniture with throws, but I do have carpets and do give them both fish flavored canned food and poultry and beef as well. I was assuming it is just an old age disease, like a cat getting renal failure or lymphoma or squamous cell carcinoma in their teens. I am against the radioactive iodine treatment since I have a well and septic system and their waste must be flushed afterwards since it is radioactive. I also read articles about a case where an officer in Washington state who was scanning cars coming down a highway for radioactive materials and pulled over a car which happened to have a cat going home from his radioactive thyroid treatment. I don’t want to bury my cat and have a satellite showing my back yard lit up with radioactive material. The methimazole is cheap now, $9.95 for 100 count of 5 mg pills.

  2. I find the possible link fascinating (and very persuasive). Now I would like to see some good theories put forth on the root cause of the epidemic of cranial cruciate ligament tears in dogs. When I worked as a vet tech in the 70’s, we never saw that problem and now it’s rampant.

  3. My kitty was just diagnosed with hyperthyroid. I think there must be more to it than just PBDEs or as suggested, it’s a result not a cause. There is no carpet in our house. Any upholstered furniture we have is only a couple years old (so presumably not treated with PBDEs) except maybe the mattresses. She does not get any seafood-variety foods.

  4. Thank you for your well written informative article! Please continue to educate for the sake of our animals.??

  5. It’s tough to know the answer to your question Amy. I’m not sure that removing the chemicals now will make a difference.

  6. My aunt’s cat is very sickly and this may be the cause. Should they replace their carpet & cover their upholstery? They keep all their kitties strictly inside and live in a house built in the 1920s.

Leave a Reply

Your email address will not be published. Required fields are marked *