Visiting Veterinarian

Feline hyperthyroidism

By Michele Gerhard Jasny V.M.D. - October 11, 2007

The entire senior class had been called down to the pathology lab to look at the postmortem on an unusual case. A partially dissected cat was laid out on the stainless steel table. He was thin as a rail and his heart was obviously enlarged, but that wasn't what we had been pulled out of class to see. The head pathologist was wildly excited.

"This cat has a very rare condition called idiopathic hyperthyroidism," he announced with enthusiasm. "We don't know what causes it, but see how his thyroid gland is several times larger than normal?" He went on interminably about the disease.

"Just great," I groused to a nearby classmate, holding my nose against the overpowering odor of formaldehyde and death that pervaded the lab. "Another obscure disease we'll never need to know about in practice."

That was in 1980 when feline hyperthyroidism was virtually unheard of. Today I diagnose cases every week and the syndrome is considered to be reaching epidemic proportions in the United States. We know that it is the result of a tumor of the thyroid gland, that the tumor is almost always benign but that it secretes excessive thyroid hormone, causing an elevation in the cat's metabolic rate. In most cases, hyperthyroid cats exhibit increased appetite and thirst, increased heart rate that can lead to cardiomyopathy, weight loss, and hyperactivity. What we don't know is the underlying etiology. What has changed over the last 25 years to cause this syndrome to go from rare to epidemic?

Connection to flame retardants?

Recently it has been proposed that feline hyperthyroidism may be caused by exposure to the flame retardant polybrominated diphenyl ethers (PBDE). These chemicals came into wide usage in the 1980s. They can be found in computers, foam cushions in furniture, carpet pads, upholstery, telephones, kitchen appliances, drapes, mattresses, house wiring, fax machines, and many other everyday items. The intent for using flame-retardants is a good one - to reduce the chance of things catching fire. The problem is that we may be getting out of the fire and, well, into the frying pan.

Scientists have found that PBDE contamination of the environment is ubiquitous. PBDEs are in the air and soil, in meat and milk, in fruits and vegetables, in house dust. They have even been found in polar bears in the Arctic. PBDEs build up in the fat tissue of animals, and in humans. PBDE levels in people here in the Unites States have been doubling every two to five years and are up ten to 40 times greater than levels in people in Asia and Europe. They have been found to cause brain and thyroid problems in laboratory rats. Since discovering that PBDEs may be harmful, many countries have banned at least some kinds of flame retardants and some companies have voluntarily stopped using them in their manufacturing but many are still in use all over the world.

So what does this have to do with cats? Since PBDE usage became common during the same decade in which feline hyperthyroidism increased so dramatically, scientists looked to see if the two phenomena were related. An article published in "Environmental Science & Technology" states that there is a "clear association" between feline thyroid problems and PBDE flame retardants. What does this mean? In one study on 11 cats with hyperthyroid, they were all found to have substantially higher PBDE levels than healthy cats.

Picture Fireball, the pampered house cat. PBDEs are particularly prevalent in house dust. Fireball spends most of her life lazing on the PBDE-laden foam-cushioned couch. She meticulously grooms herself, ingesting PBDE-laden house dust. She likes to sleep on the warm top of the computer or the stereo, appliances that may contain PBDEs. Her levels of PBDEs rise. Then one day she is diagnosed with hyperthyroidism. Before you get too excited, remember this is just a hypothesis. There is no proof that this is a cause and effect relationship.

Why an epidemic?

Back in the early 2000's several different studies were done exploring a number of factors and their relationship to Fireball's risk of getting hyperthyroidism. Researchers found that cats that ate primarily canned cat food had a higher risk. So did those that used cat litter, and those that preferred fish or liver and giblets-flavored food. Older cats were higher risk. Siamese and Himalayan breeds had lower risk. But none of these findings were cut-and-dried definitive answers. I know of multiple Siamese cats that ate only dry food and still developed hyperthyroidism.

Studies like these raise other questions. For example, cats that live completely indoors have to use cat litter, and cats that live completely outdoors never use litter. So is it the litter that increases Fireball's risk, or is it living indoors? I certainly see plenty of cases of hyperthyroidism in outdoor cats here on the Vineyard, where many people let their cats roam free.

Another theory linked hyperthyroidism to pop-top canned food. Still others point out that many fish contain high levels of PBDEs, and that canned cat foods typically use more fish than dry. So maybe it's the fish, not the cans? Maybe it's just that our cats' life expectancy has risen so much over the last few decades that they live long enough to develop hyperthyroidism. We just don't know. The fact that PBDEs have a chemical structure similar to thyroid hormone makes the theory of a correlation an intriguing one - but it is still just a theory.

Regardless of the cause, cats may be serving as our "canary in the coal mine," warning us about another environmental pollutant that may have serious effects. Scientists are concerned about the rising levels in people. How can we minimize exposure to PBDEs for our cats, our children, and ourselves? We don't know exactly how PBDEs get into the body. Suggested guidelines include the following.

Keep indoor living and workspaces free of dust. (Ha! Good luck at my house.) Try not to stir up the dust when you vacuum and clean. You can do this by adjusting your cleaning techniques. Use a damp cloth to dust, not a feather duster. Use the right type of vacuum. Keep the house well ventilated. Wash up after cleaning and dusting. Remove your shoes when coming inside. Cover or replace furniture that has exposed foam cushions or pads. Recycle or dispose properly of furniture, mattresses, drapes, electronics. Avoid construction sites, especially for pregnant women and young children. Follow recommendations for fish preparation and consumption.

Anything special to do for Fireball? If she adores Tuna Feast, Mackerel Mash, or Liver Lumps canned cat food, it may make sense to limit the amount of those flavors she eats, interspersing other varieties. Do we stop feeding canned food altogether? No. Other studies have linked dry food ingestion to diabetes in indoor cats. All the facts aren't in yet and the circumstances are complex. Try to think about environmental exposure by minimizing contact with things like uncovered foam pads. The reality is that there is not too much you can do as an individual.

We have to push for worldwide recycling of implicated products, like computers, and for worldwide controls on the use of these harmful substances. Vote for environmentally conscious legislators. Consume less. Recycle more. Open the windows. And if Fireball is eating voraciously yet losing weight, call your veterinarian. It ain't a rare disease anymore.