Yesterday I talked about some exciting new research into a potential cause for a type of kidney disease in cats. Today … on to another disturbingly common feline disease: hyperthyroidism.

First a little background. Hyperthyroidism is usually caused by a benign tumor within the thyroid gland that secretes large amounts of thyroid hormone. One of the primary functions of this hormone is to regulate an animal’s metabolism. Cats with too much circulating thyroid hormone have a greatly increased metabolic rate, which leads to the paradox of weight loss despite a ravenous appetite. Being perpetually in overdrive also frequently leads to high blood pressure and a type of heart disease called hypertrophic cardiomyopathy. Other symptoms can include vomiting, diarrhea, and increased thirst and urination.

In most cases, diagnosing hyperthyroidism is straightforward – high circulating levels of thyroid hormone in the blood stream (total T4 or TT4) in conjunction with typical clinical signs. Additional forms of thyroid testing may be necessary in complicated cases. Treatment varies depending on the cat’s overall health and owner finances, but options include radioactive iodine therapy, daily medication, a low-iodine diet, and surgical removal of the thyroid gland.

What isn’t clear-cut about hyperthyroidism is its cause. There’s been some evidence that feeding canned food increases the risk, but that is certainly not the whole story (both of my hyperthyroid cats were completely on dry food at the time of their diagnoses). A study recently published in the Journal of Toxicology and Environmental Health might shed some light on this issue.

Researchers looked into the role that polybrominated diphenyl ethers (PBDEs) might play in the development of feline hyperthyroidism. PBDEs are used as flame retardants in furniture, electronics, and other consumer products, and are known to have an adverse effect on many parts of the body, including the endocrine (i.e., hormonal) system of which the thyroid gland is a part.

The study included 62 housecats (41 of which were hyperthyroid) and 10 feral cats. The results were not conclusive. For example, there was no correlation between the levels of TT4 and PBDEs in the cats’ bloodstreams, which you might expect if the flame retardant chemicals were causing hyperthyroidism, but the research did have some important findings nonetheless.

First of all, dust samples from the homes of 19 of the housecats were analyzed for PBDEs. The levels in the homes of hyperthyroid kitties ranged from 1,100 to 95,000 ng/g. In comparison, PBDE levels in the homes of cats with normal thyroid hormone concentrations were much lower (510 to 4900 ng/g) and only 0.42 to 3.1 ng/g in 10 samples of canned cat food. Also, feral cats had significantly lower levels of PBDEs in their blood than did the housecats, regardless of the latter’s thyroid status.

So, it appears that house dust could be an important source of these chemicals for cats. They are probably ingesting PBDEs when they groom the dust out of their fur (a similar route is used to explain the linkage between environmental tobacco smoke and lymphoma in cats). But what does this mean for cat owners? Should we all get rid of our flame-resistant furnishings, clean our houses more often, kick our cats outside (kidding!!)?

I don’t know, but now I’m a little paranoid that my family is living in a house where two cats have developed hyperthyroidism. Do I want to know what our PBDE levels are?

Dr. Jennifer Coates

Image: Grooming by ex.libris / via Flickr