Ask the Edmonds Vet: The connection between chemicals and hyperthyroidism in cats

By Dr. David Gross

Is it true that flame retardant chemicals can be especially toxic to cats?

There is some evidence that Polybrominate Diphenyl Ethers (PBDEs) may be involved in hyperthyroidism in cats. Feline hyperthyroidism may be the most common endocrine disorder in cats. It is associated with benign tumor(s) of the thyroid gland and usually appears in middle-aged to older cats, without preference to breed or gender.

The signs of hyperthyroidism are weight loss, hyperactivity accompanied by a voracious appetite. Cats can also demonstrate increased water intake, more frequent urination, along with intermittent vomiting and/or diarrhea. Cats with severe hyperthyroidism suffer from increased heart rates, arrhythmias (irregular beats) and congestive heart failure. About 10 percent of cats with hyperthyroidism develop a condition known as apathetic hyperthyroidism. These animals show depression and lack of appetite with fast weight loss.

The diagnosis of hyperthyroidism is by measuring increased circulating levels of the two thyroid hormones. Your veterinarian can verify the diagnosis by the use of special thyroid imaging called planar thyroid scintigraphy. Hyperthyroidism can be treated successfully with anti-thyroid drugs, surgery or the administration of radioactive iodine. The latter is currently the most commonly employed and probably the most successful.

There are three different types of PBDE compounds commonly used as flame retardants. They can migrate out of the flame retardant products then accumulate in indoor air and house dust and eventually contaminate the environment. Since the PBSEs do not break down quickly in the environment, they accumulate in air, soils, sediments, fish, marine mammals, birds and other wildlife and well as in meat, poultry and dairy products.

We should expect a decrease in these contaminants in this country since two of the most commonly used types were discontinued in 2004 and the third will be phased out in 2013. However, exposure from existing building materials, furnishings and consumer products, especially those imported from countries still using these products will continue.

A paper recently published in the Journal of Toxicology and Environmental Health suggested a link between PBDEs and hyperthyroidism. The researchers studied 21 normal cats, 41 cats diagnosed as hyperthyroid and 10 normal feral cats with no exposure to household dust. Although the total PBDE concentrations in the serum of normal and hyperthyroid cats were not significantly different, the total PBDE in dust from homes of hyperthyroid cats was significantly higher than the dust from homes of normal cats. The levels of PBDE in dust and one of the thyroid hormones (T4) were significantly correlated.

Although this study does not prove a cause and effect between PBDE levels in household dust and hyperthyroidism in cats, it is another indication that household pets could serve as sentinels for environmental toxicants that could affect humans. A major problem with most toxicology studies is that the effects of low levels of toxicants, over long periods of time, are too expensive to conduct and therefore are almost never done.

Dr. David Gross of Edmonds graduated from Colorado State University’s veterinary school in 1960 and was in private practice for 10 years. He retired in 2006 as Professor and Head of Veterinary Biosciences, at the University of Illinois, Urbana-Champaign. Dr. Gross is the author of “Animals Don’t Blush,” which describes the unique patients and even more unique clients of a veterinary practice in Sidney, Montana in the early 1960s. The book is available at the Edmonds Bookshop.

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