Feline Hyperthyroidism and PBDEs

Thyroid Disease in Cats is Not Just a Cat Problem:
Cats in the Coalmine

In earlier times, coal miners used canaries to warn them of dangerous gas build-ups in the mine. When the canary died, they knew it was time to get out. The canaries served as unwilling sentinels for the accumulation of otherwise-undetectable toxins. Our indoor-living cats may be the canaries in the mines of our homes.

Feline Hyperthyroidism (FH) is a disease that has increased dramatically since the ’70s. Cats typically develop hyperthyroidism in old age, but I have diagnosed it in young animals as well. Hyperthyroidism is the result of abnormally high production of thyroid hormones.

Thyroid hormones drive the metabolism of the animals. Cats with abnormally high thyroid hormone levels have a ravenous appetite yet they lose weight, vomit frequently, and look scruffy. Over time, cats suffering with FH develop liver, heart and kidney diseases. FH is diagnosed by means of a blood test and is treated either surgically, to remove the thyroid glands, or with drugs to kill thyroid gland cells, therefore reducing production of thyroid hormones.

Seeking an explanation for the increase in FH, researchers looked for commonalities among cats suffering from the disease. Several studies (though not all) found that cats with FH were mostly older cats, living indoors and regularly consuming canned cat food – primarily fish-based cat foods. At Indiana University (USA), researchers found that cats suffering from FH had higher levels of a group of chemicals known as polybrominated diphenyl ethers (PBDEs). PBDEs interfere with thyroid gland function, leading to the excess production of thyroid hormones.

Introduced in the '70s as fire retardants, PBDEs are added to the plastics used in all sorts of household and office materials, such as electronics equipment, upholstery foam, mattresses, imitation-wood materials, carpets, and more.

PBDEs are also found in the plastic coating used in the lining of canned foods. PBDEs are not chemically bonded to the plastic, so they are shed with use into the environment. PBDEs accumulate in lakes, fish and wildlife, and also become a part of our household dust.

Cats like comfort. They seek the most comfortable chair with the best upholstery, or the bed with the softest mattress, to snooze all day. They will move from their favourite spot to get their favourite snack, which ideally is a canned fish treat. We keep them indoors to protect them from traffic, attacks from other cats, and disease.

Cats are fastidious and spend a long time grooming, ingesting any dust they might have collected; they love fish; and they prefer canned food over dry food. Thus domestic cats are getting a triple dose of PBDEs: in household dust, in fish-based foods, and from the plastic linings of cans.

In this way their sedentary, indoor lifestyle provides us with an early-warning system concerning the environmental quality of our homes – like the canaries in the coal mines. And it puts our cats at risk of chronic debilitating disease.

Not all indoor cats develop FH with old age. Other factors are also at play, predisposing a cat to develop FH. Genetic susceptibilities have been identified and there are other possible factors. As with most disease, it is usually a confluence of factors that ends up in the development of the disease.

It sounds like a good idea to add fire retardant chemicals to household items. PBDEs may well have saved lives, allowing people and pets time to be rescued from a burning home. When PBDEs were first introduced, no one thought they would cause any problem. This was also the case with the drug Thalidomide – it is just taking us a lot longer to realize the problems inherent with PBDEs. Cats with FH are a tragic early warning of the unintended consequences of living with PBDEs.

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