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Is Your Cat’s Food Contributing to Risk of Hyperthyroidism?

A new study found compounds linked to hyperthyroidism in the fish in cat food, but more research is needed. Here are the facts we know.

s.e. smith  |  Feb 25th 2016


In recent years, vets have struggled with an explosion of hyperthyroid disease — in which the thyroid gland produces too much of a hormone called thyroxine — in cats. It’s unclear why so many older cats are developing this condition, which is currently the most common endocrine disease in cats, so researchers are trying to learn more about its origins. A new study explores one possible cause: marine organisms used for flavoring and protein in some cat foods.

The Japanese researchers looked at organohalogen compounds, found in some plastics, fire retardants, solvents, pesticides, and other things you definitely do not want your cat eating. These compounds bioaccumulate in the environment as animals that have been exposed to organohalogens are eaten by bigger members of the food chain. Fish and other marine animals can become highly contaminated thanks to the length and complexity of the ocean’s food chain — for example, species such as tuna are known to be high in mercury, while elevated levels of flame retardants have been found in seals.

A tabby cat looking shyly away from the camera.

Photo: spazbot29

Organohalogens are linked to a variety of health problems in mammals, including humans, and long-term studies of blood samples from pet cats have shown high concentrations of these compounds. Notably, these chemicals are specifically associated with an increased risk of thyroid disease. Hazuki Mizukawa, Kei Nomiyama, and their colleagues got curious about why cats suddenly had higher levels of organohalogens in their blood, as it could explain why so many cats were developing hyperthyroidism, and they found the same compounds in cat food. In a sense, some cats may be eating themselves sick, but the story is more complicated than that.

Here’s where the study gets more interesting: The source isn’t materials used in the cans, such as sealers and liners. It appears to be the food itself, in the form of what the researchers described as “natural products from marine organisms.” That means this problem can’t be fixed with a quick can redesign or distribution of foods in other materials, such as glass. It involves the very source of the food cats are eating, which could get very complicated, especially in the case of fish flavoring, which is made from concentrate.

Addressing elevated levels of organohalogens in cat food (as well as our own diet) requires getting these chemicals out of the environment itself. Some nations have already clamped down on products that include these components, for a variety of human and animal health reasons — you won’t find much DDT around, for example, and PCBs aren’t widely used. Expanding such regulation will help, but it doesn’t resolve natural reservoirs of pollutants that humans have already introduced to the environment. That requires environmental cleanup.

While some of these chemicals remain in production, some of the worst offenders haven’t been manufactured in decades — their persistence in the environment highlights how harmful they can be. Many of the companies that manufactured or used them have gone out of business, merged, or been purchased, and it’s hard to pin down environmental responsibility and force businesses to clean up their toxic legacies. Instead, organizations such as the Environmental Protection Agency and its Superfund program are trying to identify and clean up some of the most toxic sites in America — but pollution is a problem worldwide, and everyone needs to commit to similar efforts to protect natural resources, including, yes, the fish our cats eat.

However, there is some good news for you and your cats: While testing demonstrates that organohalogens are found in pet food, we still need more data conclusively linking them to fish. Furthermore, we need studies that will confirm this one’s findings about the presence of these compounds and how they’re metabolized in cats’ bodies. That means you can put down the can of tuna for the time being.

However, it still pays to be vigilant when it comes to your cat’s thyroid health. Cats with this condition tend to be insatiable eaters, but paradoxically, they still lose weight. They also drink a lot of water and urinate frequently, and they can be more active and talkative. Some cats develop matted, greasy coats, and if the condition isn’t treated, they can eventually experience cardiomyopathy, in which their hearts become enlarged and can’t pump blood as efficiently. Thyroid disease in cats is typically measured by a blood test that evaluates T4 levels, but not all cats have elevated levels when they’re tested, so veterinary authorities recommend checking and rechecking to confirm that a cat’s thyroid gland is functioning normally.

If your cat has this condition, the first line of offense might be a pill to normalize thyroid levels, with surgery to remove the gland being an option. This is especially important in cases of adenocarcinoma, a malignant cancer. Radioactive iodine therapy, which sounds scary, is the gold standard of treatment, and here’s what one Catster writer had to say about it. Feline y/d, a food specially formulated for cats with hyperthyroid disease, recently entered the market, but veterinarian Eric Barchas, who writes Catster’s Ask a Vet column, is skeptical about it, expressing concern that it isn’t backed by rigorous independent research.

Cats develop hyperthyroid disease for a variety of reasons, and researchers are still trying to understand all of the mechanisms involved in this pernicious condition. If your cat does get sick, don’t blame yourself or the food you chose to feed her, because there’s no single known cause for thyroid disease.

Top Photo: Quinn Dombrowski, Flickr

About the author: s.e. smith is a cat-owned writer, editor, and agitator living in Northern California with felines Loki and Leila. While not mediating cat fights, s.e. explores a wide variety of subjects in writing and elsewhere, in addition to enjoying reading like a fiend and baking like an angel. Follow smith on Instagram, Facebook, and Twitter.