A serious disease affecting cats’ thyroid glands has risen to epidemic proportions since the first cases were diagnosed in 1979. It is a worldwide phenomenon, though worse in the U.S. than other countries.
The disease itself, called hyperthyroidism (hyper = too much, thyroid = a hormone-making gland), is seen mostly in older cats age 10 or more. Thyroid hormone regulates the body’s basic metabolic rate. Too much of it is like drinking too much coffee—it speeds up every reaction in the body.
Symptoms of hyperthyroidism are related to excess thyroid hormone: excessive hunger, weight loss, restlessness, yowling, vomiting, diarrhea, and heart disease. However, about 20% of cats are “atypical” and do not show these symptoms. (Dogs also develop thyroid disease—but in the opposite direction. They are more prone to hypothyroidism; a deficiency of thyroid hormone.)
Many theories have been proposed to explain the dramatic rise in feline thyroid disease. Because it affects so many cats, the focus has been on widespread, environmental causes.
Studies have found several suspects in cat food:
- One theory implicates the large excess of iodine found in many cat foods. Humans can develop hyperthyroid disease from too ingesting much iodine; might cats do the same? Iodine is difficult and expensive to test for; instead, pet food makers routinely add extra, just to make sure that minimums are met. But how much is too much? So far, nobody knows; and nobody is really looking.
- Several studies found an increased likelihood of developing thyroid disease in cats who eat a lot of canned food. Specifically, they found a higher incidence of hyperthyroidism in cats that ate fish or “giblet” canned foods. “Giblet” is another name for organ meats commonly listed on pet food labels as “by-products.” Better-quality, natural cat foods do not contain by-products, although some include specific organ meats like liver.
- Recent research suggests that the culprit may be a chemical (bisphenol A and similar compounds) found in can linings of pop-top cans that can leach out into the food and cause toxicity (the smaller the can, the more chemical exposure the food has). However, there are at least 25 different types of can linings, and the particular type used by a manufacture may change over time. It is difficult to know which foods may be affected, and to what degree. The FDA, however, has stated that the amount of chemicals that may leach into the food is unlikely to cause disease.
A new study that recently captured media attention suggests that fire-retardant chemicals known as PBDEs may be a factor in feline hyperthyroidism—even though the study itself clearly states “no association was detected between hyperthyroid cats and PBDE levels.” The authors note that PBDEs were introduced at about the same time hyperthyroidism was first described in cats. California was particularly aggressive in promoting these life-saving fabrics, which could explain why the veterinary school at U.C. Davis saw so many early cases of hyperthyroid disease in cats. Additionally, the rate of feline hyperthyroidism has roughly paralleled the use of PBDEs in other countries. This particular study looked at only 23 cats (less than half with hyperthyroid disease; and the case controls were not well-matched in age or gender.
The main route of exposure in cats was hypothesized to be the PBDEs contained in carpets, upholstery, and mattresses—and the dust mites that live in these fabrics. Electronic equipment, which attracts dust, is also a suspect. Since cats often sleep on carpets, sofas, chairs, mattresses, and nice warm TVs and stereos, their exposure could be high and prolonged. Subsequent grooming would then cause the cat to ingest a fairly large amount of dust. This may explain why hyperthyroidism is also more common in indoor cats.
Perhaps the most interesting wrinkle in this recent study is that PBDEs were also found in cat food. For two major types of PBDEs, high levels were found in canned food, especially fish- or seafood-based cat foods. However, high levels of other PBDEs were also found in dry cat food.
The double (or triple) whammy from PBDEs, can liner chemicals, and excess iodine may be too much for many cats to handle. However, outdoor cats who never eat canned food can also develop the disease—so other factors that haven’t yet been discovered are likely to be involved.
What can you do to minimize the risk for your cat? Well, it wouldn’t be smart to push your cat outdoors—the dangers outside are far worse, and most of them will kill your cat long before the age where she’s at risk for thyroid disease. Ripping out all your carpets and throwing away your furniture probably isn’t practical.
However, we also would not recommend feeding only dry food. There are many health risks for cats that eat too much dry food, including obesity, diabetes, arthritis, urinary tract disease, constipation, kidney disease, and inflammatory bowel disease. A variety of high-quality, natural foods—including at least 50% canned food—is your cat’s best bet for a long and healthy life.