"Causes. 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. Just what it is in these foods that is the culprit is unknown.
* Recent research suggests that the culprit may be a chemical (bisphenol A and similar compounds) found in can linings of easy-open "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 orhwe PBDEs were also found in dry cat food.
The combination of PBDEs, can liner chemicals, and excess iodine may be too much for many cats to handle. However, plenty of outdoor cats who never eat canned food can also develop the disease—so other factors that haven't been discovered yet are likely to be involved. For instance, many holistic veterinarians also believe that, because this disease is fairly new but rapidly reaching epidemic proportions, that vaccines may also be a factor."
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 all that practical, either!
Feeding canned food is very important to an older cat's overall health, but it may be wise to stick to poultry, beef and lamb flavors that don't contain liver, giblets, or by-products. If possible, get the larger cans that don't have a pop-top." http://www.littlebigcat.com/index.php?action=library&act=show&item=hyperthyroidism