The problem is typically a benign thyroid tumor. Because the tumor cells are relatively normal, they continue to produce thyroid hormones, resulting in a high level in the blood. The thyroid regulates the body's metabolic rate, so this increase is sort of like drinking espresso around the clock. Symptoms include increased appetite, weight loss despite eating more, increased heart rate, anxiety or "hyper" behavior, howling at night, increased thirst and urination, vomiting, and diarrhea. Not all cats will have all symptoms, and about 20% of hyperthyroid cats will be sluggish and depressed instead of hyperactive. Untreated, hyperthyroidism can cause a serious heart problem called hypertrophic cardiomyopathy that will ultimately be fatal.
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 problematic 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 other PBDEs were 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.
Treatments. There are three primary treatment options for hyperthyroidism:
- Methimazole (Tapazole) is a medication that can be given in tablet form (the tiny tablet can be crushed and mixed into wet food) or as a topical gel that you rub on the inside of the cat's ears (ideal for non-pillable cats). It requires frequent blood tests for the first few weeks as the dosage is adjusted to fit the cat, and then every six months to make sure the dose remains appropriate. Occasionally, cats become allergic to the medication. While this is initially the least expensive option, maintenance can become costly over time.
- Thyroidectomy is the surgical removal of the thyroid glands. Cats actually have two thyroids, one on either side of the throat. Often only one is involved and can be safely removed. However, there is a significant risk that the second gland will ultimately develop disease and need to be removed later. Removing the thyroids is no big deal to an experienced surgeon, but there is one major problem: four tiny parathyroid glands that are closely attached to the thyroids. Removing or damaging the parathyroids can result in severe, even life-threatening problems with calcium balance. Because of the way the thyroid glands develop in the fetus, there can be thyroid cells scattered here and there that can also become cancerous. A cat who has had both thyroids removed can therefore still become hyperthyroid again. These secondary tumors can form inside the chest where they cannot be surgically removed.
- Radioactive Iodine is the most definitive treatment. The thyroid uses iodine to make its hormones, and accumulates large amounts of iodine. A single injection of radioactive iodine will be hoarded by thyroid cells and kill them, theoretically curing the disease permanently. Most cats tolerate this procedure well, and most do not need thyroid supplementation. The downside? The up-front cost is very expensive, and regulations require that the cat be kept in the hospital for 7-10 days. However, dealing decisively with the problem may save money in the long run because no further treatment is needed in most cats.
Since hyperthyroid cats are typically older, many also have underlying kidney disease that may or not be obvious. The increased blood flow caused by hyperthyroidism can actually help the kidneys and keep them functioning at a fairly normal rate. Lowering the thyroid levels (by any treatment) lowers blood pressure and blood flow. This can "unmask" kidney disease that was always there but not detectable.
Because surgery and iodine treatments are irreversible, a few weeks of treatment with methimazole is usually a prerequisite. A cat with both kidney disease and hyperthyroidism is a challenge to manage; you'll need to work closely with your veterinarian if this is the case.
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