By Jean Hofve, DVM
A serious disease called “feline hyperthyroidism” 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. Hyperthyroidism (hyper = too much, thyroid = a hormone-making gland), is seen mostly in older cats age 10 or more.
In people, the thyroid gland is “H” shaped, and located at the base of the throat. Its position is the same in cats, but the shape is different. It’s missing the crossbar of the H, so there are actually two glands, one on each side of the trachea. Additionally, many cats have extra thyroid tissue in other places, often at the top of the ribcage just inside the chest.
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.
Hyperthyroidism is typically the result of 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 most common symptoms of hyperthyroidism are:
- Increased appetite
- Weight loss (often despite eating more)
- Increased heart rate
- Anxiety or “hyper” behavior
- Howling at night
- Increased thirst and urination
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 (thickened, enlarged heart) that can ultimately be fatal.
Diagnosis. In some cats, your veterinarian may be able to palpate an enlarged thyroid gland. However, hyperthyroidism is definitively diagnosed by blood tests. In cats, we usually look for an increase in T4, the inactive form of thyroid hormone. Normally, the thyroid produces and releases T4 into the blood. When needed, the T4 cycles back through the thyroid gland, which attaches an iodine molecule, converting it to the active form, T3. The test for T4 will report a result as well as a “normal range.” The unique thing about T4 is that even if it is normal, if it’s at the high end of normal, the cat may still be hyperthyroid. For equivocal test results, a confirming test (equilibrium dialysis) is usually performed. It is also common to also see elevated liver enzymes in hyperthyroid cats, so this may also help with the diagnosis.
Newly published research1 suggests that it may be possible to detect potential thyroid disease in cats. Thyroid stimulating hormone (TSH) should be low when there is more T4. It turns out that it may be low even when T4 is still in the normal range. Cats whose TSH was low or undetectable were likely to develop clinical hyperthyroidism within the next few years. Currently, the test for TSH is not very accurate in the low range, but hopefully a more sensitive test will be developed soon. Even now, it may be worth adding it to your cat’s senior wellness checkup. A low level is a strong predictor for later hyperthyroidism, but even more importantly, a normal level appears to rule out thyroid disease in older cats.
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.
* Research suggests that the culprit may be a chemical (bisphenol A–BPA–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.
Another study suggested 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 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 now four primary treatment options for hyperthyroidism:
- Methimazole 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. While the human drug Tapazole or its generic equivalent have been used for cats “off-label” for 30 years, there is now an FDA-approved version of methimazole for cats, called Felimazole. However, because there is now a “licensed” feline product, it may be technically illegal to use the human methimazole for cats in some states.
- 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. Additionally, 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 still become hyperthyroid again. These secondary tumors often form inside the chest, where they are inoperable.
- 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.
- Therapeutic diet is a new option as of August 2011, with the release of Hill’s Prescription Diet y/d canned and dry. Hill’s claims that this diet, fed exclusively, will normalize a hyperthyroid cat’s hormone levels within 3 weeks. The diet is not recommended for cats without the disease; and it does not satisfy the AAFCO nutrient profiles for cat food (a food does not have to meet those standards if it passed a feeding test). Remember that an AAFCO feeding test requires feeding the test diet to a minimum of 8 cats for 6 months; apparently in this case, it was fed to 14 cats for a year. y/d is very low in iodine, and relatively low in phosphorus and sodium–what Hill’s calls “kidney friendly.” Given the utter lack of scientific evidence that dietary iodine plays any role in feline hyperthyroidism (though there is a theory that it might contribute), the basis for the claimed improvement is not completely explained. As is usual for Hill’s products, the ingredients of the dry food are pretty awful: mostly GMO vegetable proteins and no meat at all (in order to reduce both iodine and phosphorus); the canned food appears somewhat better, since it contains actual chicken, in addition to the usual meat by-products and grains. Ingredients that are typically high in fire retardant contamination (fish, poultry by-products) are avoided; and Hill’s states that the cans are BPA free. It will be interesting to watch the reaction within the veterinary community, as well as from Dechra (which makes Felimazole), and the radioactive iodine industry.
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.
1 Wakeling J, Elliott J, Syme H. Evaluation of predictors for the diagnosis of hyperthyroidism in cats. J Vet Intern Med 2011;25(5):1057-1065.
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