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.

Watch Floppy Cat’s interview with Dr. Jean here (a written transcript can be found below the video by clicking “show 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.

Symptoms 

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
  • Vomiting
  • 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 (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.

Recent research suggests that it may be possible to detect potential thyroid disease in cats.1 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:

* 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, which are commonly listed on pet food labels as “by-products.” High-quality natural cat foods do not contain by-products, although some include specific organ meats, such as liver. Just what it is in these foods that is the culprit is unknown, although some suggest it may be BPA in the can, or fire retardants in the ingredients (see below).

BPA (bis-phenol A)–and similar compounds) found in can linings can leach out into the food and cause toxicity. The smaller the can, the more surface area of food is contacting it, and the more chemical exposure the food has overall. However, there are at least 25 different types of can linings, and the particular type used by a manufacture may change over time. It used to be that white can linings seemed more likely to contain BPA, but this has changed over the years, and no such generalizations can be made today. Now, some brands have BPA in their large non-pop-top cans, and not in the smaller ones, and vice versa. It is difficult to know which foods may be affected, and to what degree; although you can call manufacturers for current information. The FDA has stated that the amount of chemicals that may leach into the food is unlikely to cause disease; but FDA has been wrong before!

* 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 fire-retardant 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. However, 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), so its statistical power is very low.

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 (which may explain the canned food connection noted above). However, high levels of other PBDEs were also found in dry cat food.

* Another 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, although Hill’s Pet Nutrition is taking advantage of this theory by producing an ultra-low-iodine food (see below under Treatments).

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 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 to avoid PBDEs—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 giblets, or by-products.

Treatments. There are 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. Often only one of the cat’s two glands 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: the four tiny parathyroid glands that are closely attached to the back side of the thyroid glands. 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 may 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 several days. Dealing decisively with the problem may save money in the long run because no further treatment is needed in many cats. However, to date this treatment has used the same dose for all cats, leaving some cats in need of thyroid supplementation, and others whose disease has not been fully eliminated. Research looking at how to individualize dosages is ongoing.
  • Therapeutic diet is a relatively new option, with 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, and to one cat for five years. Given the small sample size, this study is not statistically powerful. Hill’s y/d is very low in iodine, and relatively low in phosphorus and sodium–which Hill’s calls “kidney friendly.” Given the lack of actual scientific evidence that dietary iodine plays any role in feline hyperthyroidism, the basis for the claimed improvement is not completely explained. As is usual for Hill’s products, the ingredients of the dry food are terrible: 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.

Complications

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.