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.


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.

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:

* 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. 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 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.


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.



Do you really know what's in your cat's food?

Signup now and find out! Get our free Pet Food Label Quiz!

You can unsubscribe at any time.

17 comments for “Hyperthyroidism

  1. Jean Hofve DVM
    November 21, 2013 at 11:01 pm

    It is important to monitor blood tests for hyperthyroid cats to make sure the levels are under control, especially since her numbers started out so bad. Since y/d is essentially “experimental” at this point, it is even more important to monitor her thyroid levels closely. You have no assurance that she is in the normal range even now, and if she’s not, heart disease is a very real probability.

    BTW, forget the y/d dry, the ingredients are extremely poor quality, and since she doesn’t like it anyway, Hill’s will refund you.

    If are unhappy with your current vet, please get a second opinion.

  2. joan carol
    November 21, 2013 at 9:06 am

    When a neighbor died, I rescued his cat. I took her to the vet and found that she was 8-1/2 yrs old and hyperthyroid with a level of 20. I decided that the easiest way for me to handle her disease was to put her on the very expensive Hills y/d diet. Her coat was terrible, she would pant some times, her heart beat rapidly, and she was 7.3 lbs. After going through all the things that could help her, I felt the only way I could do it was with food. My vet had me wean her off her regular food and onto the Y/d. 10 days after she came home from the vet, she had a bad case of fleas, which has been treated, along with my condo, by Fleabusters. She is vocal, but never howled. She usually just meows when she’s hungry which is about four times a day! She eats almost 2 cans a day and has the dry food available but rarely eats it. I refuse to take her back to the vet because of the flea incident, although they say they treat their cats with capstar. Since I have seen such improvement, why do I have to take her back for another blood test, stressing her out again? I could, however, have a vet nurse come to my home to do it. The food has obviously helped so much, and I feel I should just leave her on it for as long as she lives. Is there any thought about how long her life expectancy is with this disease? She’s just the sweetest cat in the world! I would love a reply. Thank you so much.

  3. alrac-catlady
    May 18, 2012 at 8:40 pm

    And, I am not impressed that the paper documenting Hill’s research is behind a paywall. http://www.tandfonline.com/doi/pdf/10.1080/00480169.1992.35700

    Their brochure hints at short tests on small numbers of cats

  4. alrac-catlady
    May 18, 2012 at 8:25 pm

    Thank you Dr. Hofve for all the excellent information. My little 11-year old cat was diagnosed (about five weeks ago) with hyperthyroid. She has had symptoms for a couple of years: excessive thirst, vomiting, and nervous behavior. I didn’t recognize these as something that needed to be checked out. When she has a sudden weight loss it caught my attention, from eight pounds to six, with wasted flanks and loss of strength.

    My vet is wonderful and has taken good care of my three cats, three dogs, and two horses. This is the first time I have disagreed with her treatment recommendation– she sold me a bag of the dry Hill’s y/d. $44 for 8.5 pounds. I understand why a food treatment would be such an attractive option, but in my opinion this product is not food at all, but overpriced floor sweepings. The main ingredient, corn gluten meal, is a byproduct. It has very low food value and is used as a natural weed killer. (Really!) The other main ingredient, soybean mill run is also a byproduct: the hulls and whatever nut meats cling to the hulls after milling. There is little animal protein in the y/d. A realistic price would be two bucks.

    My cat has eaten mostly dry food in her life, and after researching cat nutrition I’ve come to the conclusion that dry food is not good for kitties. So now I have her on homemade food made with turkey, chicken, and rabbit, a BARF-type diet except I partly-cook the meats. She likes it better than raw, and it alleviates my concern about surface bacteria. I add a small amount of iodized salt because I’m not persuaded that a complete iodine defiency is healthy. She has gained a pound, is calmer, and doesn’t vomit anymore. So far I am encouraged.

    If anyone wants to follow the progress of my little cat I’m documenting it on http://bratgrrl.com/

  5. January 9, 2012 at 9:02 am

    After only a few days on the YD food Shadow developed huge inflamed hairless spots on his face, which our vet said she’s seen only in food allergy cases or severe rubbing. I knew it wasn’t the rubbing because some of these spots happened within hours. One side of his face was a red, inflamed, bald rivulet. After quitting the YD, it took only days before the spots started to heal, and within the week the spots formed scabs, which fell off, leaving healthy-looking pink skin. It was definately an allergy.

  6. jhofve77
    January 5, 2012 at 11:34 am

    Variety is so important, I can’t emphasize that enough! Even with the potential issues with canned foods, I still do not recommend feeding dry food as anything but an occasional treat or in case of emergencies (blizzard, tornado, etc.!).

  7. January 4, 2012 at 11:36 am

    We are involved in cat rescue, and have more cats than average. We have had two diagnosed with hyperthyroidism in the last four months: Shadow (15) and Shawnee (9). Shawnee is going in for follow up blood work this week and we believe she’s stabilized on 7.5 mg Tapizole 2 times a day. Shadow never stabilized, and was up to 10 mg 2 times a day. The vet suspects malignancy. He was isolated and put on YD, but refused to eat the food, had to be assist fed. My vet is surprised that not one but two cats from one household needed more than 1-5mg tab BID. Up until this last year, we fed premium chicken or turkey flavored canned food in large cans almost exclusively. We supplemented with high-quality kibble, some home-cooked turkey, chicken, or poached fish, and some canned salmon. I appreciate your article as we’ve been feeling guilty for feeding canned food (trying to contain urinary issues).

  8. Kathleen
    December 6, 2011 at 12:23 pm

    My cat was recently (Oct 2011) with elevated T4 levels.(47.4 nml/L)
    We started him slow with 1.25 mg/day of tapazole for 7 days then up to 2.5mg/day.His levels came down to 2.4 ng/dL. This is supposed to be ‘normal’.I dose him twice daily with 1.25 mg. mixed in with a tsp of wet food.He has been doing well with no adverse reactions. I decied to try the new y/d food as a possible alternative. Our cat did not like the wet food at all and tolerated the dry. He didn’t seem very happy or lively on this food. I discontinued after 2 weeks and went back to his old w/d
    dry food and wellness canned chicken wet food (for the Tapazole administration)We lost our other cat who had also been diagnosed with hyperthyroidism in April 2011. He did not respond well to Tapazole. I think there was some other issues going on but we were first trying to get the thyroid under control.
    My advice to anyone is to do as much research as you can re food, meds and supplements and monitor your cat as closely as you can. Then make your own decisions. I am retired so have the time for this.

  9. jhofve77
    October 13, 2011 at 5:58 pm

    Sorry to hear about your kitty! Anorexia and anemia are known side effects of methimazole. If you are dissatisfied with your veterinarian’s treatment, you may wish to consider filing a complaint with the state veterinary board. For more information please see http://www.aldf.org/article.php?id=243

  10. October 13, 2011 at 2:47 pm

    My cat Houston died on canadian Thanksgiving day.
    she was on tapazole ear gel 0.2ml (5mg) twice daily. her t4 was normal but she refused to eat. She hid herself in the basement for two days, finally brought her to the emerg and her hematocrit was 9%. She had to be euthanized. there was no way to bring her back from that. She was booked for radioactive iodine in october. I feel horribly guilty. the vet denies it’s tapazole but i’m very suspicious….

  11. Madelyn
    October 12, 2011 at 10:42 am

    thanks for listening – turns out in June, Tiger did not have elevated T4’s but did seem to have elevated alamayse and not lipase so the vet assumed he did not have a pancrease problem. I guess I will always be wondering if that was the beginning of intestinal cancer. At 18 yr I know they can go down hill quickly. thanks again

  12. jhofve77
    September 13, 2011 at 11:19 am

    Sorry to hear about your kitty! It’s possible that he wasn’t tested for T4 (the most definitive test for feline hyperthyroidism), there are certainly many permutations of tests they can select. But any vet would most likely run a “senior” feline blood panel on an 18 year old cat as a rule, which would include T4. The level could have been all right then, perhaps in the grey area (high normal, but still in the normal range) but not conclusive of disease. Many feline diseases can progress very quickly. For instance, I happened to test my own cat one April, and everything was completely normal, but when we retested in June, she was in kidney failure. Sometimes you just don’t catch them at right moment. Because hyperthyroidism involves a tumor, it may grow unusually fast and create a lot of havoc. The vet should be able to give you a copy of his records if you want to check.

  13. Madelyn
    September 12, 2011 at 1:47 pm

    Thank you for the info. My 18 yr old cat passed away a few weeks ago. He had elevated liver and kidney numbers. But, in June his blood levels were good. I did tell the mobile vet he threw up a lot but had missing teeth so maybe he swallowed to much. I am realizing that perhaps he had hyperthyroidism. It does not seem like he was tested for it in June. Thanks

  14. jhofve77
    September 7, 2011 at 10:20 am

    So sorry to hear that you lost your cat and that she had to go through all that. I’ve seen many cats that were put on the full dose right away, and developed toxicity or an allergic reaction. I typically started with 1/4 tablet once a day for 3 days, then 1/4 tablet twice a day, then 1/2 tablet, etc. It took a couple of weeks to get them up to speed, but it was well worth being cautious, as I never had a cat develop that kind of reaction! The new feline methimazole tablets come in 2.5 mg, so hopefully vets will get the hint that lower doses are just fine!

  15. Sean
    September 6, 2011 at 11:27 pm

    As mentioned above, be very careful with the dosage of Tapazole for the hyperthyroid, make them start your pet with a low dose first! Recently my cat died because they started her off too high (5mg) for weeks and by the time we got her back for another blood test she was on her death bed. 5mg is standard according to manufacturers, but even less is strongly recommended to save on your pets organs.

  16. jhofve77
    January 13, 2011 at 10:13 am

    Many vets tend to start methimazole (the generic drug name) at too high a dose. (BTW, the brand name of the human product is Tapazole, but now that there is a feline-specific version, called Felimazole, Tapazole should, technically, no longer be used). In my experience, cats do better starting with 1/4 of the dose and working up to the full dose over a week or two. This seems to eliminate those reactions; which makes sense–the effect of the drug is to crash the blood pressure…which will make a zombie out of anyone!

    My alma mater, Colorado State University, does the iodine treatments, but they require a trial of methimazole for at least 2 weeks to assess the kidneys. There should not be any effects on the other cat from the iodine.

    If her kidney values are normal or only slightly elevated, a prescription kidney diet is not warranted. Cats don’t really like k/d, and many will lose weight on it due to the severe protein restriction. Read more about kidney disease here: http://www.littlebigcat.com/health/kidney-disease-in-older-cats/

    Honestly, I would not delay some type of treatment The effects of hyperthyroidism on the body, especially the heart, can be severe–even if the blood levels don’t seem that bad.

  17. Linda Taylor
    January 13, 2011 at 9:04 am

    Thank you so much for this article. I have had so much misinformation about my cat who was recently diagnosed with “elevated T4.” I have been trying to decide what treatment to pursue because she had a bad reaction to the pills (Tapazole). Perhaps the dosage was too high but she quit eating and went into a zombie like state. She returned to normal after quitting the pills. I have been concerned with the radioactive iodine treatment because I have another cat at home. I had 3 different opinions from 2 vets and a treatment center. It sounds like the pill approach to detect any kidney disease and then the radiation might be the right way to go. Her levels are only slightly elevated so I will have her checked again in 3 months before pursuing any radical treatment. The vet recommended KD diet to help her gain weight first.

Leave a Reply