Hyperthyroidism is a common disease that typically affects middle-aged and older cats.  It is caused by an excess production of thyroid hormones, which are produced by the thyroid gland, located inside the cat’s neck.  Thyroid hormones affect nearly all organs, which is why thyroid disease can sometimes cause secondary problems such as hypertension, heart and kidney disease. 

What causes hyperthyroidism?

The most common cause is an increase in the number of cells in the thyroid gland.  Groups of these abnormal cells form small nodules called adenomas on the gland.  Most of these adenomas are formed by non-cancerous cells, only a very small percentage of hyperthyroidism is caused by malignant tumors.

More recently, there has been speculation on a possible link of an increase in thyroid disease in cats and the coating used on cat food cans.  Another theory is that flame retardants used in furniture and carpeting may be linked to hyperthyroidism in cats.

What are the signs of hperthyroidism?

Afflicted cats often develop a variety of signs, and some of them can be subtle.  The most common signs are weight loss, increased appetite without weight gain, and increased thirst and urination.  Hyperthyroidism can also cause vomiting, diarrhea, and hyper-activity.  The haircoat may become matted and dull.  Some cats will begin to vocalize more frequently.  Rapid heart rates are common, and cats can also present with heart murmurs and high blood pressure.

How is hyperthyroidism diagnosed?

You cat’s veterinarian will perform a physical exam and palpate externally alongside the trachea with thumb and forefinger to feel for any enlargement of the thyroid gland.  Heart rate and blood pressure will be checked, and a complete blood chemistry will be run.  Most hyperthyroid cats will have elevated levels of the thyroid hormone T4 in their blood stream.  However, sometimes a cat with concurrent kidney, heart or gastrointestinal disease may have normal T4 levels.  If other symptoms and exam findings point to hyperthyroidism, your veterinarian may order additional testing to arrive at a diagnosis.

How is hyperthyroidism treated?

There are currently three treatment options:  medication, surgery, and radioactive iodine therapy.  Each option comes with advantages and disadvantages, and you should carefully weigh all options and make the best decision for your cat and your lifestyle in conjunction with your veterinarian.


Drug therapy, using a drug called methimazole (Tapazole), controls, but does not cure the disease.   It is typically given twice a day in either pill form or as a transdermal gel that is rubbed on the inside of the cat’s ear.  Methimazole therapy will be required for the rest of the cat’s life.   While some cats tolerate the drug well, it can have serious side effects including elevation of liver enzymes, low white blood cell counts, low platelet counts, itchiness of the face, and gastrointestinal problems, including vomiting and loss of appetite. If these signs occur, the medication has to be discontinued and other treatment pursued.


Surgical removal of the thyroid gland is an option, although many hyperthyroid cats won’t be good candidates for surgery due to the anesthetic risk caused by their elevated heart rates.  Even though removal of the thyroid gland is a fairly straightfoward procedure, it should only be done by an experienced surgeon, since there are potentially serious complications, including damage to the parathyroid glands, which lie close to or within the thyroid glands and are crucial in maintaining stable blood-calcium levels.

Radioactive Iodine

Radioactive Iodine, also called I-131, is the gold standard for treating hyperthyroid cats.  It involves a one-time injection of radioactive iodine under the skin.  The radioactive iodine will destroy the abnormal thyroid tissue but does not damage the surrounding tissue or the parathyroid gland.  The cat will have to remain hospitalized for a specified period of time (typically 3-10 days, depending on geographical location, the length of the stay is regulated on the state level).  It will be released with some special care instructions, such as limiting contact with the cat and special disposal of urine and feces for a few days following treatments.  The treatment is only available at special facilities that are typically found at large veterinary referral centers, and is somewhat costly, but it is curative, and needs to be weighed against the cost of lifelong medication.

Regardless of which treatment is chosen, unless there are other, underlying diseases complicating things, treatment is usually successful and most cats will lead normal, healthy lives.

The photo above is of Amber, taken the day she went for her radioactive iodine treatment at Radiocat in Springfield, VA.

19 Comments on Hyperthyroidism in Cats

  1. Re: flame retardants mentioned in the article. Flame retardants are associated with a class of chemicals call PFAS. They are “forever chemicals” meaning that they never leave the body and continue to accumulate. It has been suggested that PFAS are in 98% of all living creatures on Earth. (PFAS was a major ingredient in Teflon-coated cookware.)

    If you’d like to know more about PFAS I highly recommend reading “Exposure”, Robert Bilott’s 20 year battle with DuPont (Which is still on-going, I might add.) over PFAS contamination – or watch the docudrama it was based on: “Dark Waters”. Lots of information on the web, as well.

  2. The article was great! Thank you for the information. For those having this prodedure done on your cats, please know that the radiation is still present in the cat when they come home!!!! I did research and found so many scary stories about it. I actually found a website that gets rid of the radiation (bind-it hand soap). It made me breath a little easier anyway.

  3. Thank you for this very informative and insightful article – my cat was diagnosed with hyperthyroidism this spring, and we have had a very hard time getting good advice. Many Vet doctors recommended the Hill’s diet that was reviewed and I heartily agree with you, it is really not a good solution, nor a healthy product. I am currently in the San Francisco area, and would love it if you knew of anyone who specialized in treating hyperthyroidism and cats holistically, part of why I really like your blog is that I agree with your more healthy & natural approach. Thank you again, warmly, Carolyn.

  4. Yet another great article, our tortie Jackie had this about four years ago, we elected to do the iodine treatment and it was completely successful. She developed a perforated ulcer while in the hospital from the treatment, this was repaired surgically and she is doing great now except for being a bit overweight which we are working on.
    tom Mary Beth and the furries.

    • If I had it to do over again I would of gone with the radiation treatment. My boy has been on the pills for over 3 years and he is starting to fail now. I am so sad because he has such a dynamic personality. I just wish my vet had kept me better informed of my options.

  5. Hi Ingrid: I recently took my newly adopted shelter cat (a Tortie like Amber) to Radiocat for treatment. Her 1-month blood test was good except for a slight increase in BUN levels which the vet will check again on Cookie’s 3-month blood test. I did find that she had some diarrhea today with what looked like blood. I immediately panicked but then decided it could be the coloring in her dry food. For the life of me, I can’t get her off the crappy dry food from the grocery store. She won’t eat anything else. She was behaving normally, so I’m just tracking her output on a calendar and obviously will take her to the vet if I see this again. But my question to you, is how was Amber after her treatment. I understand that hypothyroidism can also mask CRF in cats. Any insight you can provide is greatly appreciated.

    • Amber did great after her I131 treatment. She briefly became hypothyroid, ie., her thyroid values went below normal. It happens to a small number of cats after the readioactive iodine treatment, and in most cases, it regulates itself within a few weeks, as it did with Amber. Her kidney values were normal until the end of her life in 2010.

      As for your little hardcore dry food addict, I encourage you to keep trying. At the very least, transition her to a better, grain-free dry food. Here are some tips on how to get your cat off dry food: http://consciouscat.net/2011/07/25/how-to-your-cat-off-dry-food/ and you can also use these tips to transition to a better quality dry food.

      All my best to you and Cookie.

  6. I’m glad you enjoyed the post, Marg, and even happier that your kitties have always been able to take the medication. That can be such a challenge.

  7. That is an excellent article on hyperthyroidism in cats.
    My friends cat had that and she opted for the Radiocat
    too in South Carolina. It was successful. I had viewed
    several articles on the disease and this one is more informative.

  8. Bernadette, wow – 25 years?! That’s amazing.

    Layla, thanks. I agree that it’s no different for our animals than it is for us – when it’s time to leave, it’s time. I also believe that ultimately, our animals decide when and how they’re going to leave, and if we’re in touch with their spirit, and I know you are with yours, it can become a mutual decision rather than us playing God. I wrote about this extensively in Buckley’s Story – it was a little easier to apply it then, because with Buckley’s heart disease, I had plenty of advance warning, and there was time to connect and say our good-byes in a peaceful environment over a longer period of time. With Amber’s sudden illness and the emotional devastation it wrought on me, it was much harder to keep checking in her with her on that level to see if she wanted to to keep going with treatment, or if she was ready to leave. I still haven’t come to terms with how everything unfolded in the final days of Amber’s life.

  9. Great information Ingrid on the hyperthyroidism. I have had several cats that have had it and once they get on the medicine, they get along just fine. The cat I have now is very good about pills so it is easy to treat her. And most of the cats I have had, eventually learn to swallow the pills. But that was a really good post about this disease.

  10. Ingrid, Amber was blessed with your stellar care and I’ll second Bernadette’s response. Well done. And wow about her Stanley living close to 25! There is no doubt cats are living longer and but despite our medical advances I subscribe to my spiritual beliefs; regardless of the best care given, when it’s time to go, it’s time to go. It’s so hard to know when to and pull the plug and euthanize, to prolong treatment or not, to evaluate quality of life. To me it’s like play God deciding if my pet lives or dies and it’s one role I’d rather not play.

  11. Ingrid, thanks for this clear and concise article on hyperthyroidism. Many people, without clear explanation, fear the disease is a death sentence when it’s so easily controlled it’s hardly noticeable. A friend sent her 13-year-old cat to Ohio State for the RI treatment, which was successful and without incident, and Maximillion Diamond lived another seven or eight years. My Stanley was diagnosed with it at the same time as his renal failure when he was about 21, and he lived almost another four years with both; any treatment but medication was out of the question for him. Namir was also treated for it though it was minimal in comparison to everything else. With the medication alone for both, we had to keep slowly increasing as the disease did progress as they grew older.

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