PetMeds® Treating Hyperthyroidism in Cats

Hyperthyroidism or an overactive thyroid is the most common hormonal disease seen in older cats. The most common symptoms include weight loss (despite an increased appetite), as well as increased thirst and urination. Some cats may become more restless and vocalize more, as well as develop inappropriate urination or defecation. If left untreated, hyperthyroidism can lead to high blood pressure, kidney damage or secondary heart disease.

Diagnosis is usually made by blood measurement of Thyroid levels, usually a T4 in conjunction with a free T4. There have been traditionally three options of treatment. The most common form of treatment is with a prescription medicine known as Methimazole.  This medicine must be given daily to twice daily, and periodic thyroid level  blood monitoring is needed every 3-6 months after initial control is obtained, , as well as monitoring of the blood count, and liver/kidney function  to make sure there are no ill effects of the medication.

Occasionally cats will become lethargic, vomit, or lose their appetite on Methimazole. However, with dose adjustments, most cats do tolerate Methimazole quite well, which needs to be maintained for the life of the cat. Other options of treatment include using radioactive iodine, which is preferred by many as a single dose curative treatment in most cases.  Surgical removal of the thyroid gland is no longer as common, due to surgical risk in many of these older cats.

In my experience there are not that many effective supplements for cats to treat the overactive thyroid, however, if your cat will accept cooked brassica veggies like broccoli or kale, these can sometimes have a thyroid-lowering effect. The herb bugleweed has been used by some, but data as to efficacy is limited.

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  1. Dr. Michael Dym, VMD veterinarianJuly 9, 2013 at 9:52 pm

    You are very welcome. Sounds like you take wonderful care of you kitty.

  2. Dr. Michael Dym, VMD veterinarianJuly 9, 2013 at 9:58 pm

    Hi Mrs Loughry. Great to hear from you. Sorry to hear of Ceasar’s thyroid issue. Of the allopathic traditional option, probably radioactive iodine is the safest on the short term. However many cats can develop kidney disease shortly after, and given Caesar’s urinary tract issue predisposition that would be a concern. With the methimazole therapy, if kidney values start to go up, then therapy can be reduced or stopped which is not possible with the radioactive iodine once it is given. Depending upon how high thyroid values are, homeopathy and nutritional supplements may also be an option. I would be happy to consult with you further on this, as I do offer phone consultations. STAY AWAY from any vaccinations in cat like this. To learn more about my current practice, see

  3. I also wanted to add that the compounded methimazole has the least side effects. Starky has been on this for a year with little or no side effects, but he was in perfect health otherwise, so I don’t know if this played a role or not but it has had little effect on his kidneys. The compounded methimazole is a liquid form which I give him 1cc a day. His levels were over 5 times the normal. He started out taking 1cc twice a day for 6 months until his levels dropped to 4 which is very low, so they changed it to once a day. I don’t know if this helps or not but I just wanted to let you know.

  4. Dr. Michael Dym, VMD veterinarianJuly 10, 2013 at 6:27 pm

    Thanks for sharing…..

  5. Hi again. I just wanted you to know that both of my older cats will no longer be on y/d. I had them on a raw diet for roughly 8 years, so it was a tough decision to move them to a processed diet in attempts to lower T4 levels. At the time, the y/d seemed the lesser of two evils (compared to tapazole). Now, I’m leaning the other way because I want to protect her kidneys and of course support her immune system and all that other good stuff that comes with feeding raw (like better breath!).

    The one cat’s kidneys are functioning well enough to process y/d so they should be even healthier when she’s back on her easy-to-digest, raw diet. If I can get her BUN to remain normal while her T4 is taken to 2.0 or below, she can have the radioactive iodine. Hopefully her sister will have the same luck. Please cross your fingers for us. dies.

    Thanks for being an open minded healer who cares about animals and their owners. A big hug to you, from me, and sandpaper kisses from my sweet, goofy cats.

  6. Dr. Michael Dym, VMD veterinarianJuly 11, 2013 at 6:12 pm

    You are very welcome. Raw diets are the way to go. I work with a renowned feline nutritionist Anitra Frazier, author of The NAtural Cat on improving cats’ health over time through diet, nutritional supplements and classical homeopathy. There are other health options. Hyperthyroidism is not easy disease to treat holistically but there has been recent progress.

  7. Dr Dym. I really love what you have to say. I am going back and forth on the I-131 treatment option because I am not sure yet if renal function in these cats is sufficient. Since I can only have them on a very low dose of carbimazole, I am getting creative and thinking of ideas that might work to increase it’s effect or otherwise help me minimize the amount needed to lower T4. I am using broccoli powder (added to meals) as part of my effort to lower T4. I’m getting ready to juice some broccoli for them instead of feeding the powder. I feel this will maximize glucosinolate content.

    What do you suppose would happen if I applied transdermal carbimazole to a shaved patch of skin right over the thyroid gland? I don’t know if this skin is as vascular or thin as the skin in the pinnae, but if it is, it should allow me to use less carbimazole .

  8. Dr. Michael Dym, VMD veterinarianJuly 27, 2013 at 11:04 pm

    Hi Nik. Great question about the medicine being placed closer to thyroid gland. I am not familiar with any evidence that this would be more effective so I would stick with the ear. I would certainly continue the broccoli as you are doing. I have also found the supplements CANINE thyroid support and feline hepatic support from Standard Process company to help balance thyroid gland when added to food . The canine thyroid support formula is fine, as they dont make one for felines.

  9. my cat’s blood work has been and is perfect normal since on y/d.

  10. Dr. Michael Dym, VMD veterinarianAugust 25, 2013 at 9:26 pm

    Thanks for sharing the great news.

  11. When in a rush to get to work or some other situations I have accidentally forgotten 1 or 2 dosages of my cats anti thyroid meds. Can this cause more of a problem for him?

  12. Dr. Michael Dym, VMD veterinarianSeptember 1, 2013 at 12:36 am

    HI Priscilla, Missing occasional doses of thyroid meds should not cause a problem.

  13. Just to let anyone know in case they are interested, Starky is doing well since he started on the compounded methamizole. He has been taking it now for 15 months with no side effects from the methamizole. He was really constipated and we had to take him to the hospital cuz he lost almost 2 lbs in a week. He was on fluid therapy for 3 days and started eating again after not eating for 5 days. He is home now and doing well but he seems to want to drink water on the walk way outside or in the shower floor. I don’t know what that’s about. He is 20 years old now and he is doing ok.

  14. oh yeah, I did try giving him broccoli but he would not even touch it. I even made juice out of it and still he did not want it. I tried putting a little on his mouth but he hated it!! I guess it won’t work for him.

  15. Dr. Michael Dym, VMD veterinarianNovember 12, 2013 at 12:55 am

    Hi Sandra. Thanks for sharing. Glad he is doing beter.

  16. Dr. Michael Dym, VMD veterinarianNovember 12, 2013 at 12:56 am

    Many cats wont eat broccoli although worth a try. Try new prescription food Y/D from Hills, which also may lower thyroid levels.

  17. I have given him the y/d in the past and he did not like the wet food but he will eat the dry now and then but not every day. He has to have different food every day. I figure at his age it’s ok to spoil him. Thanks for letting me know about it and if they do have a newer version of the y/d, I would be willing to try that for him. Thanks again Dr. Dym

  18. Dr. Michael Dym, VMD veterinarianNovember 13, 2013 at 1:29 am

    You can also try bloodtwig dogberry, an effective natural liquid gemmotherapy which also may help with thyroid condition.

  19. Dianna~So sorry that you lost Rogue, Im sure she knew how much you loved her. I myself have a kitty diagnosed with Hyperthyroidism January 2013~After awhile of greasy looking hair and losing ALOT of it and throwing up occasionally. Doc put him on y/d food and he wouldnt touch it. After alot of reasearch I took my kitty about 2 hours away and left him at a ficility for 4 days to inject “IODINE 131”. “STINKER” seemed to get better for a few months BUT after approx months started throwing up again~Another trip to the vet and they said his blood was “SOMEWHAT” normal~It is December now and Stinker has been diagnosed with Liver failure and Kidney (RENAL) failure~We will be letting him drift to kitty heaven in a couple of days. I feel like I have let him down by having him injected with IODINE 131 but thought it wa the right thing to do. (It want easy on the pocket book either) AND my point is……Be careful with your decisions as thy are sometimes HARD to live with~R.I.P. Stinker.

  20. Dr. Michael Dym, VMD veterinarianDecember 29, 2013 at 5:23 pm

    Excellent points Jill, especially when considering treatment of hyperthyroidism in senior cats, who by their age, are prone to multiple geriatric issues. What I usually do in practice is try the cat on methimazole for a few weeks, and watch kidney function closely. If BUN and creatinine are going up, then probably not a good candidate for radioactive iodine, although radioactive iodine typically the easiest and best treatment, as long as kidney disease not develop once thyroid levels return to normal. Benefit of medication is that doses of meds can be adjusted to allow optimal thyroid values for the individual patient, which is of course not possible with radioactive iodine, which is literally a one shot deal

  21. Thank you Dr. Dym, I have not yet tried these things you suggested as our cat was very sick for a couple of weeks that had nothing to do with his thyroid problem. He has been on the compounded methamizole for 18 months now with no side effects. He had a bowel infection from being so constipated and was not able to poop at all. He almost died. He was walking around crying constantly and could not rest at all even though he tried. He would not eat or drink so we took him back to Hesperia Animal Hospital and Dr. Ahmed gave him IV therapy and cleaned him out and he did okay for a while but had a tooth infection and had to have his teeth cleaned and have 2 extractions. He is doing much better now and still no side effects from the compounded methimazole. His blood tests are all normal which is why Dr. Ahmed gave him the fluid therapy. He said Starky is his oldest patient and keeps on ticking. We didn’t know what to do about these problems but the Dr and his staff took such good care of him, we knew we made the right decision. Yes, it did cost quite a bit but he is worth it cuz he is a member of our family and had nothing else wrong with him which made his chances of surviving the surgery that much better. Dr. Ahmed is a really great vet and the most caring and devoted Veterinarian we have ever known. We have seen all the letters from other people who brought their pets there. I put a pic of him with Starky on facebook because he has saved our cat so many times when we thot he was going to die. I don’t know how long he has to live but I know that giving him the compounded methamizole was the right decision.

  22. Dr. Michael Dym, VMD veterinarianDecember 31, 2013 at 1:44 am

    HI Sandra. As a fellow veterinarian, i am sure that Dr Ahmed greatly appreciates your words of kindness and support. I am sure clients like you make his love and passion for his work that much stronger, as he continues to help be an important part of your health care team and in promoting the deep human/animal bond that is clearly so strong between you and Starky. Have a wonderful New Year.

  23. Thank you Dr. Dym…..just to let you know, Starky is doing good and still kicking. Starky loves Dr. Ahmed, cuz he spoils Starky so much. He had another scare last month, but thanks to Dr. Ahmed’s quick thinking, he was able to save Starky again. We made a huge mistake. Starky’s blood tests have been really good, no damage to any organs from the methamizole. He continues to take the compounded methimazole. The only problem was because he has a megacolon now and he was so plugged up we had to give him a fleet emema so we could save money not keeping him in the Hospital, but we gave him too much cuz we thought he was more constipated than he was and we ended up severly dehydrating him so he had to have fluid therapy for 2 days but he got 2 come home at night. We must be doing something right cuz he is almost 21 and still kicking and sometimes more vocal than we would like him to be but he is doing so good, I can’t believe he is still alive. Dr. Ahmed says Starky is a miracle cat. He loves Starky, Starky loves him and I told the DR that Starky liked him more than us but he told us he couldn’t afford Starky……..who knew he would go through our whole life savings? lol …..j/k. it’s already cost almost $3,000 the last 2 years but its ok, cuz he is worth it. Anyway, gotta go cuz he is holloring for me now, and is purring and howling at the same time….lol

  24. Dr. Michael Dym, VMD veterinarianMay 1, 2014 at 9:49 pm

    Hi Sandra. You are very welcome

  25. Hello, my 13-year-old kitty had what the vet later determined was a blood clot when his back legs momentarity collapsed. I took him in to the vet and he was diagnosed with hyperthyroid. He is now on the felimazole ear gel twice a day. I have a few questions: 1) If his thyroid problem is stabilized, will that prevent a future blood clot episode? 2) How long does it take the transdermal felimazole to work (he’s now been on it a week). 3) I’ve read conflicting advice about the necessity of cleaning the cat’s ears so the medicine will work. Thank you for any information.

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