PetMeds® Hyperthyroidism in Cats

The most common hormonal disorder seen in cats today is an overactive thyroid gland known as hyperthyroidism. While it was only recently characterized and identified as an illness in cats in 1979 at the Animal Medical Center in New York City, the incidence of this disorder has expanded tremendously over the past three decades. While it is most commonly a disease of cats over the age of ten, we are now identifying this disorder in younger and younger cats due to more accurate testing, including measurement of both a T4 and Free T4 thyroid levels.

Environmental factors are believed to be the major cause for benign tumors associated with hyperthyroidism in cats The most common cause of this hormonal imbalance is due to a benign but functional tumor of the thyroid glands that leads to an overproduction of thyroid hormone. The debate in the veterinary community rests on what causes this benign tumor to develop.  The largest consensus rests on environmental factors, including potentially contaminant metals in certain screw top canned cat foods, to environmental toxins in carpets, as well as over vaccination where immunity to most viruses lasts for years to life of the pet.

As with most chronic disorders in our pets, it is believed to be an autoimmune reaction, where the immune system reacts against its own tissues leading to the development of this functioning tumor of the thyroid glands. Symptoms of this disease include increased thirst/urination and appetite, however, with concomitant weight loss at the same time. Also, greasy or matted coats including excessive shedding can develop as well.  Behavioral changes include easy overheating and panting, restlessness, vocalization, aggression and even changes in litter box habits resulting in feline house soiling.

If left undiagnosed, high blood pressure, subsequent kidney and heart damage, and secondary disease of the heart muscle called cardiomyopathy can develop as well.   Treatment options include the prescription drugs Tapazole or generic Methimazole. 15 to 20% of cats may develop side effects on these drugs including lethargy, loss of appetite and vomiting. A lesser percentage can develop problems of the bone marrow, liver failure and even less common intense itching of the head and face.  In roughly 20% of cats treated with Methimazole, elevation of kidney enzymes and possibly future renal failure can be seen. That’s why it’s so important for cats treated with medications for this condition to be adequately monitored through periodic exams, blood testing and blood pressure measurements. And while surgical removal of the thyroid glands used to be a popular treatment option, it has for the most part fallen out of vogue because of the close proximity of the parathyroid glands, as well as the surgical risk in many older cats.

The preferred method of treatment by many feline experts includes the injection of a single bolus of radioactive iodine, which selectively destroys only the functioning tumor of the thyroid glands. Most cats tolerate this treatment quite well, with the only down sides in my opinion, being the initial expense of $1200 to $1500 dollars for the workup before the treatment, as well as the cat being isolated for at least a few days away from the guardian during the treatment period, in order to avoid radioactive exposure of the human guardian family. As with all other hormonal diseases, I always recommend good vitamin supplementation,  including Soft Vitachews for Cats, Vetri-DMG, fatty acids such as Be Well Cat or Nordic Naturals Pet Cod Liver Oil, as well as liver support such as Denamarin or Denosyl, in order to protect the liver against the toxic effects of long term drug therapies.

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  1. Starky seems to be doing okay, so I am going to the vet today to get a prescription for his methimazole since he did have a refill but when they called 1800petmeds, they said they had no info on Starky or this medicine. Seems strange to me since they just sent me a reminder that he is due for a refill. Kinda makes me think they just wanted me to get it from them. I remember Ryan telling me that its the same as the meds we take so I looked it up at the Wallgreens Pharmacy, and found out it was only $29.00 instead of $72.00, and I don’t have to wait for it. Thanks Ryan for the info.

  2. I’m so sad & heartbroken to report that I lost my very best friend Rascal on Monday October 8th at 2pm. I found out a couple months ago that he had early kidney disease. He seemed to be getting better, he gained a pound or two back & I was giving him subcutaneous fluids twice a week, which he hated but knew it was needed. I was with him when he passed away, & he didn’t suffer. He fell over with a spasm & I grabbed him so fast as I was 2 Feet away. I attempted CPR & mouth to mouth & rushed him to his vet but there wasn’t anything they could do. He’d saved my life & took such great care of me when I was sick & in pain. My life will never be the same. I’m truly lost after more than 16 wonderful years with my precious cat. Part of me died with Rascal on Monday & my heart will Never not be broken, I have no. Doubt!! He was an amazing cat & showed me more love than any human even considered. He will forever bee missed!!! Good luck to all of you, & thanks for all of the information & support!

    • Dr. Michael Dym, VMD veterinarianOctober 11, 2012 at 7:20 pm · Reply

      I am sorry to hear of your loss. My heart and thoughts are with you during this difficult time.

  3. Ryan, words cannot convey how truly sorry I am to hear about Rascal dying. I know how close you 2 were and it really saddens me to hear of this. Please know that I am thinking about you and I know how difficult this is for you.

  4. is it ok to mix the standard process thytrophin and simplex F with Methmizole? I have 2 cats on methmizole 2x/day. Both are older and have lost weight. My holistic vet prescribed me methmizole saying that is the only thing we can do for hyperthyroid. I am not sure why she didnt have me try the thytrophin and simplex F. Please help.

    • Dr. Michael Dym, VMD veterinarianJuly 8, 2014 at 9:38 pm · Reply

      I have used standard process products in practice, but thus far have not been that impressed with them in treating or reversing hyperthyroidism in cats by themselves.

  5. My cat is 19 years old, taking methimazole twice a day for hyperthyroidism.
    He eats Iams dry food (free feeds) & gets wet food twice a day with his medication.
    He is becoming increasingly constipated.
    Will Metamucil help or should I change his diet, giving him more wet food daily?
    Any suggestions will be greatly appreciated.
    Thank you.

    • Dr. Michael Dym, VMD veterinarianJuly 18, 2014 at 4:20 am · Reply

      I would definitely give more wet food, as well as possibly adding one pinch of metamucil to each meal. On occasion a pinch of Miralax can help as well.

  6. Hello all!
    I have a 15 yr/old Bengal, diagnosed with hyperthyroidism last year. He has been on the Hill’s Prescription Diet for approximately onye year and has saved his life (Y/D formula). But, he is having problems pooping, getting constipated, more times than not. Can I use Vetasyl with this food, even though he is just supposed to eat the Hill’s Diet and nothing else? Please help!

    • Dr. Michael Dym, VMD veterinarianAugust 6, 2014 at 2:19 pm · Reply

      I dont think using vetasyl will be a problem. You can also consider a pinch of miralax in with each meal. See vet for recommendations.

  7. Starky decided he no longer wanted to live, he couldn’t live like this anymore. I know this because for the last 3 days before yesterday, he just stayed curled up in our bathroom so he could be close to us. Before we always took him to the vet when he stopped eating. This time it was different and we knew it. When I called Dr. Ahmed to tell him Starky was tired of living and was ready to go, he had already left the office early for lunch but when he got the call about Starky, he came back and he was there when we got there. He told us about our options and since Starky had lost so much weight by this time he weighed only 4.5 lbs. He gave Starky a strong sedative, then a lethal injection. it was so heartbreaking, but the Dr stayed with us the whole time and explained everything that was going on and we told him Starky came to say goodbye. He was always amazed at how strong his will was to survive and how much he always loved Starky. He wrapped him in paper bag and took him to our car so we could take him home to bury him. It was so hard to bury him even though we had a spot picked out for him when we made a rock garden 4 years ago when his friend died. It was so hard to dig with so many rocks and the DR had told me earlier that day I could no longer kneel to do anything in the yard. I am sharing this story because I have been writing about Starky since he first got hyperthyroidism, and I kept thinking I should have taken him in sooner and maybe he would have had a better chance but tha’ts not really the case because he had a good 21 years and he was ready to say good bye. We will miss him very much. Dr. Ahmed is the best animal Dr we have ever known. He always went to great lengths to save Starky, and was always there for him. I have heard other ppl telling stories about what a great DR he is but I have experienced this first hand, and I know for sure if I traveled the whole world I would never find a DR who cared more for his patients than DR. Ahmed. I am putting this on facebook on his page at Hesperia Animal Hospital. Thank you DR. Dym for all your help and support. It really helped a lot and meant a lot to me. I will forever be grateful to everyone connected with Starky’s life.

    • Dr. Michael Dym, VMD veterinarianAugust 6, 2014 at 2:11 pm · Reply

      HI Sandra. I am so sorry to hear of your loss. Dr Ahmed does indeed sound like a special veterinarian. Please know that STarky would not have lived for as long or as comfortably without your love and devotion, as well as Dr Ahmed. 21 years is a very long and happy life for most cats

  8. Thank you Dr. Dym for you comments.
    We were really surprised when we received a sympathy card in the mail yesterday from Dr. Ahmed and Hesperia Animal Hospital telling us how much they all loved Starky and how much they will miss him. They were voted best animal hospital in the desert, and I can really see why.

  9. My 14 y/o female(n) cat has hyperthyroidism and is on Methimazole. As expected she has lost weight and is hungry. Her biggest problem is diarrhea. What can be done to help control this? She is a fussy eater so changing her food is probably not an option.

    • Dr. Michael Dym, VMD veterinarianSeptember 24, 2014 at 10:55 am · Reply

      Make sure thyroid levels are under control with proper dose of methimazole. As for diarrhea, vet exam and evaluation to rule out chronic pancreatitis, IBD, etc is needed. You could try probiotic like fortiflora from 1800petmeds and naturevet enzymes added to meals

  10. Thanks for the info. My vet currently has her on Prednizone which has helped some. I hate having to give that to her. She loves Fortiflora so I will order some more of that. Again, thank you.

  11. Hello,
    I have a 17 year old cat diagnosed with hyperthyroidism. He’s been on methimazole for 5 days. Prior to this diagnosis, two months before I took her to one get who ran bloodwork a everything seemed fine. She had been loosing weight but also had been itching, scratching, biting, grooming excessively and vomiting. All I got was antibiotics and Pepcid and suggested something wrong with kidneys but required more testing. She stopped vomiting but would still itch and lick and continued to lose weight. Three weeks ago she woke up with a bright red face. She is a black cat by the way. I immediately took her to another vet who gave her an allergy shot and antibiotic shot, and did some cytology test. Tested for bacteria, mites, parasites, what not, everything came back negative.
    She did not respond to allergy shot and antibiotic, her face and spots around her body were very red still. Took her back 3 days later and more skin tests were done, blood work and fungal growth were done. She was given prednisone. 3 days later I was told she had hyperthyroidism. Those three days she stopped eating but then started eating again and a lot. 5 days ago we started methimazole and continued prednisone. There’s still redness and still itching a lot. Still waiting for the fungal growth results. I feel so bad for cuz she still itching, some normal skin is starting to peek through tho. Is the skin reaction part of the hyperthyroidism? I just don’t understand why it will not go away, not sure what to think anymore

    • Dr. Michael Dym, VMD veterinarianDecember 25, 2014 at 9:49 am · Reply

      Sounds like very complex situation. I would ask your local vet to refer you to a veterinary specialist or internist who can put whole picture together. History and treatments sound very suspect to me. Sometimes hyperthyroid cats can have coat problems, shedding, dull greasy coats, etc but not usually red skin eruptions, etc like you describe. To make matters even more confusing, is that in a small percentage of cats treated with methimazole, can actually develop excessive itching and eruptions around their head and neck, so you want to be watchful for that. Another option, other than conventional medicine, is consulting with a homeopathic vet, who looks at the whole symptom picture as one disease or illness, rather than parts, like you are finding out is very frustrating here. To learn more about constitutional classical homeopathy, see the booklet on the website as well as my website Many homeopathic vets do offer phone consultations. Not a quick fix, and will take time and patience, but may be worth exploring for you.

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