Hyperthyroidism Treatments in Cats


Hyperthyroidism Treatments in Cats

- a veterinary site dedicated to the treatment of Hyperthyroidism


Continue with Tapazole or not

Question: Dr. Mike: My cat had a Free T4 of 86 and after two weeks on 1/2 m/twice daily of Tapazole (liquid), her Free T4 has come down to 37 (normal, in the lab's range of 10-50). Her kidney function is normal as well. My question to you is, should I proceed with I131 treatment, or should I continue with Tapazole for the rest of her life. The problem with the Tapazole is the need to give it to her twice a day, especially if I'm away for a day or so. Is there any danger in long-term use of Tapazole? Is there any danger in the I131 treatment in an 11-year-old cat? Thanks very much, Jeanne

Answer: Jeanne- I would recommend giving the methimazole (Tapazole Rx) for at least one month, regardless of what you want to do over the long run. This will give you some idea whether problems will develop with its use and it will also allow evaluation of kidney function, which is important when considering whether to use radioactive iodine therapy. Many cats tolerate methimazole long term with no problems and it works well to control the hyperthyroidism in these cats, as well. About 20% of cats can not tolerate methimazole for one reason or another. Most of these cats have gastrointestinal problems such as vomiting, due to the medication. A few cats will develop more serious signs, including damage to the kidneys or anemia induced by the methimazole. Usually, stopping the medication will allow recovery from these problems, as long as they are caught early. Due to this, we recommend drawing blood after one week of therapy and again after a month, to rule out these problems. As long as none of these side effects occur in a patient, methimazole seems to be tolerated long term pretty well. Cats can be hard to keep on medication long term, though. For this reason, and also due to a smaller number of side effects, radioactive iodine therapy is considered to be the first choice in treatment in cats for hyperthyroidism. It also has the advantage of controlling the small percentage of cases in which thyroid cancer is present better than methimazole. However, some cats who have kidney failure that is partially masked by the hyperthyroidism will develop acute kidney failure following radioactive iodine therapy. As long as everyone is aware of this possibility and watching closely for it, treatment is usually possible to reach a state of compensated kidney failure again. Using methimazole for a month or so prior to considering radioactive iodine therapy can help to reveal cats who may have a problem with radioactive iodine therapy. It is a little confusing, since it is unclear whether the problem with the kidneys is directly from the methimazole or due to treatment of the hyperthyroidism. It is sometimes necessary just to make an educated guess and then base treatment decisions on this best estimate of which problem is occurring. Whatever treatment decision is made, it will be obvious that monitoring the kidney function will be important and doing that and treating for problems found will usually allow control of the kidney problems for some time. If the cost of radioactive iodine therapy is not a problem for you and you wish to do what is best, I think that the radioactive iodine therapy is the best long term choice for most cats and I am pretty sure this is the opinion of most endocrinologists at this time. If the cost is prohibitive, surgery and long term methimazole therapy are both reasonable options, as long as it is possible to give the methimazole and there are no adverse effects from its use. For an eleven year old cat you are likely to be looking at more than five years of giving pills, though. I hope this is helpful. Mike Richards, DVM 9/14/2001

Hyperthyroidism treatment - Radioactive iodine therapy vs Tapazole

Question: My cat has been diagnosed with hyperthyroidism. I need to make an informed decision concerning her treatment. Her Free T4 is 84; last year it was 50 (borderline). The doctor wants to give her Tapazole; is radioactive iodine a better treatment in an otherwise healthy cat?

Answer: Jean- Radioactive iodine therapy is considered to be the best treatment for hypothyroidism. I don't think that there is much disagreement over this statement among internal medicine specialists and/or endocrinologists. There is a facility in Springfield that offers this treatment. You can find the information on their facility by going to www.radiocat.com It isn't necessarily a bad idea to start with methimazole (Tapazole Rx), though. This can help to identify cats who have compensated renal failure that may be dependent on the increased blood pressure associated with hyperthyroidism. This is the one thing that seems to cause problems in cats treated with radioactive iodine -- the presence of compensated kidney failure that may not be detected prior to treatment, without careful examination or possibly the short term use of methimazole. If your cat responds well to Tapazole and doesn't show any signs of kidney failure after a month or so, that would make the use of radioactive iodine even more likely to be the best long term option. Mike Richards, DVM 8/25/2001

Hyperthyroid treatment

Question: my siamese cat who is sixteen y/o is on tapazole. in humans we usually radiate the thyroid then give synthetic thyroid for the remainder of life! Is it very expensive to do this with a cat? I give her on the Vet instr. 3.5 mg. qd or about 3/4 of a 5 mg. tab a day. Still she seems tachycardic and i know that this is not good for her. What is a better solution? is it cheaper and easier to also give her a B blocker. please advise me on this!

Answer: James- The consensus of opinion among endocrinologists who treat hyperthyroidism in cats and write about it, is that the best options for treatment of hyperthyroidism, in order, are radioactive iodine therapy, surgical removal of the affected thyroid gland or glands and then long term medical treatment with methimazole (Tapazole Rx) or other agents, if necessary. The exception to this rule is a cat with pre-existing kidney failure. Some cats with this problem will experience a sudden worsening of the kidney failure after radioactive iodine therapy, presumably due to lowering of the heart rate and blood pressure more quickly than the kidneys can compensate for it. Many veterinary clients are reluctant to pursue radioactive iodine therapy due to the cost, which tends to run between $1000 and $1500, depending on where it is done. Surgery costs less but has higher risks due to the potential for damage of the parathyroid glands leading to low calcium levels post-operatively, which can be fatal. Long term medical therapy is considered to be acceptable but often does not completely control the problem and does require monitoring of the effectiveness and for side effects, on a fairly regular basis. When necessary, it is acceptable to treat the heart problems induced by hyperthyroidism as a separate entity, especially if it seems like they might really be independent of each other. Most of the time, though, it is better to concentrate on treating the hyperthyroidism and making sure it is under control, if possible, before resorting to additional medications to treat problems that are known to be related to the hyperthyroidism. There are a number of places in the U.S. where radioactive iodine therapy is available, so please let me know if you are having difficulty locating a facility that can help your cat. Mike Richards, DVM 3/23/2001

Methimazole (Tapazole Rx) prior to radioactive iodine therapy for hyperthyroidism

Question: Dear Dr.Mike, As a quick reference and a medical history for my cats, Rusty and Pipo: Both are from the same litter and 8 and half year old female tabby. They were neutered and are kept indoors and given vaccinations for leukemia and FVRCP annually. I have stopped given them rabies vaccination since 1997 since Pipo got allergic reaction for the shot. They both eat same dry food. But Rusty also likes to eat no salt canned tuna in water, canned sardine in olive oil, turkey, chicken and dried sea weed. Rusty occasionally coughs and has wheezy sounding breathing and vomits hair balls more often than Pipo. And she is more aggresive and dominatig. The vet took her upper respiratory x-ray last year and diagnosed her symptoms as allergy. She also gets one eye watery occasionally. (sorry that it became rather long than brief) So here is my concern for Rusty: She started coughing and vomiting once or twice a day about three weeks ago. Most of the time she seemed to vomit in late night like 2 or 3 am. At first I thought it was allergy again since it happened last year about the same time in spring. But her vomiting became more intensified like 2 or 3 times every time after she ate. She did not loose her appetite, but could not keep the food in her stomach. So I took her to vet and vet gave her 1 pt of fluid each in two days and took her lung and stomach x-rays and found no problems. So the vet drew her blood to test and also suggested to do beryllium dye x-ray to check her bowel. And meantime I gave her 5 mg Pepcid a day for three days as the vet suggested. And she seemed to be vomiting less. The vet found that her blood test showed that she has a hyper active thyroid which measured 8.8. So instead of checking further for her bowel, the vet gave me 5 mg Tapazole pills and suggested that I should consider alternative radioiodine treatment performed by Dr. Turrel in Pacifica, CA. I was very concerned about the side effect s by Tapazole but the vet encouraged me to start her on the pill. So she has been taking the pill for 5 days and so far she has not threw up and been acting pretty normal. She has lost a pound but does not looking like loosing more or getting sicker. Since radioiodine treatment seems to be more effective and less risky for her long term health than Tapazole pills, should I go ahead to have her hyper thyroid treated by radioactive iodine? Or is there any reason to stay on the Tapazole pills longer? Or could it be possible that her thyroid hormone was elevated temporarily by her diet like eating sea weed which contains some iodine? Should she be tested further more before the treatment? And what kind of test should be done? I will appreciate very much for your suggestions. I already learn a lot more about hyperthyroidism through your website. Thank you very much! Mari

Answer: Mari- Many vets start cats on methimazole (Tapazole Rx) prior to radioactive iodine therapy to try to get some stabilization in their condition as soon as possible and to ensure that they will respond to therapy and that it seems like the only problem, prior to the time they receive radioactive iodine therapy. There is some controversy over whether or not methimazole use has to be discontinued prior to the radioactive iodine therapy, but the standard recommendation has been to stop methimazole about five days prior to I 131 therapy. If your vet, or the vet who will administer the I 131 feels this isn't necessary there are definitely studies to support their position, too. I don't see any need for retesting unless you just want to be sure there wasn't a mix up in the lab samples, or a breakdown in testing procedures. The T4 level is high enough to be certain of the diagnosis, especially with the improvement in clinical signs. It isn't a bad idea to monitor kidney and liver damage and to check for anemia about a week after starting methimazole, though. You might want to ask your vet when he or she likes to do follow-up testing to make sure that there aren't adverse reactions to the Tapazole. I would treat a cat of my own with radioactive iodine therapy as I really think it is the best of the therapies for this condition so I would still recommend that option. Good luck with this. Most cats do very well after treatment for this condition, so hopefully Rusty will stick with the crowd and do the same. Mike Richards, DVM 4/29/2000

Hormone replacement - l-thyroxine

Q: my 16 yo cat was successfully treated for hyperthyroidism, and is now very slow and fat. Can small doses of thyroid replacement be used? A: Jud- Some cats do require thyroid hormone replacement after treatment for hypothyroidism. Usually a low dose of l-thyroxine is used on a daily basis in these cases. Mike Richards, DVM Radioactive iodine therapy for hyperthyroidism Q: my cat was diagnosed with hyperthyroidism 2 years ago and has been taking tapazole with no problems except her dosage changes with every blood test. my vet also had me give her a high dose daily vitamin and potassium supplement. she said tapazole lowers potassium levels. i have moved to another state and we have a new vet. he says there is no need for the supplements and has taken her off of them. what is your opinion on this? he has also recommended the radioiodine treatment. i am very nervous about doing this but was encouraged when i read your comments. i am trying to gather all the info i can before i decide to have it done. i am worried that she may become hypothyroid, is that a possibility? she is 13 years old and has a very sensitive system, she has many allergies and does not handle things such as valium very well, it takes her several days to get back to normal when she has to be sedated. could any of these things cause problems with the radioiodine treatment? any info you can give me or other sources to look up would be appreciated. thank you. jan

A: Jan- You might want to check out the Pet Care Forum's Veterinary Hospital area. You might find information there on this problem, as well. I am not sure I'd worry a lot about the use of methimazole (Tapazole Rx) in a cat that has been on it for two years with no problems but since radioactive iodine therapy is considered to be the best therapy, I can understand the recommendation. I can not find any references to hypokalemia (low blood potassium) associated with the use of Tapazole and have not supplemented it when using Tapazole. I am not sure why this recommendation was made. It is possible your first vet knows something I don't. Hypothyroidism is unusual following radioactive iodine therapy. It can occur but I think it is seen very infrequently. Once in a while it takes two doses of radioactive iodine to work but that is an infrequent occurrence as well. Mike Richards, DVM

Surgery for hyperthyroid

Q: Dear Doctor - thank you for being online. We have a 15 year old male cat who lives indoors, travels with us, and, in short is our daughter's little brother. Last October, as we were preparing to move from Portland, OR to Washington, DC, we took the cat to have his shots updated. The cat is what everyone terms "fractious" so we spend as little time in veterinary offices as possible. However, he had been sick once before in Oregon, and blood work had been done. At the time, the vet there was suggesting that we were "unfit" pet owners if we didn't immediately fly our cat to some clinic in Eugene where, for the sum of $1,000+, they would do some surgery that would save his life. To tell you the truth, at the time, we didn't have health insurance for ourselves. And though I love my cat and would do anything for him, we simply didn't have the money for any one of us to get sick or go to the doctor, let alone deal with a flight and hundreds of dollars of vet bills. So, we left the office stunned, took him home and committed to feed and take care of him as best we could until he either got well or left us. It was a horrible hurtful choice at the time - one of the reasons we left Oregon was because I found a decent job in Washington that included health benefits for my family and myself. We decided that once here and on our feet, we would look into what could be done for our feline friend. Financially, we are still pretty strapped but things are getting better, and I do have a good steady job that I could commit to paying for the necessary fees. However, we are here and new to Washington and I don't know who to contact about the surgery. The doctor in Oregon suggested that Tufts Veterinary School was excellent, but that's in Boston. Could you please give me some insight into where I might find a good doctor that could do this surgery here in the Washington area? I would go Monday morning and see if I could make some payment arrangements if only I knew where to go. Again, I thank you for being online. And look anxiously for your reply. J.

A: If you wish to pursue surgery as the treatment option, Dr. Bradley in Manassas is who I chose to operate on my cat years ago before radioactive iodine therapy became available. I have the information on that treatment in the Washington DC area at my office. I will try to find it and send it to you tomorrow. It is safer and only slightly more costly -- perhaps $50 to $100 compared to a surgical specialist. This surgery is becoming more widely available and it may be possible to find a general practitioner who does in it your area as well. As long as they are experienced it can be significantly cheaper without a great deal of increase in risk. A veterinarian near us does this surgery for $350 and has done well on all the cases we have sent. We're only 3.5 hours from DC -- it could be worth the trip if you can't find someone up there! Mike Richards, DVM

Hyperthyroidism treatment

Q: Dear Dr. Mike, My cat has just been diagnosed with hyperthyroidism. He is 18 years old, and has had kidney disease for about 6 years. I am unsure about what the best course of treatment for him. He gets very hyper when he has to go to the vet, so I am looking for the least intrusive option for him. P.

A: The best treatment for hyperthyroidism is radioactive iodine therapy. Unfortunately, this is not available everywhere and it does involve two weeks of hospitalization. So far, even cats we thought might be too shy or nervous for this treatment have adapted well to the hospital stay and done fine. We have to send people on a four hour trip to have this treatment so we usually end up treating in some other manner, though. Surgery is probably the next best option but it is frightening to consider in older cats for many owners. There are risks involved in surgical removal of the thyroid gland due to its proximity to the para-thyroid glands which are responsible for calcium regulation. Most cats, even older ones, do well with surgery, though. The last treatment option is long term medication with methimazole (Tapazole Rx). This works for many cats and avoids the complications of surgery but there are also a number of cats which either do not respond well to this medication or who react to it adversely. It is important to monitor kidney function in a cat with a previous history of kidney disease when using this medication and it also can cause bone marrow suppression, so any sign of illness needs to be reported immediately to your vet when using methimazole. I know that this information doesn't make the choice easier for you. I would take my cat quite a distance for radioactive iodine therapy if I was facing this choice, if that helps any. Mike Richards, DVM

Tapazole for hyperthyroidism

Q: Hello Dr. Mike Our cat is Blue Persian, 7-10 yrs., Female. She has been diagnosed with Hyperthyroidism and we have been giving her medication for about 10 days. She also has an ear infection as well. Her symptoms are: Pacing around the room, she is very vocal (a yowling kind), nervousness, vomiting (not always but once a night), she also has trouble maintaining her balance (similar to drunkenness and topples over when her attention is taken away from staying in balance). We started giving her 5MG of Tapazole and for about 4 days she was noticeably better. However the past week now our Doctor here has increased the dosage to 5MG twice per day. Also Dexatethasone 2MG/ML three drops per ear twice per day. We have noticed no allergy to the pills. She is eating and using the litterbox normally. 5MG tapazole used for about 10 days. She has started the 10MG Tapazole about 4 days ago. No improvement yet ARE WE DOING THE RIGHT THING? Can you recommend other avenues to test? Or do these symptoms match with other disease? Your comments would be greatly appreciated!!!!!! Thank you very much! Scott

A: Scott- It sometimes takes awhile to see major changes when using Tapazole to treat for hyperthyroidism. Some cats require as much as 15mg twice a day of the medication. We have not exceeded that dose but have had two cats that did not respond at that dosage level to the medication. The other alternatives are surgical removal of the affected thyroid gland and radioactive iodine therapy. At this point, it is probably best to be patient and to let your vet recheck your cat weekly. Sometimes symptoms such as increased heart rate will resolve before there are noticeable behavioral changes. Your vet will be able to assess those sorts of changes. It is also possible to recheck the T4 levels in the bloodstream to be sure they are dropping. Mike Richards, DVM Last edited 01/30/05


Michael Richards, D.V.M. co-owns a small animal general veterinary practice in rural tidewater Virginia. Dr. Richards graduated from Iowa State University's College of Veterinary Medicine in 1979, and has been in private practice ever since. Dr. Richards has been the director of the PetCare Forum...