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Tapazole not working.

Species: Cat
Breed: Domestic longhair
Age: 11-15 years
Miss Kitty who is spayed and about 13 yrs old (we rescued her) was recently diagnosed with hyperthyroidism and placed on methimazole. At her first blood test, her thyroid level had almost DOUBLED instead of decreasing. It started at 10 and was up to 18. My vet didn't know what to make of this so we doubled her dose to twice a day. She is now very lethargic, sleeping in very odd places (tucked among times on the kitchen counter) and she doesn't seem to be hearing us whe we call to her. She has always been very responsive to us. It doesn't seem like she is ignoring us. If she has her eyes closed she jumps when we touch her. When she is asleep, she is difficult to rouse, which is a little scary! Also, her balance has deteriorated; she seems wobbly and has fallen in places she formerly went with ease. She is still eating a lot of food, and when awake she is quite alert, and still loves to be held.
1). What could have caused the anomalous lab test result? 2) are hearing loss, lethargy and deteriorating balance possible side effect of the medication? One of my other kitties was also diagnosed hyperthyroid just a couple of weeks previously and even after increasing his dose, he doesn't seem to have any of these problems. Also, apart from gum diseas, she has always been very healthy and the most active and inquisitive of my four kitties.

Thanks for your response!


Online vet, Dr. Marie

Dr. Marie replied:

This is very unusual! I can't say that I have ever had a cat's thyroid level double after going on medication. In fact, I don't know if I have ever seen one even increase after starting medication.

It is possible to have a cat fail to respond to thyroid medication if the cause of their thyroid problems is something called a thyroid carcinoma. Most cats with hyperthyroidism have a benign thyroid tumor that responds really well to methimazole (tapazole). But, if they have a carcinoma this is much more serious. Thyroid carcinoma is really not common so we don't have a lot of information in regards to what all of the known symptoms are.

The other things I can think of would all be sort of human error issues. I'll mention a few and see if any of them seem to fit.

Is Miss Kitty on oral Tapazole (pills) or on a transdermal gel that goes in her ear? If she is on a transdermal gel then there are some cats that it just doesn't work for them. You may need to switch her to oral or you could consider radioiodide therapy (but that is expensive) or you could consider Hill's y/d.

Is the medication actually Tapazole, or is it made up by a pharmacy (i.e. made into chewable treats or liquid). If it is compounded it is certainly possible that there was a mistake when the first dose was made. If your vet suspects this then it may be worthwhile to go back to the original dosage but use actual Tapazole (not compounded).

Do you still have the first tablets that you were dispensed? It may be worthwhile to take them back to the vet to make sure that they actually were Tapazole. It does happen occasionally that there is human error. Perhaps the veterinary staff that counted out the pills picked out the wrong medication? It may be worthwhile to take the pills in and have the vet ensure that they actually are Tapazole.

The symptoms you are describing certainly could be due to an overdose of Tapazole. The next step here is likely to have the thyroid level checked again and I would likely want to be repeating other blood tests to see if there is anything else going on.

I really hope things are better soon!

Dr. Marie.


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Disclaimer: Although Dr. Marie is a qualified veterinarian, the information found on this site is not meant to replace the advice of your own veterinarian. AskAVetQuestion.com and Dr. Marie do not accept any responsibility for any loss, damage, injury, death, or disease which may arise from reliance on information contained on this site. Do not use information found on this site for diagnosing or treating your pet. Anything you read here is for information only.

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Dr. MarieDr. Marie is a veterinarian who practices in a busy animal hospital in Ottawa, Ontario. She created Ask A Vet Question as a resource for good, accurate veterinary advice online. Dr. Marie treats dogs, cats, hamsters, guinea pigs, and rats. She has been a vet since 1999.

Is an online vet visit just as good as a trip to your veterinarian? No! But, many times, asking an online veterinarian a question can help save you money. While Dr. Marie can't officially diagnose your pet or prescribe medications, she can often advise you on whether a vet visit is necessary. You can also ask Dr. Marie for a second opinion on your pet's condition.