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What is Methimazole Allergy?

Because of the seriousness of the disease (hyperthyroidism), the side effects of the drug (methimazole) have to be weighed against the benefits. Sometimes the side effects will dissipate within a few weeks or can be treated with a different medication to lessen the effects. However, if your cat has any serious side effects that have to do with the liver such as yellow gums, skin, and eyes, vomiting, lethargy, skin lesions, and swollen lymph nodes an evaluation of the use of this medication must be done. It is important that your veterinarian do a blood test to determine the lowest amount of methimazole possible before starting the medication. In addition, blood tests to keep track of liver and kidney enzymes, thyroid hormones, and other abnormalities should be done on a regular basis.

Methimazole is a drug prescribed to treat hyperthyroidism in cats that subdues the processing of thyroid hormones. It is not specifically regulated for animal use but has been accepted for veterinary use by the FDA. This drug should only be used for a short time in cats because of the risk of serious allergy symptoms such as swelling of the face, scratching, and hives, which can lead to seizures, shock, and even coma. Other complications include appetite loss, vomiting, weakness, and liver damage. Some of the less common side effects include anemia, lymph node swelling, abnormal bleeding, and jaundice. If your cat has any of these symptoms when taking methimazole, you need to see your veterinarian right away.

Symptoms of Methimazole Allergy in Cats

Since some of these symptoms (vomiting, weight loss, weakness) are also signs of hyperthyroidism, you may not notice them right away. The most common signs you should watch for include:

  • Vomiting or regurgitating undigested food
  • No appetite (refusing to eat)
  • Rash
  • Swelling of the face
  • Excessive scratching (especially head, neck, and face)
  • Bleeding disorders
  • Cold limbs
  • Shock
  • Seizures
  • Liver failure (yellowing of the skin, eyes, and gums, vomiting, lesions anywhere on the body)
  • Coma
  • Death

Types

Methimazole is a prescription drug made for humans that reduces the amount of thyroid hormones (T3 and T4) your cat’s body produces. It has been accepted for use in cats with hyperthyroidism. This drug also goes by other names such as:

  • Felimazole
  • Tapazole

Causes of Methimazole Allergy in Cats

The cause of methimazole allergy is the consumption of methimazole. In some cases, the body attacks the drug and this causes histamines to be released, which creates the symptoms such as vomiting, diarrhea, swelling of the face, scratching, hives, shock, and even coma.

Diagnosis of Methimazole Allergy in Cats

The allergy symptoms of methimazole are usually pretty straightforward, but the veterinarian will probably need to rule out other issues before making a diagnosis. Because the drug has many side effects besides the allergy symptoms, the veterinarian may decide to stop the drug immediately anyway. However, a comprehensive physical examination, blood tests, urinalysis, and radiographs will all be needed to ensure there are no underlying conditions besides the hyperthyroidism.

Treatment of Methimazole Allergy in Cats

The treatment for methimazole allergy is the immediate stoppage of the drug. Because hyperthyroidism is a life-threatening condition, other treatment will need to be decided on right away. There are two other choices that veterinarians use for hyperthyroidism, which are radioiodine or surgery.

Radioiodine (I-131)

Radioiodine is a one-time injection that is effective and safe. Because it is radiation, your cat will have to stay in the hospital for several days (by law), but there are no other medications or injections required. It does not damage any other organs or tissues, will not damage the healthy parts of the thyroid glands, and has no serious side effects like the other drugs. The only drawback is the cost, which averages about $1,000, depending on where it is done. Unfortunately, some cats are unable to get the medication because they are not healthy enough. Radioiodine treatment is only able to be done at certain hospitals that have permits for radioisotope use.

Surgery (Thyroidectomy)

Removal of the thyroid glands is a procedure that is usually only done on cats that are relatively healthy (besides the hyperthyroidism) and under 15 years old. Most often, both thyroid glands are removed, but if the veterinarian believes that one is healthy, it will be left to do its job. It is an effective surgery that is done often, but the parathyroid glands are sometimes wrapped around the thyroid glands, making it a difficult process.

Recovery of Methimazole Allergy in Cats

Hyperthyroidism is a fatal condition that can cause organ failure and death quickly without treatment. If your cat cannot be treated with methimazole, radioiodine will be used, if possible. However, surgery may be the best choice if your cat has allergies.

Methimazole Allergy Questions and Advice from Veterinary Professionals

Yanke
Domestic short
12 Years
Serious condition
1 found helpful
Serious condition

My cat had an allergic reaction to the thyroid meds (itchy, facial swelling, lethargy) so we took her off the meds. It's been 1 week and she's doing a lot of vomiting and is still very lethargic. How long would it usually take before they are back to themselves?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
Any adverse reactions from medications should be handled by your Veterinarian and they should be informed as they may wish to pass the information back to the manufacturer as part of pharmacovigilance and to adjust treatment. Overall it depends on the how long Yanke has been on methimazole, however you should visit your Veterinarian regardless. Regards Dr Callum Turner DVM

My cat, age 10, had 2 doses of methimazole, and vet recommended I stop because my cat stopped eating and was clearly worse off. She had been zooming around the house and usually woke me to get her breakfast. She didn't come down for breakfast or treats. That was 3 days ago, and she not only has not recovered, she seems quite a bit worse off. Barely drinking, vomiting what she eats, very lethargic. I am heartbroken. It seems this drug has brought her to death's door.

My kitty past away 2 days ago from allergic reaction to this drug. My heart is broken and I feel I can never trust a vet again to make such decisions. Beware of this drug.

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Courage
Domestic shorthair
10 Years
Mild condition
1 found helpful
Mild condition

Has Symptoms

Face swelling

Medication Used

Methimazole

My cat Courage has been taking methimazole for seven months. He has gained weight and seems much better. However, his face swells every few days for about twenty minutes and he has to keep his mouth open to breath. I have read that this can be a symptom of an allergy to methimazole, but he had this "face swelling" before he ever took methimazole. Can it be a symptom of hyperthyroidism? I have kept a food diary to see if there is a cause that I can find, but it seems entirely without a pattern. In fact, the swelling seems to disappear quicker if I give him food. I bought him an air purifier and I run it day and night, but it doesn't make a difference. He gets the swelling in the winter when the windows are closed and in the summer when the windows are open (we live in Minnesota where the winters are very cold and snowy and the summers are very hot and humid) I was wondering if it is an allergy and might respond to Zyrtec. Is that safe for cats with hyperthyroidism? Or should I ask my vet for hydroxyzine? Is there any allergy medication that is preferred?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
If the swelling occurred before the start of methimazole, it is not going to be the cause; there are many possible causes for facial swelling and I don’t think it is related to the hyperthyroidism or methimazole; I cannot be sure it is an allergy due to the quick onset and resolution of the swelling in a short time frame without any treatment. You should speak with your Veterinarian to see if there is a specific cause which may be identified. Regards Dr Callum Turner DVM

Thank you! Courage is going to the vet soon for a CBC and a new prescription of methimazole.

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Tabi
Domestic short haired tabby
14 Years
Serious condition
1 found helpful
Serious condition

Has Symptoms

vomiting and diarehha

My cat started Methimazole early December, her thyroid went too low and they decreased the dose. Still showing even lower at her next blood test the vet took her off for a month, now thyroid is high again. Now trying the food but my cat is refusing to eat it, and when she does its very small amounts. The vet said if she wont eat the food (shes really fussy) they'd try an even lower amount of the methimazole. My concern is the fact she was scratching her head above her eyes and there was bleeding and scabbing, is this something that will likely return with any dose as its an allergy? Or it may not happen due to the lower dose and it was just a matter of too much methimazole?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
Treating hormonal conditions like this are difficult as the problem is never the same in each animal and you need to balance the medication to hit within the reference range (or as near as you can to it); it is possible that there was some scratching as a side effect of the treatment but I cannot say if it will return with a lower dose or not, it may not be related to treatment. Regards Dr Callum Turner DVM

Thank you for the reply. The scratching was from the medication for sure as it has been the only time she has ever done this. Within one week of taking her off she stopped and it healed up. My concern was that it may be an allergic reaction in which case I'd be worried if she had to go back on, even at the much lower dose.

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Bella
Lynx Point Siamese Mix
16 Years
Critical condition
1 found helpful
Critical condition

Has Symptoms

excessive urination
Crying
excessive hunger
Sneezing
refusal to eat medicated food
excessive energy
aggressive at times
excessive bad behavior
spinning

Hi Dr.'s,

I'm not sure what to do. My cat has hyperthyroid and is extremely allergic to meds (eyes bleeding with yellow pus, severe vomiting all day, scratching to bleeding in ears and eyes, hair falling out in clumps and weird bleeding sores on paws). She also has stage 3 kidney disease (beginning at 2.9), and my Vet says she will not be able to do the iodine treatment. After 5 days of Hills y/d canned food, she refuses to eat at all. Unless it's her usual Nature's Variety Raw canned foods- I only feed her the poultry options (turkey, duck, chicken), and occassionally venison, rabbit or lamb. She is howling often now in the afternoons and evening and wakes me up constantly every evening. She has excessive urine and thirst. I am supplementing Mary Ruth's organic liquid cat probiotics, Pet WellBeing Kidney Support Gold, Aminavast for cats (1x per day, she cannot tolerate the twice per day recommendation), plus one drop of ionic liquid zinc per day in her water. All this seemed to help for about 3 weeks, but now she is exhibiting bad behavior - clawing into leather purses, exercise equipment- all for attention to get her salmon flavor greenies (I stopped feeding her any fish meals) all day long- she could eat them as meals. Is there any help for Bella? She is a just turned 16yr old Lynx Point Siamese Mix. I don't know what to do and I don't want her to suffer. I feel I have no more options if she can't withstand the iodine, surgery and is severely allergic to medicine. She seems to be going a bit crazy spinning around sometimes, she has beginning dementia (dx by vet) as she sometimes stares at a corner for long periods, and running all over and crying. When she is her "old self" she's playful with her toys, naps and purrs- these times seem to be first thing in the morning till about noon. After that starts the onslaught of crying and chaos intermittently throughout the day until evening. I would be heartbroken to put her down, but if she's in pain and suffering, and I can't tell, I would be sad as well. I don't want her to suffer. I just don't know what to do anymore. I read a study where the homeopathic tx Nature muriaticum 200 ck helped the hyperthyroid in some cats, but I've been afraid to give her the one pellet dosage as it did make some cats worse. Thank you so much for any help you can offer.

Dr. Michele King, DVM
Dr. Michele King, DVM
1409 Recommendations
I'm sorry that Bella is having so many problems - kidney and thyroid disease can be a very tricky combination to manage. I am unable to comment on whether she may be suffering, but if she won't cooperate with the Y/D food, she needs to eat. There are different formulations of methimazole that she may tolerate better, such as the topical formulation. It may be worth having a consultation with an internal medicine specialist to get their opinion on her situation. I hope that you are able to find a solution to keep her comfortable.

Thank you Dr. Michelle... I appreciate your time! Unfortunately, my veterinarian said Bella would not tolerate the topical either. I'll check on a consultation with an internal medicine specialist for a second opinion. Thank you so much again!

She also gets a 1/2 the dosage of liquid CoQ10 for cats.

Also, sorry, I forgot to add.. she weighs 5lbs now, down from 5lbs 4 oz. Thank you.

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Miss t
Russian Blue
12 Years
Mild condition
0 found helpful
Mild condition

Has Symptoms

Losing weight,

My 12 yr. old cat was diagnosed a month ago with thyroid disease. After being on Methimazole for that time, her levels were retested. Her Alt and Alt which had been normal were thru the roof so we stopped the meds
Vet said she looked like a good candidate for iodine therapy. Other than the cost, would her reaction to the Methimazole be a contraindication to the procedure. Oh yes, her thyroid levels had come down while on the gel

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
Radioiodine is a treatment option for hyperthyroidism in cats and is generally considered to be safer than other options, if you are considering using radioiodine you should withdraw Miss T from methimazole therapy for around five days before switching to radioiodine; you should discuss this with your Veterinarian. Regards Dr Callum Turner DVM http://veterinarynews.dvm360.com/hyperthyroidism-cats-can-live-normal-life

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Gosha
tabby
13 Years
Moderate condition
0 found helpful
Moderate condition

Medication Used

Methimazole

My cat has been on tapazole for about 5 years now with no allergy symptoms about three weeks ago he started being really hyper and scratching a lot! He doesn't go outside he is an indoor cat recently had a health check up could his new dry food or tapazole causing this? Thanks

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
Whilst it is possible that the medication is causing this response as cats may develop allergies to medicines and other substances overtime; if there is a correlation of the symptoms presenting and the changing of the food I would look more into that. Try to change the food back to the old food to see if there is a decrease in symptoms, otherwise try a hypoallergenic diet to see if that helps; if the issue continues visit your Veterinarian. Regards Dr Callum Turner DVM

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Apollo
dsh
11 Years
Moderate condition
0 found helpful
Moderate condition

Has Symptoms

Vomiting

My 11yo DSH neutered male cat was diagnosed with hyperthyroidism. We began a Methimazole pill regimen (2.5 mg twice a day). After four days, he began vomiting regularly. The vomiting was not concurrent with the pilling or with feedings. Generally, the vomiting occurred between 4-6 hours after his pill.

He also became jaundiced, then stopped eating completely. The Methimazole pills were discontinued immediately. An exam revealed probable pancreatitis (not fatty liver).

After he recovered, we began transdermal Methimazole, in roughly the same 2.5mg concentration, applied twice a day (alternating ears with each application). After 9 doses, his vomiting resumed. As before, it occurred 4-6 hours after the Methimazole application.

He is now on Hills y/d food, but has shown a lack of interest in the food (dry kibble) at times. He has not stopped eating, but it's clear he does not like the y/d food.

I am looking for other treatment options.

Dr. Michele King, DVM
Dr. Michele King, DVM
1409 Recommendations
Thank you for your email. Apollo may benefit from radioactive iodine treatment - that is one option for treating hyperthyroidism in cats, but it is only beneficial if he has adequate kidney function. Your veterinarian can discuss this option with you, whether it is possible, and where it may be performed, as only specialty hospitals tend to administer the treatment.

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Titty Baby
tabby
12 Years
Moderate condition
1 found helpful
Moderate condition

Has Symptoms

Keeps her ears down and sleepy
Keeps her ears down and seems sleep

Medication Used

Methimazole

My cat started methimazole about 3 weeks are and yesterday she started keeping her ears down and she seems sleepy all of the time. Are these serious side effects from methimazole? Should I discontinue her methimazole?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
You should visit your Veterinarian for an examination since the amount of methimazole administered may need to be adjusted or an alternative treatment should be considered (radioiodine for example); side effects are usually vomiting, loss of appetite, facial excoriation, thrombocytopenia, leukopenia, increased liver enzymes, lethargy among other side effects. Regards Dr Callum Turner DVM

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Amy
American Shorthair
13 Years
Mild condition
0 found helpful
Mild condition

Has Symptoms

Twtiching, itching
Twtiching, itchin

My 13 year old cat for the past three plus months seems to get these “fits” where she “hits and wacks” her ears and then sometimes her body completely twitches. She could be sleeping and then all of a sudden start hitting her ear and squirm and shake about. This can last from a few minutes to an entire evening. I brought this up to our vet to see if it is something neurological or an allergy.

Now for the past year my Little Amy takes daily Methimazole in Lipoderm 5 mg./0.05ml T-A-D Trnsdrm gel 1.5ml for her thyroid. It is applied to her inner ear flap, alternating ears each day and she has been on this medication now for one year. She also gets her ears cleaned 2X a week to prevent buildup. Could it be the meds causing the twitching? I don’t want this to escalate into something I can’t reverse or control. First, I don’t even know what this is and what is causing it. So should I see a specialist or return to my regular vet and ask her to check further into it based on what you might think this is.

email: [email protected]

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2996 Recommendations
Facial excoriation is a noted side effect of methimazole therapy, facial excoriation is the repeated hitting of the face to the point of self trauma; the twitching of the muscles may be related to low blood calcium, this is something you should discuss with your Veterinarian after blood tests and an examination. Regards Dr Callum Turner DVM

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