Thursday, August 9, 2012

How to Dose and Monitor Hyperthyroid Cats on Methimazole


Antithyroid drugs, including methimazole and carbimazole, are medications that treat an overactive thyroid (hyperthyroidism) by blocking the thyroid gland's ability to make thyroid hormone. Carbimazole is a "pro-drug" since, after its absorption, it is converted to "active" methimazole in the body; therefore, these drugs can be considered equivalent (1).

Over the 25 years since we first reported the use of methimazole for treatment of cats with hyperthyroidism (2), I have had repeated questions from veterinarians and cat owners about the use of once or twice-daily dosing regimes. In addition, I've had many questions regarding the best time to monitor a hyperthyroid cat's serum thyroid concentrations during methimazole treatment.

The purpose of this post is to review how these drugs actually work to lower serum T4 and T3 values. In addition, I'll provide an overview of my current protocols for dosing and monitoring hyperthyroid cats treated with these drugs.

How Do Antithyroid Drugs Work? What's the Mechanism of Action?
After administration, the thyroid gland takes up and concentrates methimazole from the circulation. Once within the thyroid, methimazole works by inhibiting the production of T4 and T3 from the hyperthyroid cat's thyroid tumor.

More specifically, methimazole blocks thyroid hormone synthesis by inhibiting thyroid peroxidase, an enzyme involved in the oxidation of iodide to iodine, incorporation of iodine into thyroglobulin, and coupling of tyrosine residues to form T4 and T3 (2-7). Methimazole does not block the release of preformed thyroid hormone, so there is generally a delay of 2 to 4 weeks  before serum T4 concentrations return to normal after initializing therapy.

It is important to remember that antithyroid drugs, such as methimazole, do not destroy the thyroid tumor, decrease it's size, or slow the progressive thyroid tumor growth that is characteristic for this disorder. Because the thyroid tumor continues to grow larger over time despite methimazole therapy, the need for higher doses should be anticipated during long term treatment in many cats (6-8).

How Long Does a Dose of Methimazole "Last" After Its Given? 
After oral administration, studies show that methimazole has a relatively short half-life in the circulation—only 2.3 hours and 4.7 hours in hyperthyroid and normal cats, respectively (9,10).  But we must remember that this drug does not work to lower T4 by remaining in the circulation. Rather, this antithyroid drug is taken up by the thyroid gland, the site where it acts to inhibit the production of thyroid hormones.  In accord with that, normal cats show sustained suppression of serum thyroid hormone concentrations for up to 24 hours after a single dose of methimazole (11), proving that this drug inhibits T4 production far beyond its known half-life in the circulation.

Unfortunately, once-daily dosing with methimazole will not be adequate in most cats with hyperthyroidism. In one study of 40 hyperthyroid cats (12), once-daily dosing was much less effective than use of twice-daily administration (the total daily dose given was the same in both groups of cats). In that study, only 54% of cats become euthyroid (i.e., normal thyroid state) after 2 weeks with once daily treatment, whereas 87% of cats became euthyroid with twice-daily administration (12).  Therefore, I strongly recommend twice daily dosing of methimazole to help ensure better control of the cat's hyperthyroid state.

If once-daily treatment is insufficient and twice-daily treatment causes compliance issues, a sustained carbimazole formulation (Vidalta, MSD Animal Healthcan be tried at 10 to 15 mg, once a daily (13). However, this drug is not approved for use in the USA and is currently only available in Europe.

For more information about carbimazole and Vidalta, see my last post on Antithyroid Drug Treatment for Hyperthyroidism: Brand Name, Generic, or Compounded Drug? 

Initial Methimazole Dose 
The initial methimazole doses used vary depending on the cat’s pretreatment serum T4 value and the size of the thyroid tumor. In general, however, most cats are started on 1.25 mg to 2.5 mg of methimazole, administered twice daily (4-7). This dose is adjusted according to T4 measurements (see below), as well as the cat's clinical response.

In cats that fail to respond and remain hyperthyroid, the daily doses are slowly titrated upwards to lower serum T4 concentrations into the desired range.  For more information about making dose adjustments, see my blog post on: Treating Cats with Hyperthyroidism: Antithyroid Drugs.

Best Time to Collect Blood (Post-Pill) for Monitoring Cats on Methimazole?
Although one might expect that protocols for monitoring cats on methimazole would be standardized among veterinarians, this is not the case.  Some have stated that the time of serum T4 sampling in relation to the administration of the antithyroid drug is not important, even in cats on once-daily methimazole (11).

However, I strongly disagree with that recommendation. We know that the suppressive effect of methimazole on thyroid secretion is highly variable among hyperthyroid cats, depending on the severity of their disease and size of their thyroid tumor. In addition, we also know that twice daily dosing is more effective and that once daily therapy is less likely to be successful in treatment (12).

The protocol that I use for my hyperthyroid cats is to have the methimazole dosed twice a day (morning and night) and then collect the post-treatment serum T4 sample sometime during the day. Ideally, it would make the most sense to me to take the sample around 6 hours post-pill (half-way between the two methimazole administration times), but the "exact" timing does not appear to  critical.

However, I would recommend that one be consistent in the timing of the post-pill test sample. In other words, do not have your cat tested once in the morning (an hour after the methimaole was given) and then compare those results to another testing time when the sample was collected late in the afternoon (11 hours after the methimazole was administered). In other words, we should try to monitor the post-methimazole serum thyroid hormone values at the same time each hospital visit.

What's the Ideal Serum T4 Concentration for Cats on Methimazole? 
It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values on methimazole. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L). Recent research indicates that hyperthyroidism may contribute to the development or progression of chronic renal disease in cats (14-16).  Leaving a hyperthyroid cat untreated (or poorly regulated with methimazole or carbimazole) may therefore be detrimental to long-term kidney function and is never recommended.

During long-term treatment, it is also important to avoid inducing hypothyroidism, which may be deleterious to the cat’s kidney function (25,26). If hypothyroidism is suspected, a complete thyroid panel is recommended, including determination of the serum concentrations of total T4, free T4, T3, and TSH (see my previous blog posts on diagnostic testing for more information about these tests). The findings of low serum free T4 with high TSH concentrations is diagnostic for iatrogenic hypothyroidism; in those cats, the daily dose of methimazole  should be decreased.

References:
  1. Peterson ME, Aucoin DP. Comparison of the disposition of carbimazole and methimazole in clinically normal cats. Research in Veterinary Science 1993;54:351–355. 
  2. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150-157. 
  3. Peterson ME: Hyperthyroidism, In: Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Fifth Edition). Philadelphia, WB Saunders Co. 2000; pp 1400-1419. 
  4. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22–28.
  5. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North American Small Animal Practice 2007;37:775-788.
  6. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110. 
  7. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012;571-592. 
  8. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
  9. Trepanier LA, Peterson ME, Aucoin DP. Pharmacokinetics of intravenous and oral methimazole following single- and multiple- dose administration in normal cats. Journal of Veterinary Pharmacology and Therapuetics 1991;14:367–373.
  10. Trepanier LA, Peterson ME. Pharmacokinetics of methimazole in normal cats and cats with hyperthyroidism. Research in Veterinary Science 1991;50:69–74.
  11. Rutland BE, Nachreiner RF, Kruger JM. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats. Journal of Veterinary Internal Medicine 2009;23:1025-1030. 
  12. Trepanier LA, Hoffman SB, Kroll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 2003;222:954–958.
  13. Frenais R, Rosenberg D, Burgaud S, et al. Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism. Journal of Small Animal Practice 2009;50:510-515. 
  14. Lapointe C, Bélanger MC, Dunn M, et al. N-acetyl-beta-D-glucosaminidase index as an early biomarker for chronic kidney disease in cats with hyperthyroidism.  Journal of Veterinary Internal Medicine 2008;22:1103-1110. 
  15. van Hoek I, Lefebvre HP, Peremans K, et al. Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine. Domestic Animal Endocrinology 2009;36:45-56.  
  16. van Hoek I, Meyer E, Duchateau L, et al. Retinol-binding protein in serum and urine of hyperthyroid cats before and after treatment with radioiodine. Journal of Veterinary Internal Medicine 2009;23:1031-1037. 
  17. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. Journal of Veterinary Internal Medicine 2010;24:863-869. 
  18. Williams T, Elliott J, Syme H. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine 2010;24:1086-1092. 

200 comments:

Michael Petty said...

Can you cut up (or crush) 2.5 mg pills of methimazole (Felimazole)? The manufacturer's data sheet advises against this. Presumably crushing will removed the sugar coating, so our cat will experience the full bitter taste of the medicine. But is there any other reason for this advice. Will the pills loose their effectiveness if cut/crushed?

Dr. Mark E. Peterson said...

See this post about the handling precautions:

http://animalendocrine.blogspot.com/2012/09/methimazole-handling-precautions-for.html

Michael Petty said...

Many thanks for this. Interesting comment from Mr Poston.

gocatgo said...

My 19 year old cat has been treated with Methimazole transdermal gel for 4 years. Her T4 is 1.2. She is now showing signs of CKD and vet wants to reduce the dose of Methimazole in order to bring her T4 higher. I am not understanding what this will accomplish. Can you explain this to me? Thank you.

Dr. Mark E. Peterson said...

We want to avoid overdosing the methimazole because that can lead to hypothyroidism, which also can cause the kidney function to worsen.

In general, I like to keep the serum T4 above 1.5 ug/dl in a cat treated with methimazole. However, to best monitor your cat, we would also monitor a complete thyroid hormone panel, which would include a total T4, T3, and TSH level. That would better define if hypothyroidism is really present or not,

J Dressel said...

Dr. Peterson
My 18 y/o male cat has been diagnosed with both eosinophilic granuloma and thrombocytpenia. He is on the mend, now as reported in his most recent blood test.1/28/14
After a free T-4, he is also hyperthyroid. Methmizole has been precribed..We don't want to backslide..doesn't Meth exacerbate these problems?..Thanks

Dr. Mark E. Peterson said...

The finding of a high free T4 alone in a cat with other problems of nonthryoidal illness is not diagnostic in itself. Up to 30% of euthyroid cats with illness can have a falsely high free T4 value.

Ask you vet if they can palpate the thyroid tumor. If they can't and the T4 was within the normal range, I would just recommend monitoring at this time.

tabbiecat005 said...

Dr Peterson -
My 13 yr old girl is on 5mg, twice a day, and my vet assures me her thyroid level is normal now. She has been on Meth for 2 months .. put on some weight .. and was just retested in January, tho I don't have her actual numbers. Her coat is looking better .. and I'm hopeful .. I'm concerned that her appetite is still very voracious. She is on wet food only, so her HUGE appetite increases her urine output too. I've been researching hyperthyroidism so much, I feel like a walking billboard for it! I can't seem to locate any information about whether her appetite is normal .. will it ever slow down .. so even tho I'm hopeful .. and so so grateful for such a brave, strong girl .. I'm worried that she is driving herself crazy if she's feeling unsatisfied. I just lost my beloved boy in September, and can't even think about her suffering now too .. thank you so much for your time, and this wonderful site. I may just be a crazy cat lady, but its clear I'm not alone! Thank you ..

Dr. Mark E. Peterson said...

Your cat's appetite should remain high until the weight and muscle loss has been restored. This will certainly take a few weeks to months to do. As long as the thyroid values are in the desired range, I would not be concerned unless your cat becomes overweight or obese.

Dr. Mark E. Peterson said...

Your cat's appetite should remain high until the weight and muscle loss has been restored. This will certainly take a few weeks to months to do. As long as the thyroid values are in the desired range, I would not be concerned unless your cat becomes overweight or obese.

Patty Vanderhoef said...

My 14 year old male cat Tommy has been on 0.1 ML Methimazole Lipoderm gel rubbed on his inner ear flap every 12 hours now for 4 months. His coat is still dull and looks unkempt otherwise seems to be maintaining weight and eating,drinking normal. Why does his coat still look this way?

Dr. Mark E. Peterson said...

Have you checked the thyroid function to make sure that his is euthyroid? How about a CBC, chemistry panel and urinalysis? Talk to your vet to find out what is wrong.

makatak said...

Hi Dr Peterson.. You are providing a wonderful service to the animal lover's community. I don't know if you can help us or not..Coco is being syring fed (primarly) for almost 15 months because of cholangitis hepatitis.eat a little on her own. She was further diagnosed with hyperthyroid January, based on blood tests tt4 >90nmol/L (her liver ALT was 1717 U/L)and prescribed transdermal methimazol 2.5 mg/.5 ml twice per day. We discontinued after a few days because of bleeding and scratching. We tried Y/D for about a week didn't like, vomet etc. We are struggling to find a good quality protein food from a vet or pet food store.We have the pill Tapazole5 mg and doseage is 2.5mgX2 per day, havn't started it yet (scared to) Coco is very thin and is down to 6.9 lbs .lost a lb in 6 months. Could we start her on a smaller dose at first? Sorry for rambling on and on . john m

Dr. Mark E. Peterson said...

Yes, I'd try a lower dose of 1.25 mg twice a day to see if your cat tolerates that dose first. Then recheck a serum T4 value in 2-3 weeks.

makatak said...

Thankyou we will give it a shot john m

makatak said...

Hi again Doctor.. what is a good diet for gaining with Coco. would be nice if she could put on 1/2 lb or so..
thanks j m

Dr. Mark E. Peterson said...

See this past blog post of mine on the best diet to feed hyperthyroid cats.

http://endocrinevet.blogspot.com/2011/09/best-diet-to-feed-hyperthyroid-cats.html

J Dressel said...

Dr Peterson
Thank you for your earlier reply.
We seem to have gotten beyond the
EGC and the thrombocytopenia. His most recent blood test 2/26/14showed that his platelets are up to 600K and he tests negative for EGC.
The treatment for both was Prednisolone (not taking anymore)
Unfortunately, now he has very high (500)blood glucose caused by the Prednisolone and is being prescribed insulin.
My choices are Glargine or PZI.
Is there a preference?
Meanwhile,his free T-4 taken 2/26/14.. shows..T-4 is 3.2..
Free T-4 dialysis is 82.
He has also been prescribed 2.5mg Methimazole twice a day.
I would like to know of any connection between diabetes and hyperthyroid and your opinion on how to proceed.
Thank you for your time and expertise..MUCH appreciated

Dr. Mark E. Peterson said...

The diabetes could be due in part from the insulin resistance secondary to the hyperthyroidism but is likely primarily due to the prednisone.

In any case, you need to start insulin (either Lantus or ProZinc twice a day is fine). You also need to get the T4 and free T4 values normalized.

Talk to your vet about what to do next, or get a referral to an internal medicine specialist.

makatak said...

Hi again Doctor.. I have Coco on a combination of kitten food, a good quality high protein chicken and turkey based from pet food store and just added max cal today, with a can of each. She is scratching her ear after ony 2 applications of 1.25 mg X 2/day of methimazole. Skipped 2 doses since. Thinking of cutting the dosage in half again. She appears to be very comfortable and has almost stopped scratching. Thanks Doctor and have a great weekend john m

sierrasue said...

Hi Dr. My cat has been on twice daily hyperthyroid pills for over 6 months now and is doing well. My question is how terrible is it for her if she misses a dose now an then. I travel a lot. Usually just for 2-3 days and have just found out that the petsitter has trouble giving her the pill ( I think she is pretty easy to pill). So, I believe she might miss a dose now and then. Also, I ordered some meth in gel form but have not started it as my vet said it is better to use the pills. That the gel does not allways absorb right and give a proper dose. What is your opinion on this??? Thank you !!!

Dr. Mark E. Peterson said...

Ideally, the serum T4 would be held at a fairly constant level and not go up and down in and out of the normal range. This can only be done by consistently giving the medication every day. Some feel that such fluctuations may contribute to development of kidney disease but we don't know this for certain.

So, is missing an occasional dose a big deal? Probably not. But not giving anything for 2-3 days once a month certainly is. I'd recommend that you talk to your vet about a form of treatment that removes or destroys the thyroid tumor (surgery or radioiodine), so you can cure your cat and not worry about this issue.

Sarah said...

Hi, my cat has been on methimazole for about two weeks: 1/4 of a 5mg pill, twice daily. She's been vomiting for 3 days. I'm waiting to hear back from her vet, but I thought I'd ask your advice too. I'm wondering if I can work out a feeding schedule- taking account of the half life of the medication- that would reduce the vomiting? (Or does the nausea linger after the medication has cleared the system)?

Dr. Mark E. Peterson said...

The drug lasts about a day and most cats continue to vomit for longer than that. If the vomiting is severe, it might not work in your cat or you might be better switching to transdermal medication. Talk to your vet.

Kyle Micciche said...

Dr. Peterson - Six months ago I adopted a 14 year old cat. She's hyperthyroid and was on 5mg of Methimazole once a day. Her last blood test was in late August and the Dr. said her T4 level was 11.0. She showed absolutely no signs of an increase in her level. I was surprised when I received the call to increase her dosage to 5mg 2x daily. (I've had hyperthyroid cats before and can usually notice when their T4 levels go up). Six weeks have past since we increased her dosage and she is miserable. She doesn't want to play, she's sleeping too much, acts like it hurts to move and moves very slowly, decreased her food intake, doesn't produce stool daily anymore and looks swollen. However, she's managed to maintain her weight at 11.lbs. We went to her Dr. this past Tuesday and the Dr. thinks Kitty's back is sore and explained that as the reason for the changes. So she prescribed Prednisolone 5mg 1x daily. My husband and I think something else is wrong or the Methimazole dose it too high. I'd like to decrease her pm dose to 2.5mg to see if her behavior changes. What are your thoughts?

Dr. Mark E. Peterson said...

Not all cats do well on methimazole. If the dose needed to lower the serum T4 value to normal makes your cat not feel well, I'd think about another form of treatment.

You could stop the methimazole for a few days and see if she feels better - if not, then it make be a neurological issue and steroids (short-term) may help.

Adam said...

Hi
I am so happy I found your blog as my cat recently passed on and was hyperthyroid. I have a couple of questions I hope you can help with.
Oscar was a 17 male who was on the streets for some time (he was loved once as he was desexed, microchipped and very affectionate), He was found severely anorexic. They found he was severely hyperthyoid. He was started on treatment and in 8 months went up to 2.8 kilos. About 8 months after being found I adopted him. I had trouble giving pills and almost lost him as his weight decreased again, so I switched to the antithyroid cream. He seemed OK on it. About a month into his treatment he became very alert and started regaining his kitty-litter training. We thought he was getting much better.
I was giving him 3 doses a day of the cream. When it eventually ran out, it was about 3 days later that we started retreating him as we couldn't get to the vets. He was treated for 3 full days before having a stoke on the 4th which left him partially paralysed and with a severely dialated pupil. The vet recommended euthanasia (which we did). Do you think having missed 3 days would have contributed to his stroke? The vet did not think so, but for me it is a bit of a coincidence to ignore.
Thank you.

Dr. Mark E. Peterson said...

Sorry about the loss of your cat. No, it's highly unlikely that missing a few days of methimazole would lead to a cat's stroke.

In most cases, the cause of strokes in cat is not clear, and hyperthyroid cats almost never get them.

Shauna Price said...

I have a 17 year old female cat diagnosed March '14 with hyperthyroidism, moderate renal disease and elevated liver enzymes. she was taking 5mg methamazole in the am and 2.5mg methamazole in the pm. last thyroid/kidney/liver function blood test 4weeks ago reported her liver enzymes in normal range, her kidney functions were the same and stable but her thyroid levels were on the low side. the vet recommended lowering her methamazole dose to 2.5mg am and 2.5mg pm. My cat didn't show any signs or change in symptoms until just recently. She seems to be sleeping a lot more and has lost much of her appetite. she is still eating but only 1/4 to 1/2 of what she was eating. she is drinking and peeing much more recently also. I am bringing her back for more bloodwork next week when I get paid again but in the meantime I was hopeing you could offer your opinion as to what may be happening? Could her kidney functions declined bc of the lower dose of methamazole which is now causing her to not feel well and loose her appetite? Should I try increasing her dose back to the 5mg am and 2.5mg pm? what type of foods do you recommend? I have tried 2 different prescription diets for renal disease but the dr. never recommended a thyroid prescribed diet. She wouldn't even go near the food dish with the 1st food but she does eat the 2nd one (RENAL LP MODIFIED-P made by Royal Canin) but not always. She seems to only lick the gravy off the morsels in the can food and she grazes over the dry food at least once a day so recently most days I will offer her her old junk food favorites (fancy feast,friskies,whiskas)just to see if I can stimulate her appetite but she is not even enthusiastic about those either and she wont go near Nutrical.
Any advice, thoughts or opinions would be greatly appreciated and helpful. thank you for your time and service.
p.s. I am most greatful for this site and blog for the information provided and more specifically learning that handling Methamazole with out protective gloves could lead to harmful affects on my own health. I wasn't warned and I had no idea this could happen.

Dr. Mark E. Peterson said...

Sounds like the methimazole is not agreeing with your cat - not eating well and lethargy are not good.

If the lower dose hasn't helped, you may want to do a test and stop it for for a few days and see if the appetite goes back up. If so, another treatment is recommended. Talk to your vet about this.

Kristin Mathis said...

My 17 1/2 year old cat was diagnosed being hyperthyroid on 10/15/14. On 10/16/14 she began taking Methimazole 5mg two times a day. Over the weekend she began having spells of vomiting up white clear liquid. She had problems with vomiting before but it has increased since starting the medication. I took her back for bloodwork again today.They told me her levels went from a 0.8 to a 5.0. Which now is way to high. They also told me that now her kindney levels are high and they were not 2 weeks ago. I now have to reduce her dosage. My question is when they had me start her at such a high doseage could it have had

an effect on her kidneys?

Dr. Mark E. Peterson said...

Yes, if your cat had underlying, concurrent kidney disease (not unusual in a 17-year old cat), then giving methimazole could unmask the kidney disease. This is especially true if the methimazole dosage was too high, leading to hypothyroidism (underactive thyroid).

See this blog post: http://endocrinevet.blogspot.com/2014/10/unmasking-kidney-disease-in.html

Kristin Mathis said...

Thank you for the link for the reference material. Im just upset due to I feel they started her out with such a high dosage to begin with instead of smaller doseage to see how that went. Taking into consideration she is a small cat and has always been 6 pounds. Again thank you for your help. Im hoping the lower dosage and new diet will help her out and also stop the vomiting.

Humboldt Hanna said...

My 18 year old cat was diagnosed with a thyroid condition a few months ago, her value was 7.5 We now have her at 3.2 and the vet wants to increase her to 1 whole pill in the morning and a 1/2 pill at night, She was on 1/2 pill twice a day. My father just died and Im leaving my kitty with a friend who is very good with animals while I go help the family and my mom for a few weeks, the friend asked what the symptoms are of too low a value/ a overdose? I think thats a really good question and I was also wondering how often should the values be tested, do we really need to be testing every week? Thankyou for your answer in advance.

Dr. Mark E. Peterson said...

What's the reference range for the T4 at the lab your vet uses? That value could be high, high-normal or midnormal so giving me a number doesn't help.

You may not want to raise the dose until you get back in case the higher dose is too much. No specific signs that cats will show, but overdosage can make the kidney tests worse.

In general, once we stabilize a cat on methimazole, testing every 3-6 months is adequate, but you do want to check the T4 and kidney values 2 weeks after making a dose change.

Kathy Flynn said...

My 16 yr old has been on 2.5 mg 2x/day for 3 mo. She was tested 10 day ago and her numbers are good. She does have kidney problems and is in Sience Diet K/D. She has recwntnly begun to cry after she urinates. Her amount is normal. She has had the crystal problem before, and this is different. Does methimazole have a side effect of any bladder or uretha inflammation? I haven't read about it anywhere.

Dr. Mark E. Peterson said...

No effect of methimazole. But your cat might have a urinary tract infection or cystitis. Talk to your vet about getting a urinalysis and urine culture done.

Jo said...

Dr. Peterson,
You wrote: "It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values on methimazole. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L)."

Do YOU adjust your T4 goal range upwards any for elderly cats whose kidney values worsen to stage 3 and beyond as they continue with longterm methimazole treatment? If so, what is your modified goal range?

Background: After >1 yr on 1.25mg BID Felimazole, my 20 yr old's T4 has risen from 1.9 in Oct. to 3.1 (0.8-4.0) last month. His BUN is now 52 (14-36) & crea 4.4 (0.6-2.4). He is on all the typical treatments for CKD. He is showing some signs of possible hyperT effects (howling, increased drinking, restlessness), based on the signs I observed in him when he was first DX'd.

I would, therefore, very much like to make him feel better by getting his hyperT better controlled with a small increase in Felimazole to bring that 3.1 T4 down a bit. But do I dare do it with his latest kidney numbers? I no longer know what my T4 goal should be under these circumstances!

His case is a recurrent one...developing hyperT again 7 yrs following his first I131 treatment, which *appeared* successful. Unfortunately, I was told by the university who did the first treatment that he was no longer a candidate for I131 or surgery because of his advanced age, Stage 2 CKD and poor candidate for isolation. I now wish I would have fought them harder on that, as I believe the potential benefits would have outweighed the risks. I hate to see him battling this debilitating disease and the harsh effects of methimazole. He's a tough ole' bird!

Loads of thanks for your continued guidance. You are STILL our American Idol!!!

Jo & Lucky Gunter

Dr. Mark E. Peterson said...

Well, this is a tough place to be. If you did a free T4, it would likely be a bit high. But as you say, increasing the methimazole dose could cause the kidneys to decompensate further. You could increase the dose by very small amounts and evaluate both appetite and other clinical signs. I'd also check serum creatinine every week during any dosage increases.

Good luck!

Jo said...

Dr. Peterson,
I forgot to tell you that his Free T4 WAS tested, as well, in Jan and it was 36.6 (10-50).

Jo & Lucky

Dr. Mark E. Peterson said...

Then I'd probably not change the dose or, if you do, by only a tiny amount (1.25 mg per day).

Jo said...

Thank you SO much!!! Will do!

vivamiiwaa said...

My moms cat is on methimazole 5mg. Wecut the pill in half and melt it in Water then syringe it to her.She is 15 in people years and she eats all day. She throws up alot as well. She is not gaining any weight in fact it looks as if she is losing weight. We are really concerned she does not act like herself. Any suggestions?

Dr. Mark E. Peterson said...

Methimazole has a bitter taste and that could be a problem in your cat. It would be better to pill directly so that the mediation doesn't have to be tasted. Or you can use transdermal medication instead. Talk to your vet

vivamiiwaa said...

Thanks Dr. Peterson......fortunately the cat likes to take the meds...she sits and waits for it then laps it up.....lol....the main concern is the weight loss I dont understand why she is losing so much weight when she eats all day.

Dr. Mark E. Peterson said...

Have you repeated the thyroid levels and adjusted the dose as needed to lower the T4 into the mid-normal range? This testing needs to be done periodically-- the dose can change as the thyroid tumor grows in size. Talk to your vet.

Juicy said...

Hi, My 13 y/o cat just started on Methimazole 2.5mg twice a day, for about the past 2 weeks. She had her vaccinations & was also found to have an unexplained seroma on her neck (no surgery). I've had it drained twice (it was filled with reddish clear serum), it comes back within a day, and this time am "watching" to see if it goes down. In the past few days (it was last drained Thursday), she seems to not be feeling well. She seems tired, and sleeps all day except to eat/drink/use litter box, just very lethargic and uninterested in anything - not her usual self. She is also occasionally sneezing. The day of the vaccines (2 weeks ago) she felt ill, but was her usual active self within a day. Now she seems very uncomfortable. Palpating the seroma does not seem to bother her at all. She isn't supposed to be rechecked for another 2 weeks (after a month on Methinazole). Should I be concerned about the medicine being too high, or the seroma? Should I have her thyroid levels rechecked sooner?

Dr. Mark E. Peterson said...

I'd recheck now, The seroma, if that's what it is, is not related to the methimazole but your vet needs to determine what that is and what's the cause.

Juicy said...

Thanks for your reply. The seroma is not related, it was found 2 weeks ago before putting her on methinazole. The vet said it was a seroma, drained it 2 weeks ago, and then ago last Thursday and checked the fluid that came out. Last Thursday I was told that though it's odd, it is a seroma, and I don't need to keep getting it drained, that it would reabsorb.
I worry about the meds because she just looks to be feeling so ill. Will checking her thryoid now provide adequate results even though she's only been on the meds for 12 days? Could she be feeling ill from the vaccines and not the methinazole(she was given RCP & Rabies 2 weeks ago), or a combination of these things? I read some cats can experience depression or lethargy upon starting methinazole. My other cat (14y/o) started methinazole a week ago but seems fine.

Dr. Mark E. Peterson said...

I normally recheck a T4 in 2 weeks after methimazole is started.

Puffy Daddy said...

Dear Dr Peterson,

Thank you for the above information and for the Q&A! It is very helpful in understanding this condition.

My 14-year-old male Maine Coon gets 2.5 mg methimazole 2x/day. I do my best but I may miss 2-3 doses a week. (I'm pretty faithful about dosing him, but he periodically hides where I can't reach him, shakes out a pill [and hides], or I forget a dose).

Is it safe to double the next dose if I miss one? Or do I just skip it?

Thank you!

Dr. Mark E. Peterson said...

If your cat needs a total daily dose of 5.0 mg per day to maintain euthyroidism and you miss a 2.5 mg dose, it's fine to give 50 mg the next time.

La La said...

Dr. Peterson, I apologize if this comes thru multiple times--I couldn't seem to get it to work right.

Our kitty Bear was dx with stage 2 CKD in May of 2013 & hyper-t at the end of July 2014 with a T4 of 4.9 (.8-4.0). She started on liquid meth. (.125mL or .625mg 2x/day), and her T4 came down to 2.9 (.8-4.7) by 8/23/14. However, b/c she was biting constantly at her tummy/legs, the vet lowered the dose to half of that (.625mg total per day). Her T4 climbed up to 6 on that dose (the vet told us to take breaks of a week off here and there over several months to see if she felt better). It also seemed the first batch of compounded liquid was more effective than the subsequent bottle.

We finally switched Bear to meth. pills about 2 months ago. The biting stopped, and at her last check on 3/5/15, her T4 was 2.7 (.8-4.7), BUN 34 (16-37), and CRE 2.1 (.9-2.5). The problem is, just like when her T4 went into the 2's back in Aug of 2014, she lost her appetite almost completely, started hiding and became VERY lethargic. So--the vet had us lower the meth. pill dose to .625mg 2x/day, and she started feeling better and eating a few days after this change.

It doesn't seem like the medicine itself is causing the inappetence, but rather when Bear's T4 drops into the 2 range. Bear has held her weight all along--she never had the classic weight loss of many hyper-T kitties.

It has been a roller-coaster of trying to get the right med form and dose so she doesn't feel lousy and not want to eat. It just looks like she feels most normal when her T4 is somewhere in the 3 range. She acts like a kitty in a much higher stage of CKD when her T4 dips into the 2's, even with a CRE of 2.1.

What do you think is going on here? Would it be harmful to her kidneys for her T4 to hover more in the 3's in her particular situation? We certainly don't want to wear out her kidneys faster, but we also don't want her to feel horrible and eat nothing, either. Very concerned...

Dr. Mark E. Peterson said...

I'd try transdermal methimazole next. If that causes the appetite to be poor, then you may have to go with another treatment. The CKD is rather mild now, so I wouldn't be afraid to do I-131 if we had to.. just give a very low dose.

If you haven't done so, a low phosphate diet or use of a phosphate binder would be a good idea now.

La La said...

Dr. Peterson,

Thank you for your reply. We've heard so many people say that they couldn't get their kitty regulated on the transdermal: it either absorbed too well and made the kitty hypo-t or absorbed not well enough, leaving the kitty chronically under-treated. Why would she start feeling poorly only when the T4 goes into the 2's, though? It can't be the med since when we decrease the dose, and the T4 climbs back into the 3's, she feels fine (no hypo or hyper-t symptoms to speak of). Have you ever heard of this happening before?

Dr. Mark E. Peterson said...

It may be a result of the dose rather than the T4 level per se. Sometimes they respond better to the transdermal methimazole.

Gina Di Cataldo said...

More then a comment I would like to know whether feeding Y/D is detrimental for a cat that takes 1/2 a pill a day of Tapazole 5mg. One vet says the two together destroy the effect of the one, another says it is possible. In fact I did follow this procedure in the past and it seemed to work, but for a mistake I made now everything needs to be rethought. The new vets I have spoken to are strongly against feeding with Y/D while the cats is given the 1/2 pill

Dr. Mark E. Peterson said...

Giving methimazole and feeding y/c commonly leads to severe hypothyroidism and is not generally recommended.

Gina Di Cataldo said...

Dear dr Peterson, Thank you indeed for your answer. I believe you meant Y/D and not Y/c, I guess the last has been a misprint. I do not understand why it should, but I am really greatful for your answer. I believe I should inform the vet who thinks I can give both: Y/D and Tapazole. However I wish I could understand why this leads to worsening hypertyroidism. Gina

Dr. Mark E. Peterson said...

It does not lead to worsening HYPERthyroidism. The feeding of y/d with methimazole can lower T4 and T3 to almost zero and can lead to severe HYPOthyroidism.

Dr. Mark E. Peterson said...

It does not lead to worsening HYPERthyroidism. The feeding of y/d with methimazole can lower T4 and T3 to almost zero and can lead to severe HYPOthyroidism.

Paul Brown said...

HI, We have a ten year old cat that lost a little over one pound in the last two months. Just 3 days ago she started having difficulty walking and listing to the right in her hind quarters. We took her to the vet Friday and they did a broad spectrum of blood tests. Diabetes test was negative but the vet did say she had a very overactive thyroid and prescribed 5 mg of Methimazole once a day. She has taken it for two days, yet symptoms are worsening. She is disoriented, weak, and will not eat or drink. She only has taken a couple sips of water by syringe. We plan on taking her back to the vet tomorrow as we are worried about dehydration. Any ideas?

Dr. Mark E. Peterson said...

Hyperthyroidism does not cause difficulty walking or anorexia. Stop the methiamazole and contact your vet about what to do next. If they don't know, ask for a referral to an internist.

Cherokeegirl326 said...

I am going on vacation and having someone take care of my 2 cats that require twice daily thyroid medication. I use pill pockets for the pills and watch them to ensure that they are taking it. My concern is that they will not take in with a stranger. I could have the pills crushed and placed on food to at least hope they get it. What would happen if they don't get their proper dosage for the week?

Dr. Mark E. Peterson said...

If you don't give the thyroid medicine for a week, the thyroid values will again rise to the peak, high hyperthyroid values.

J Crittere said...

Dr. Peterson,

This may be a duplicate, I'm not sure my first went through. For twice a day dosing of Methimazole, what is the minimum number of hours between doses? And, does it cause diarrhea? If so, is it likely to resolve with time and is there any way to reduce this effect? My cat has been on the lowest dose of 2.5 mg daily for 3 months with positive results. But, he developed sporadic liquid diarrhea after several weeks on the meds, but no vomiting.

Thank you in advance.

Dr. Mark E. Peterson said...

Methimazole does not generally cause diarrhea, but it can. You may have to stop it for a week or so and see if the diarrhea resolves. If it does, then restart the methimazole to see if it returns. If it does, then I would be more likely to blame the drug and then another treatment will have to be used.

The timing between twice daily methimazole doses is not critical and some cats can be treated once daily.

Maria josé Ramos Ramos said...

Hi Dr. Peterson,
My 12 years old cat, nero, was dignosed with a hyperthyroid 11 months ago. We can't seem to get him balanced, he has been on 5mg + 5mg a day and got normal values, but after 4 weeks, this last sunday he had to be rushed to the clinic as he couldn't breath, his tongue went blue and needed oxygen, now his Free T4 levels are higher (3.3, normal value ranges between 0.7 - 2.6) Total T4 normal , so the vets think that's what caused the crisis. They did x-rays and they don't think it's asthma but he has had abdominal fast breathing the whole 11 months whether he was on normal levels, below normal levels or over levels so I am worried he might be asthmathic (he coughs often and is short of breath with any efforts).

Also, is it normal a cat doesn't ever get a balanced T4 whatever the methimazol dose? (he has been on 20mgs a day and got hypo thyroid levels, 15 mgs and got normal but then after a few weeks high, then 10 mgs and got hypo thyroid levels, then 10 mgs hypo, 5 mgs high, and then back to 10 mgs normal but after few weeks high. Thanks a lot.

Dr. Mark E. Peterson said...

It can indeed be difficult to completely reestablish normal thyroid levels on methimazole. Your cat is on huge doses of the drug, suggesting that the hyperthyroid state must be severe and the thyroid levels (off methimazole) must be extremely high. That said, I really doubt that a slightly high free T4 value (with normal T4) caused your cat's severe respiratory distress. Coughing is very rare in cats and asthma could be causing it. That problem definitely needs further workup.

You should also consider another form of treatment (either radioiodine or thyroidectomy) once he is stable.

Dr. Mark E. Peterson said...

Your vet MUST monitor your cats with examinations and blood tests during this treatment. You cat has been on the medication for about a month and we normally recheck in 10-14 days. Talk to your vet and do so immediately!

Nina said...

My cat was diagnosed with hyperthyroidism. She was doing well with 1/4 of a 5mg methiimazole pill twice a day, but she has worsened. The vet changed the dose to 1/2 a pill twice a day, but she still isn't eating much. Is there anything I can do to get her to eat? Her prescription only changed a couple of days ago. Should I give it a bit more time? I'm really worried about how little she's eating, but she still seems alert.

Dr. Mark E. Peterson said...

Talk to your vet. You can't have your cat not eat. If you determine it's from the methimazole, then you have to use another treatment.

Agnieszka Chrobak said...

Hi Dr. Peterson
My cat was diagnosed with hyperthyroidism.His T4 and Free T4 levels were really high: T4 167,3 (reference range 10-60), fT4 77,2 ( reference range 9.0- 33,5). He's been on methimazol( 5mg- 1/2a pill twice a day) for about 1,5 month and recently got rechecked ( unfortunately my vet didn't tell me to retest him sooner). His T4 level is normal now ( 19), but Free T4 is much too low (5.0 - reference range 9.0-33,5), so the vet changed the dose to 1/4 a pill twice a day.Do you think that was the right decision ( because the vet wasn't 100% sure)? What's your opinion on this?
Thank you

Dr. Mark E. Peterson said...

You need to monitor again in 2 weeks and recheck the thyroid levels to continue to adjust the dose. This dose adjustment is a life-long process and the dose will continue to change (most likely) as the thyroid tumor(s) continues to grow.

Caroline Brown said...

Dr. Peterson,

Our 13 year old female was recently diagnosed with HYPERthryroidism. We give her 1ml of Methmazole liquid every morning. We are still in the first month. We forgot to give it to her this morning and just realized it by her behavior. It is almost 8pm and I'm wondering if I should give her 1/2 the dose for this evening?
Thank you so much for this page and information.
Caroline

Dr. Mark E. Peterson said...

You could give the half or even the whole dose tonight if you wish. In any case, it's always better to dose twice daily over once daily anyway.

Caroline Brown said...

Thank you so much for your response, Dr. Peterson. I also appreciate your advice about dosing twice daily. We have been following our vets instructions but I will discuss this with them asap, or I am guessing that we could divide the dose and give it twice daily. Thank you!

Caroline Brown said...

Thank you so much for your advice and response.

Dr. Mark E. Peterson said...

That is correct.

Eric Johnson said...

My cat, female, 15 years old had normal range T4 but many symptoms of thyroid disease including enlarged thyroid upon palpating by vet, excess drinking and peeing, restlessness, increased vocalization, and weight loss. Vet put her on 2.5 Methimazole twice a day with positive results re: symptoms. However she started vomiting so I took her off Methimazole with the intention to retest to confirm hyperthyroidism and possibly get radio iodine treat treatment. Iodine tx clinic needs hormone levels for tx decisions. How long does she need to be off Methimazole before testing? Symptoms are getting worse. What tests should be done? T4, free T4, T3? Thanks.

Dr. Mark E. Peterson said...

If cats with questionable hyperthyroidism, the longer the better but at least 1 week.

Eric Johnson said...

Thanks! Any suggestion for best testing regime to confirm hyperthyroid in this case?
I appreciate your help!

Dr. Mark E. Peterson said...

Start by stopping the methimazole for at least a week and doing a full thyroid panel (total T4, free T4 by dialysis, T3, and TSH). See my other posts for how to interprete these tests.

If thyroid scintigraphy can be done, that would be the best test, especially if the complete thyroid panel is not conclusive.

Agnieszka Chrobak said...

Hello again,
I just got my 13yo cat retested (after switching from 2,5 mg twice a day to 1,25 mg of methimazole twice a day) and his total T4 and FT4 levels are way to high (again).Vet decided to go back to a pervious dose (2,5 mg twice a day) and see what happens.
I've been wondering, was it really necessary to lower the dose in the first place. His total T4 levels (on 2,5 mg 2x/day) were normal - 19(reference range 10-60), but free T4 value was to low- 5.0 (reference range 9.0-33,5).
Any suggestions?
Also, could you tell me, how to prepare a cat for a T4 test (is fasting necessary? Before or after the pill? )
Thank you in advance

Dr. Mark E. Peterson said...

No fasting or other preparation needed for serum T4 testing.

Jersey's owner said...

Thank you for having this blog. My main question will be in ref to feline hyperthyroidism treatment with transdermal methimazole rubbed in ear. Brief history; 6-7 months ago (Mar-April), 12-13 yr old male cat was having loose, really smelly stools. He had been on a c/d formula for 6 yrs after a renal blockage/failure & surgery. His BM looked like his food. I switched to grain free dry food & this seemed to clear up the loose stools & some of the smell. He appeared to be loosing weight, (was usually ~16 lbs). In June he was on 2nd day of vomiting, took him to vet. ALT & ALP were very high, BUN & Creatinine normal, wt now 9.28lbs, Vet could not feel any lumps around thyroid, but suspected hyperthyroidism, ordered a T4. Gave some meds for vomiting symptoms in mean time. T4 came back at 16 ug/dl (ref range 0.8-4.0 ug/dl). Prescribed pills of methimazole 5 mg 2x daily. After 2 weeks, cat started vomiting again. (does not take pills well). Returned to vet wt now 8.69 lbs & coat looking unkempt. Prescribed anti-nausea & appetite bills. After 1 week, wt 9.97lbs, repeat T4=13.3 and ALT & ALP - returned to upper normal range. Started on transdermal cream in ear 1.25ml/5mg 2x a day. Eating well, dry & wet foods, BM's not smelling & return to pre Mar-April consistency (ball shapes). After 2 months, Vet on 9/29 wt now 10.09 lbs. Creatinine 2.39 (ref 0.8-1.8).T4 NOW <0.5 gu/dl (low) ref 0.8-4.0 ug/dl. FINALLY MY QUESTION: Is his dose too high? Reception at Vet said, continue on cream & repeat T4 in 6 months. I am waiting for call back from Vet (whom I trust & don't want to 2nd guess...but) Given that his T4 is now so very low (assuming no mix up in samples), shouldn't we decrease dosage & test again in 4-8 weeks? I am concerned about over dosing & his kidney functions. Thank you in advance for your time and advise. If I was closer I'd make an appointment.

Dr. Mark E. Peterson said...

Yes, of course the dose is too high. A low T4 will cause the kidney values to go up so we don't know if kidney disease is present, but even if it is, it is probably not too bad.

At 12-13 years of age, wouldn't you rather have the disease cured?

Hyperthyroidism is a progressive disease. The thyroid tumor will continue to grow and hyperthyroidism will get worse will time. WIth a T4 of 16, that's already pretty severe. If he lives for another 2-3 years, the disease will likely be very bad and the methimazole may no longer work at all.

Krystyn Evans said...

Dr. Peterson,
My 14 y/o cat has been on methimazole for about a year. Over the past couple months she's had blood in her urine. She was treated for a UTI with two antibiotics but it has not resolved. I took her in again the vet did not think the blood in her urine could be a result of methimazole. I had bloodwork, X-ray, and an ultrasound done. They were unable to get a urine sample. Though my vet did not think it was a result of the drug, I have stopped her methimazole for the time being because I am concerned it may be the cause. She is on amoxicillin - her wbc count was high but the vet said after two rounds of antibiotic is shouldn't be high (it had been about two weeks since last given). I have read about interstitial cystitis and I'd like your opinion. Would you discontinue methemazole and switch to the iodine free diet? Thank you so much for this forum, I have been reading it and learning since my two cats were first diagnosed.

Dr. Mark E. Peterson said...

We have no evidence or studies that indicate that methimazole causes cystitis.

I would not switch to y/d -- that will likely make the stress worse. It doesn't taste very good and if I had to eat that diet for the rest of my life, I'd certainly be stressed!

Unknown said...

Do you know if Methimazole can increase blood sugar in a hyperthyroid and diabetic cat? We just increased my 15 year old cats dose from 1 pill to 2 pills a day and his blood sugar has been high ever sense.

Dr. Mark E. Peterson said...

No, it would not increase the blood glucose.

Irina said...

Dr. Peterson, my 14 yo male Ragdoll’s T4 was 6.7ug/dL (ref 0.8-4.7), SDMA was 12ug/dL (ref 0-14), creatinine 0.8 mg/dL (ref 0.9-2.5), all other bloodwork normal. In retrospect, he had had all clinical symptoms of overactive thyroid for at least 6 mo, may be longer. He was given methimazole, 5mg, ½ pill twice a day. First wk of methimazole, howling stopped, vomiting and diarrhea stopped, even coat looked somewhat better, excessive water and food intake remained. Second wk- he started vomiting, water and food intake significantly decreased. He was spaced out, slept in unusual places, tried to hide, lost all his affection, became aggressive. Stopped methimazole because he was vomiting and was hoping to resume the next day. But the next day he started excessively licking all over and scratching his neck, ears, and face. Howling is back. Now vomiting stopped, he eats and drinks moderately. He is now 3 days off methimazole after only 14 days on it. He is irritated/looks worried; excessive licking all over and scratching persists. Does this look like allergy to methimazole or overdose? Should I resume methimazole? May be at ¼ pill twice a day? If it is allergy, when should I expect the scratching/licking to stop? Should I look into other treatment options? His blood work check is mid December.
Thank you so much for your time and for your wonderful website. Irina

Dr. Mark E. Peterson said...

Vomiting and scratching are not uncommon side effects. You can try lowering the dose but I'd recommend another form of treatment.

AM Husk said...

hello Dr. Peterson, my cat has been prescribed 1 ml compounded drugs I put onto the skin of her ears, she is improving but I plan to get Iodine treatment. in the mean time I am wondering because of the holidays if the twice a day administration of the drug has to be crucially 12 hours apart? could it shift two hours?n - i .e. the evening dose be at 6 or 10 PM if my usual dosing time is 8 am and 8 PM? AMny thanks in advance for your input!

Dr. Mark E. Peterson said...

No the timing is not critical with twice daily methimazole.

Annie Laurie said...

Hi! I was initially going to ask about the timing as well. The vet said every 8-12 hours, but I try to do it every 12 hours. Please correct me if the every 8 hours is totally off-base. The question I have now is, what are the benefits of giving the subdermal gel instead of the pill? My cat also has IBD and while he tolerate the Methimazle without vomiting, I wonder if he's even able to digest it with the thickness of his intestines and his liver and kidney problems. Also, it the gel really about $8 per dose or is that an urban legend? Thanks, Dr. Peterson! This has all been very helpful at a very confusing time.

Dr. Mark E. Peterson said...

Once absorbed, methimazole is taken up by the thyroid and stays there working to block T4 secretion for up to 24 hours. So giving every 12 hours is fine... no need to give the medication 3 times a day.

We use transdermal methimazole because vomiting and anorexia are common sider effects from oral administration. This is a strong drug with a bitter taste.

joanng said...

Happy New Year Dr. Peterson! thank you for all your insightful advice.
I read your web pages all the time and they have become gospel for me with my 15 year old kitty who is suffering from hyperthyroidism. Once she went on the felimazole (5mg. 1/2 in AM, 1/2 in PM). she stopped the vomiting. she's been on it for about a year but now she is vomiting pure water, or it appears to be straight water. puddles of it all around the house. what should I do? is is this just the beginning of the end? she is still very playful and adventurous I just don't know what all this water means. I don't know that you'll get this, but I really would love to hear from you.

Dr. Mark E. Peterson said...

You need to call your vet and have your cat checked. This doesn't sound like it's from the methimazole or hyperthyroidism but you need to have her checked ASAP.

Ismahan Sharhan said...

Hello Dr. Peterson,

My 13yr old cat was recently diagnosed with hyperthyroidism and was put on methimazole a month ago, 1.25 mg in the morning and 2.5 mg at night since she's a fairly small cat. She recently began having a bad reaction to the drug, severe facial scratching and lethargy. I took her to the vet since it had been a month of her using the drug, and my vet told me that her T4 level is normal and to take her off the drug for a month and then run blood tests again. I'm a bit worried by this method because I fear that her hyperthyroidism will rear its ugly head again and was wondering if this was normal procedure. I was under the impression that methimazole use would be long term so I'm not sure why she was taken off the drug altogether instead of being given a replacement.

Thank you for your time and I hope to hear from you soon

Dr. Mark E. Peterson said...

No, it doesn't make any sense to me either. Talk to your vet but it they do not have an explanation, I'd get a second opinion.

J Crittere said...

Hello Dr. Peterson,

My 16 year old cat has been on Tapazole for about 2 years. What are the symptoms of hypothyroid in a cat on long-term treatment? My vet has told me it isn't possible for hypothyroidism to occur, so I'd like to know if some of the issues I'm having with him could in fact be this and get the full panel of tests if so.

Thank you

Dr. Mark E. Peterson said...

It is indeed possible for a cat on methimazole to develop hypothyroidism. This is ruled out by measuring a T4 and TSH level.

AM Husk said...

hi again Doctor, my cat developed allergy to the methimazole even as a transdermal - i took her off the drug and she seems much much better, herself again, the only thing is she is eating a lot, peeing a lot and drinking a lot of water. she is off the drugs for almost three weeks and put back on some of her weight - is it ever possible for the hypothyroidism to be cured by short treatment of the methimazole - or is this just a temporary situation that she seems fine? does hypothyroidism ever pass or once the cat has it - it has to be cured?

Dr. Mark E. Peterson said...

No, Methimazole can never cure hyperthyroidism.

capebass said...

Dear Dr. Peterson, August2015 my 14 year old female cat was diagnosed with hyperthyroidism. Her thyroid level was 7.1, and her vet started her out with .1 milliliters Tapazole gel twice a day. Three weeks later she began vomiting,diarrhea and fever. Blood work showed her thyroid had dropped to 1.4. Vet prescribed Flagyl and decrease Tapazole to .01 ml twice a day. She recovered, but thyroid level gradually increased again to 3.4 and then back up to 7. Her vet said increase Tapazole gel to .1 milliliters in morning and.05 milliliters in evening. About three weeks later she began the vomiting,diarrhea(this time bloody)decreased appetite and spiking fever. Vet prescribed Flagyl again.Thyroid was 4.9 so no Tapazole dosage change made. She did not respond to treatment like before. Dr.stopped Flagyl and gave convenia injection. Also perscribed Kao/Reglan and Prednisolone. Our vet said he's treating her for IBD and fuo for now but gastrointestinal lymphoma is another possibility. She was not better in a week so vet perscribed Veraflox and Mirtazapine. This resulted in bout of severe vomiting and diarrhea, so he stopped the Veraflox and Mirt. An emergency visit to the clinic resulted in US seeing a different vet on regular vet's day off. He said Tapazole can weaken immune system, and he reduced her dose to .05 mil. two times a day. He said it would take two weeks to see results. Could it be possible her symptoms are result of infection, or not tolerating Tapazole well?

Dr. Mark E. Peterson said...

It may all be due to methimazole. IF you stop the drug for 2 weeks and no fever or vomiting or diarrhea, then that's likely. IBD or lymphoma aren't going to get better by stopping methimazole alone. Some cats just can't tolerate methimazole.

gr eightaa said...

Hi, I apologise if this is now in duplicate.
I would appreciate your opinion
My female cat is 14 years diagnosed 35 days with hyperthyroidism
Methimazole gel 10mg/ml at 1ml / 1 per day.
Medication is given at 12.00 mid day.
What is the best time to take blood for follow up blood test.
I appreciate the help you are giving
Regards

Dr. Mark E. Peterson said...

No best time, but I would try to take the sample early the next morning (9 am) if possible.

JamieLeigh said...

Hi! My cat just recently started on tapazole, as in last night. We noticed immediately that he was very lethargic, couldn't keep his third eyelid open and appeared dizzy. He is normally very happy and talkative. Could side effects occur that quickly? And would they resolve with time?
Thanks!

Dr. Mark E. Peterson said...

Yes, side effects can develop after a single dose. Talk to your vet.

Mark Mancini said...

Hi there,desperately need some advice.Our 19 1/2 year old male cat was diagnosed with hyperthyroidism in April 2013.The vet put him on Methimazole gel twice a day..dose of 5mg/0.05mg.They did blood work and all seemed to be going along nicely...until now. Over the last few weeks his appetite has increased dramatically,drinking a lot of water but the worse thing is the constant howling.He is waking us up through the night wanting food.He seems restless with a lot of anxiety. We have increased his dose to .1mg twice a day to see if it helps.How long will it take to see a change? It has only been a week and he still seems very anxious.Can it get to a stage where the treatment stops working? What can we do to make him more comfortable.He is really driving us around the bend.I know it's not his fault but is there anything that can be done.Or is this the decline and is it better to come to terms with this could be the end?Thank you.

Dr. Mark E. Peterson said...

You should be checking T4 values every 3-6 months. Are you doing that? Talk to your vet.

Irina said...

It takes my cat about 10 days to react to a dose change. Dr Peterson's site explains in detail how this drug works. but you do need to take him to the vet and get him checked out. it is so hard to see them suffer. may be your cat can get a couple of more good yrs. good luck!

BERT THE said...

HI My Long term buddy of 17 years Slate a cat I saved back in 1999 Just after I had back surgery long story short he developed hyperthyroidism a few years ago he has been taking the orange Felimazole brand of medicine recently 10 mg once daily because of schedule he has gone skinny again so I had my mobile vet come draw blood and check levels and he said that they were at 18 so I was wondering if there is something I can do to help him The vet did not seem to have the knowledge about what should be done . I suffer from health issues and chronic pain and can`t afford the regular veterinary`s here in Spokane Our daughter will be heart broken as well the wife & I as well. Do you have any suggestions? I would be so greatly appreciated if you could give some advice! Thanks in advance for you time

Robert Potts rcpotts47@gmail.com

Dr. Mark E. Peterson said...

I'm confused... your vet doesn't know that the dose should be increased if the T4 is high? This is a progressive disease and the thyroid tumor will grow with time; this generally requires an increase in daily dose to block the production of T4 by the tumor.

BERT THE said...

I appreciate the quick reply. He said he was going to look into it, he probably is about seventy something. He is a licensed mobile vet and he is usually real good but that kind of confused me also. So maybe I will point him to your blog so he can take the time to learn about it cause he is getting paid to know this stuff being a vet and all. So if my Cat Slate was doing ok on 10mg once a day should I increase it to 15mg split into 7.5mg twice a day? or would that be to much?

Dr. Mark E. Peterson said...

That's a reasonable increase in dosage. Just recheck the T4 again in 3-4 weeks.

BERT THE said...

+Thank you for your help and advice! I am so pleased that I found your blog I have received better help and faster response to helping my cat Slate from you than I have ever received from any vet here in Spokane, even after spending upwards of a thousand dollars for these people that I am sorry to say they don`t care about Slate just there income. Now I have the problem of getting the Felimazole prescription and having it filled. The mobile vet guy will do it but It takes for ever cause his assistant never passes on the message or when they do it takes them forever to respond, then when I get upset they always say he has lived a long life and it no big deal if he die`s. So I just wanted to say Thank you so much for your time and help, it is nice to find someone that loves animals as much as we do.

You Are The Best...

gr eightaa said...

Hi again,
I appreciate your previous advice.Again I am not sure if the post is being made so I apologise if this inquiry is now multiple posts. I also apologise if I now misrepresent what you have written

Your opinion seems to be that the dose is best given in 2 doses in 24 hour period.
Our 14 year old cat followed the prescription document 1ml/10mg given 1 dose daily.

After the first blood test, reduced to 0.75ml/7.5mg in a single daily dose.

Is it your opinion that generally the dose is best split, as in this instance into 2 doses of 0.375ml/3.75mg 12 hourly?

Have you had experience of cats being given transdermal methimazole going off their food due the dose being too high.

Have you had experience or would you consider that a dose of 7.5mg in a 2.5kg cat, 1 month into treatment, could make a cat go off their food. Sorry appear to have no interest in food for several days?
Sorry if these are open ended questions without complete information.
I appreciate your assistance
Regards


Dr. Mark E. Peterson said...

Most cats do better on twice daily medication but some can be controlled on once daily.

Methimazole commonly causes the appetite to decrease, and it's not always related to the dose. If it's given in food, the tablet has a very bitter taste. Transdermal methimazole can work better in some cats, especially if they don't tolerate oral dosing.

Remember that with time, the thyroid tumor will grow and the dose must be increased. We don't dose based on body weight but give the dose needed to block thyroid secretion from the thyroid tumor.

gr eightaa said...

Thank you very much. Your help is appreciated.
Have a good day
Regards

BERT THE said...

Thanks so much for you expertise...

Gail Wanner said...

My 12-yr old male Manx-Siamese cross was diagnosed as being severely hyperthyroid about 3 months ago when he had surgery to remove a tumor on his abdomen. He had been losing weight (weighing approx. 7 lbs.) and was always hungry. He gained back a couple of pounds almost immediately but seemed stalled and still starving so I started him on the methimazole lotion (5mg/.1ml td) and he was up to almost 10.5 lbs. We live in the woods and I usually dose both cats with Frontline Plus to prevent ticks in the spring and when I asked my vet, he said it would not cause any negative reaction with the methimazole. However, it's almost a week since I gave him the Frontline and he has not eaten more than a few nibbles of food at a time since then and he's almost groggy most of the time, which is a real change from his usual "busy" self. Is it a coincidence or was there some kind of reaction? I'm ready to quit giving him the lotion to see he will perk up again. Do you have suggestions? Thank you!

Dr. Mark E. Peterson said...

I don't know of an adverse relationship between the 2 drugs. In any case, you need to see your vet to figure out what's wrong. A cat that hasn't eaten for a week is a very sick cat that needs immediate attention and further workup.

Gail Wanner said...

Thank you. I will take him in as soon as possible (vet's only open 3 days a week in this remote area).

crissy noffke said...

Can you administer oral methimazone via wet food for cats?

Dr. Mark E. Peterson said...

Yes, you can but the tablet has a bitter taste and many cats will not take the pill this way with time (or they stop eating the food).

Dora the explorer said...

Hello and thank you for taking the time to do this. My 13 year old cat was recently diagnosed with hyperthyroidism. She is a very sweet cat but hates the vet. She becomes extremely aggressive so whenever she has to have blood work, they must sedate her slightly, get the blood sample, then bring her out of sedation. From what I have been reading here, she will going through this traumatic event every 3 months or so to check her T4 levels. She is a mess when she comes home from a trip like this because the effects of the sedation leave her lethargic. She sleeps and can barely rise up and eat for 12 hours. So here is my question, what about radioiodine treatment for her? Is it safe? What is the success rate? The thought of putting her through this for the rest of her life just sickens me. Thank you.

Dr. Mark E. Peterson said...

You can read about radioiodine for cats on this blog or on my website (animalendocrine.com). Sounds like the way to go with your cat.

Terri Zais said...

If the kitty is on 2.5 mg Methanizole tables 2x per day and sometimes doesn't take the meds (crushed up in food), can pills be alternated with the same dose of transdermal gel so no doses are missed?

Dr. Mark E. Peterson said...

Yes it can.

Terri Zais said...

Thank you Dr. Peterson. I asked my vet the same question and they said the dosing and/or absorption was different and they didn't recommend it. Usually my kitty takes the pills fine in wet food, but occasionally has an upset tummy or doesn't eat when people are over who are loud. I'm hoping we can alternate, but if we have to stick with the gel at least we know he's getting it.

Jennifer said...

Hello! I want to thank you for having the most informative website. My cat was dx with hyperthyroidism at 14yo 2 years ago and was started on Methimazole 1/4 tab of 5mg twice daily. Last year his T4 was slightly elevated so the dosage was increased to 1/2am and 1/4pm. Values continued to stay around 1.5-1.8 until this past July where he suffered from dehydration and needed subcutaneous fluids, his t4 came back at 4.3. Nothing was changed with his methimazole because the vets attributed the elevated value to the dehydration. We had his t4 tested again in August because he was exhibiting the typical hyperthyroid symptoms of ravenous appetite, increased thirst, excessive shedding, etc and also had some weight loss (about 1.2 lbs) and level came back at 3.2. His dosage was then increased to 1/2 tab twice daily, to recheck 1 month later. His follow up has revealed a significant t4 increase to 4.7! Cat is asymptomatic and seems to be doing well otherwise. We feed him high quality, grain free, single protein wet food twice daily with one or two small 'snacks' in between which is about a 1/2 3 oz can of a similar food, but with a higher fat/calorie content to help him gain his weight back. The vets are all very puzzled as to why the t4 had such an increase despite upping his methimazole dosage. They were worried about compliance but I can assure you the cat takes his medicine faithfully 2x day, everyday. What could be going on?? His health, otherwise, seems to be fine. No vomiting, potty issues, etc
Can you help?

Dr. Mark E. Peterson said...

With time, the thyroid tumor (all hyperthyroid cats have a nodule or tumor of the thyroid) continues to grow, This leads to an increased tumor mass and the need for a larger dose of methimazole. This almost always happens if the cats are closely monitored and live long enough (at least 2 years).

Jennifer said...

Thank you for your response. If you don't mind, can you please clarify what you meant regarding the 2 year time period for cats with hyperthyroidism?

kleomia said...
This comment has been removed by the author.
Dr. Mark E. Peterson said...

After 2 or more years, most cats have an obvious increase in the size of the thyroid tumor(s).

Eeebee said...

Hi there,

Our cat isnt into pills but accepts wet food that's laced with the dose. We prep the doses a few days in advance. We aren't removing the efficacy of the methimazole by dissolving it in wet food a few days before it's "served" are we?

The cat (17 yrs old) has become a lot more playful than he was before we started doing this (when we put the pills into the food directly before he ate it which he's since rejected but his levels were satisfactory) so I was wondering.

Thank you!

Eliza

Dr. Mark E. Peterson said...

Honestly, I've never heard of this approach and I don't know. If your cat's T4 is normalized on the medicine given in the food this way, that would mean it's working.

R Chahal said...

My 16 yr old cat is being treated with Methimazole but few days ago she stopped eating completely, took her to the vet he gave her antibiotics said her mouth was sore but she won't take the pills either for last 2 days and he has told me if she doesn't eat there's nothing he can do , her hyperthyroidism was 180 according to his blood work and was told normal cat is st 60 he wants to increase the dose to 3 tablets a day instead of 2 . Is that too much ? Is there anything that be done to get her eating again ? Thank you for this blog

Dr. Mark E. Peterson said...

Nothing else that the vet can do? First of all, I'd look for another vet and get another opinion.

I'd consider switching to transdermal methimazole if you haven't already done so. You could also treat with surgery, radioiodine, or a low-iodine diet.

Jennifer said...

Hello. This is a follow up to my previous post. With Oscer's T4 increasing from 3.4 to 4.7, Vet changed 5 mg Methimazole from 1/2 pill twice a day to 1 pill am and 1/2 pill pm. If the T4 didn't change after 4 weeks, then vet said he may likely have malabsorption.
After 5 weeks, T4 checked and it is now decreased to 0.9. Oscer also gained 1 1/2lbs. We are happy he gained his lost weight back. Vet concerned with T4 being too low now and wants to bring his meds back down to 1/2 pill twice a day. I am concerned it is going to spike again but I don't want him going into hypothyroidism either. Your thoughts? Does the change in T4 rule out malabsorption?
Also how often should we check CBC and organ function?
Thank you!

Dr. Mark E. Peterson said...

Yes, this rules out malabsorption. If you lower the dose, then the T4 may go high again so you will have to keep retesting at 2-4 week intervals until the you work out the dosage. Remember on the long-term, the thyroid tumor will slowly grow and the dose change (increase) over time. Talk to your vet about dosage and monitoring.

R Chahal said...

Dr. Peterson she hasn't eaten in 2 days and I have tried all types of food, she ate her 2 pills of methimazole . Vet gave her shot of antibiotic yesterday and told me if she doesn't eat there's nothing he can do for her. Could you please help It's very disheartening for me too see her this way . She 16 yrs old . Is there anything else I can ask the Vet to try.

Thank you

Dr. Mark E. Peterson said...

Stop the methimazole to see if the appetite returns. She may need supportive care (SQ fluids etc) for a couple days until the methimazole is out of her body. Also, other diseases need to be ruled out. I'd get another opinion, It doesn't sound like your vet knows what to do with your cat.

Karin N. said...

Hi Dr. Peterson,

I have written to you before on your other blog and I desparately need some advice! To recap, my 13 year old cat was diagnosed with hyperthyroidism in June with a T4 of 11.8. He did not have any of the classic signs he was overweight and had no hair loss. He was started on the transdermal gel which did not work at all. He was switched to methimazole pills 2.5 mg twice a day and T4 came down only to 10. Dose was increased to 5mg twice a day and T4 came down to 5.3. Dose was increased to 7.5 mg twice a day and T4 went to 4.7, increased to 8.75 mg twice a day and down to 4.2. Two weeks later T4 was checked and it increased to 5.2 and his dose was increased to 10 mg twice a day. His T4 was rechecked yesterday and it is 7.9. I am distraught. Now here is the problem - when hisdose was increased to 7.5 mg, I was having a problem pilling him. So, I was quartering the pills and putting them in small pieces of food that he really liked and watching closely that the pills went down - or so I thought. Somehow, he was getting some of the pills up without me knowing it. I found pill fragments everywhere - in and around his bed and in other parts of the house that he frequents. So, during this time I have no idea how much methimazole he was getting. His T4 was coming down nicely until this time and now I feel terrible wondering if my technique had something to do with it. I was since shown how to pill him and the pills are now going down without question. My vet is perplexed and she is going to consult with another vet to see what we can do next. She also did a CBC and a chemistry panel on him and all of his other bloodwork was fine except for the T4. I am considering the radioactive iodine treatment but the facility that is closest to me is closed until January. I don't know what to do at this point. I am scared that I am going to lose him if we can't get his T4 under control. My vet wants me to the keep him at the 10 mg dose twice a day until I hear from her. One other thing is that they also told me that it doesn't matter what time the T4 is checked for cats so his bloodwork times were never consistent which I have questioned more than once. I read on one of your posts that ideally the level should be checked 6 hours post pill. Could that also have something to do with it. Any thoughts you have on all of this would be greatly appreciated!

Dr. Mark E. Peterson said...

Methimazole doesn't work if the cat will not swallow the pills. But is some cats, it just doesn't work. It's not critical when the samples are taken on methimazole.

I'd do either surgery or radioiodine. Add in a beta-blocker if the heart rate is high in the meantime.

Karin N. said...

Thank you Dr. Peterson! I found a specialty clinic about an hour from me that does I-131 treatments. We are working on a referral and hopefully he can have the treatment done on the 17th. My concern is this and I can't seem to get an answer on this - my cat's main symptom of hyperthyroidism is panting when he is stressed (he never did this prior to his diagnosis). Of course, I am concerned to have him hospitalized for 4 days for fear of him having any respiratory distress issues. Have any of your feline patients that you treated with radioiodine had these issues and if so how is this handled?

Dr. Mark E. Peterson said...

We don't see that at our facility very often, especially after the first day. If the heart is fine, then we would use beta-blockers to block the catecholamine effect and that would help.

Karin N. said...

Thank you so much for this blog and all of your information! This is my first experience with a hyperthyroid cat and I have learned much from your site! I am nervous about having the I-131 teatment done but it seems like it will be the best option for him. The methimazole just doesn't seem to work for him. They did split up his dose to 3 times a day which seemed to help a little but his T4 is still 5. With hindsight being 20/20 if I ever have another cat that is diagnosed with hyperthyroidism I will certainly look into radioiodine first. It is a big expense up front but when you weigh out the amount spent on the pills and consistent labwork and also the potential side effects of methimazole, I-131 seems to be a much better option.

Karin N. said...

Hi Dr. Peterson,

I attempted to post something and was having trouble so if this is a duplicate I apologize! I followed your advice and had the I-131 treatment done on my cat on November 17th. His T4 was up to 8 pre-treatment. He was also tachycardic and had developed a heart murmur. He had his post-treatment check up today. His heart rate was normal and his heart murmur was not present. His T4 was 0.9. What is your opinion about his T4 post treatment? Is that a concern for him becoming hypothyroid or do their levels tend to normalize once they adjust to being euthyroid again? When would you recommend he has another T4 drawn? Also, do you recommend any other thyroid labwork or is just the T4 sufficient? Any input would be greatly appreciated! Thanks!

Dr. Mark E. Peterson said...

After radio-iodine, you can't diagnose hypothyroidism based on T4 alone. I recommend monitoring with both T4 and TSH at 1, 3, 6, and 12 months after treatment. If T4 is low to low normal and TSH is high (>0.6 ng/ml), your cat is hypothyroid.

Karin N. said...

Thank you! I will discuss this with his vet and request a TSH be drawn along with a T4.

Andy Emerson said...

Dr. Peterson:

I have a 9 year old cat that quit eating. I took him to the vet the second day without eating and he was diagnosed with hyperthyroidism. He had also lost weight, increased drinking, but I didn't noticed increased appetite. I was surprised with the diagnosis since it sounds like there is an increased appetite. All the other tests were normal other than elevated thyroid. Today is his second day of twice daily methimazole 2.5 mg. After not eating for 2 days, we have also been administering fluids and since he still won't eat, we are syringe feeding Hills a/d. Do some cats with hyperthyroidism stop eating before diagnosis? If so, now that he is on methimazole, how long before his appetite returns? I might consider radio iodine but if he isn't eating, I wonder if there are other issues.

Dr. Mark E. Peterson said...

Anorexia is NOT a sign of hyperthyroidism. If you vet thinks that hyperthyroidism is the problem, you need to find another vet and get to the bottom of this problem. (Your cat may indeed be hyperthyroid, but the that's not the main issue and methimazole commonly causes a DECREASE in appetite!).

Andrea Notnow said...

Dr. Peterson,

I am so lost. My 13 yo cat was diagnosed in May of last year with hyperthyroidism. His level at the time was a 6. We tried liquid methimazole 5 mg once a day and the first week he was fine. After the first week he was constantly vomiting and wasn't eating as much. The vet took him off and said we should watch him since his level was slightly elevated. I took him to another vet last month and his level had reached a 10. The new vet decided we should put him on methimazole again, this time 2.5 mg twice a day in pill form. Same thing happened. After about 5 days he started vomiting (every 1/2 hour-45 mins)lost his appetite and just was not himself. I was told to keep giving it to him and eventually his body would adjust and he wouldn't vomit as much. I should state that before he got sick he initially weighed 18 lbs for years. He is now down to 10, maybe 9 after the last 2 weeks. I picked up a prescription from the vet for anti-vomiting meds, had to give it to him the last time he was on the drug months ago, and I asked for a prescription for Hill's y/d. I took him off the medicine 3 days ago and he is slowly starting to eat again with a little bit of vomiting. I believe, as well as the first vet, that my cat is in the percentage that just can not handle taking methimazole. Do you believe that the y/d food could help? Right now I simply can not afford the radioactive iodine treatment.

Dr. Mark E. Peterson said...

The y/d is not ideal but may help. You obviously can NOT use the methimazole in your cat. Talk to your vet about thyroidectomy.

Andrea Notnow said...

Thank you for responding so quickly. Unfortunately at the moment I can not afford the surgery or iodine procedure which is why I asked my vet for the y/d. I do have one more question to ask if I may. I took my cat off of the methimazole Sunday morning, he is still vomiting, even while being given Metoclopramide 1/4 tab of 5 mg once a day, so we will be going to the vet for a shot to stop it as I had to do the same thing the previous time he was on the meds. I know that vomiting is a side effect of methimazole. How long should it take to leave the system once stopped and/or how long does the vomiting last after stopping? I just feel like the vet I have now has no interest in helping and giving out advice/options. Thank you in advance.

Irina said...

Dr. Peterson, if oral methimazole is not tolerated, can transdermal methimazole be tolerated better?

Dr. Mark E. Peterson said...

Yes, it usually is tolerated better than oral pills.

Dr. Mark E. Peterson said...

Andrea, it can take up to a week before your cat stops vomiting from the methimazole, but generally is only 2-3 days.

Elize van Rensburg said...

Dr Peterson
I have a 20 year old cat with hyperthyroidism. She is still jumping on the bed and have a healthy appetite. Im from South Africa where this condition is not so closely monitored. She is currently on transdermal methimazole, 2.5 mg/ 2x p day. While on leave I arranged for the liquid meds to put on her food. She looks a lot better after 10 days. Is it possible? For this is much more expensive than the cream. Shes to old for a thyroidectomy. Any suggestions on food? She only eat soft food now.

Dr. Mark E. Peterson said...

It should work fine in the food. However, some cats dislike the taste and will not eat the "tainted" food. You should probably work that out before you go away.

Jennifer said...

Dr Peterson,
My 16 year old cat has hyperthyroidism that is controlled with methimazole; his current value is 1.2, which the vet is happy with. Is it normal for his coat to be dull and somewhat greasy?

Dr. Mark E. Peterson said...

No, it's not normal to have a dull and greasy coat. Talk to your vet or get another opinion.

Quin Sweetman said...

Not sure if my previous comment posted. I very much appreciate your site and the advice I have found here since our cat was diagnosed hyperthyroid a couple of months back. I have found it hard to find good information otherwise. Two days ago we returned from a 5-day trip and realized our cat-sitter had given our poor little guy a full milliliter of transdermal methimazole, instead of the .1 ML he was prescribed. I was concerned as his eyes were red in the white areas and his ears looked terrible. I cleaned up his ears with a warm wash cloth and have held off giving him anymore. He now seems mostly fine. Do you think this short-term overdose usually does much harm? I feel like I should give him his prescribed dose again. He is a 13-year old.

Dr. Mark E. Peterson said...

The effect of methimazole wears off after 2-3 days. A 10-fold overdose is not good but hopefully hasn't harmed your cat. Call your vet if you have any concern if your cat isn't right. Otherwise, go back to the old (correct) dose.

Quin Sweetman said...

Thanks so much, Dr. Peterson!

Irina said...

Dr. Peterson, my 15 year old cat is on 1.25 mg felimazole BID. I would like to switch him to transdermal mithimazole and am wondering about appropriate dose. Is there any commonly used conversion factor? How is the transdermal dosage estimated?

thank you so much for your wonderful highly informative site!

Dr. Mark E. Peterson said...

There is no direct conversion. Transdermal medication is made up individually by each pharmacy so the absorption characteristics can vary tremendously. I would go with the same dose and then test your cat in 2-3 weeks and adjust as needed.

Eric Sharps, Ph.D. said...

Dear Dr. Peterson,
Thank you for an excellent overview of methimazole use in cats. Our 14 y.o. cat's T4 seems to have varied quite a bit over the last three years (re: your comment about time-of-day blood sampling?). Still, his appetite and weight are stable. He was getting 7.5mg, b.i.d., but then seemed to start drinking water and urinating a lot more than normal. He now gets 7.5mg in the morning and 5.0mg in the evening...religiously. He is still drinking (and urinating) much more water than historically. My question: In the absence of a near-term T4 assay, should we lower his dosage to 5 and 5? Can this do any harm? We can titrate down now or spread the titration over a week or two. Thank you so much for a clearly written, evidence-based discussion of this widespread problem.

Dr. Mark E. Peterson said...

The increased thirst is not related to methimazole overdosage. I'd make an appointment for a complete checkup with your veterinarian.

Niki Milligan said...

My cat's dose of Felimizole is 7.5 mg, which is currently being given as 2.5 mg in the morning and 2.5 mg in the evening. Is that adequate for control, or would it be better to split the dose into 2.5 mg every 8 hours?

Brilliant blog - really glad I found it :)

Dr. Mark E. Peterson said...

Not really necessary to dose three times a day. The intrathyroidal residence time of the drug is almost 24 hours.

cjcollier said...

Dear Dr. Peterson:
Our 17-year-old cat was diagnosed three months ago with hyperthyroidism and prescribed methimazole. Started at 2.5mg twice per day with no improvement. Dose was doubled to full 5mg twice per day. She's gained no weight back, seems to be starving constantly, and always seems cold (she's taken to sleeping on top of the floor vent whenever the furnace kicks on. She also seems to have developed pain in the joints of her front legs (licking at those joints) and has been prescribed Dasuquin for joint health. Your 9/13/16 Q&A response to Jennifer includes the phrase: "(all hyperthyroid cats have a nodule or tumor of the thyroid)”. At NO point have the words "tumor" or "nodule" been mentioned to us. We'd like to take her to another vet for a second opinion, but aren't sure how to find a vet who might have more experience in endocrinology issues. Is there an online source where we could do a search for a local specialist?

Thank you so much - and thank you for this blog...it's provided so much useful information!

Dr. Mark E. Peterson said...

You should have rechecked your cat's T4 and kidney values 2-3 time (at least) by now. Have you done that? If not, you must base dose modifications on the thyroid levels, not the lack of weight gain!

There is no online list of vets who treat hyperthyroid cats.

cjcollier said...

Yes, they've re-checked levels twice, which is what resulted in the dosage change. Please know that we are NOT modifying her dosage on our own! My bigger concern than the lack of weight gain (although she is "bony" skinny at this point) is the fact that, while we were told she's hyperthyroid, there's never been a mention of nodule or tumor, nor of it being a progressive condition. I'll definitely ask about that when we take her back in, but we also want to make sure we're getting all of the information we need to treat or manage her condition, and understand potential outcomes.

Irina said...

In Massachusetts, we have so called referral or consulting veterinary hospitals,they have most of the specialists including endocrinologists. These specialists have additional training in one or more narrow specialties of veterinary science. Better yet, if you have a veterinary school nearby, you can try to find endocrinologists and good internists there.
Good luck to you and your cat!

Dr. Mark E. Peterson said...

Good. Many vets simply do not know (or choose to ignore) the fact that hyperthyroidism is caused by a thyroid tumor that will grow over time. In some cats, the tumor will grow very slowly and in others quite rapidly (over a year or two).

At 17 years of age, this tumor growth is not as important as it would be in a 9-10 year-old cat, in which the tumor would have many more years to grow and transform.

cjcollier said...

Thank you, Dr. Peterson - and thank YOU, Irina. We're in Washington state, but I'll look for the same type of vet hospitals out this direction if need be.

John S said...

Hi Mark
My 14 year old male had been on Tapazole for several months... 5mg/day at first and 7.5mg/day later... on November 9th we switched from the Tapazole pills to Methimazole liquid... he was getting approx. 1.4ml/day (7mg/day)... after 7 weeks on the Methimazole he seemed to be fine... on Christmas day he had no visible issues... appetite normal, energy levels normal, weight normal, good fur, good purr... 4 days later he was displaying labored breathing... I took him into the vet on Dec 30th... his gum coloration was normal but x-rays revealed fluid in the chest cavity (pleural effusion)... the vet suggested severe heart disease or cancer as the most likely culprits... but fluids would need to be sent to the lab to help determine the underlying problem... because of the New Year's holidays the clinic was closing at noon the next day Dec.31st and would not be reopening for 3 days... the vet drew 800ml of fluid total from his right and left sides to hopefully give him some relief and buy us some time... I took him home for the night to be with his sister aware that it could be our last night together... I asked the vet if I should give him his evening Methimazole and she said yes... we had a very rough sleepless night... there were lengthy periods where he was struggling to get sufficient air... there appeared to be no options and although he seemed to be in slightly less distress between 7:30 am and 11:30 am we had run out of time with the clinic closing at noon... the vet convinced me that I had to let him go... taking him home and risking having him die by suffocation was a gamble I could not take... he was put to sleep at 11:34 am... for 32 days now it has been bothering me terribly that I will never know what the exact cause was... I've been reading a lot about pleural effusion and also methimazole... what I have learned is that pleural effusion is not always a death sentence depending on the underlying cause and that methimazole has been known to cause severe adverse affects over time.
I would greatly appreciate your thoughts... the timing of this unexpected and sudden development just 7 weeks after introducing the methimazole has me wondering what the heck happened here. Sorry for my lack of brevity... thank you for your time Mark.
John

Dr. Mark E. Peterson said...

Methimazole doesn't cause pleural effusion, but long-standing hyperthyroidism certainly can (due to heart failure). That's likely what your cat had.

John S said...

Thank you Mark... that gives me some peace of mind.
John

Jess Cade said...

Dr. Peterson, I would be so grateful to get your thoughts on this situation. My 9-yr-old male cat (overweight but otherwise healthy) started to display some strange symptoms last week -- unproductive gagging, licking lips/swallowing hard, refusing food, third eyelid showing, lethargy. We took him to an emergency vet last Sunday (regular vet closed). They were concerned about an abdominal mass, but xrays came up clear, so vet thought pancreatitis. They prescribed buprenorphine for pain and mirtazapine for appetite stimulation, and recommended Pepcid as well, all of which we've been giving him.

On Monday he was still refusing food and had started to shiver/tremble, so we went to our regular vet this time and they ran a full panel of bloodwork. It came back positive for hyperthyroid, with a result of 3.5 out of 4.7. Vet has started him on methimazole twice a day (he's had four doses) but we haven't seen much change in him so far. I know it can take a while to work.

My concerns are that he has NOT displayed the typical symptoms of hyperthyroidism (weight loss, extra activity, begging for food, increased thirst). I don't dispute the findings of the bloodwork, but I'm puzzled by the fact that the symptoms don't seem to match the diagnosis. His predominant issues seem to be nausea and pain (confirmed by second vet), not weight loss and frenetic activity. He's not begging for food, he's refusing it. Could the diagnosis still be accurate and he's just atypical? Or do you think there's something additional going on? Please, any help you can give would be so appreciated. Thank you!!

kelsey moline said...

Dr. Mark! You're great. This thread is amazing....

I have a question for you. My 9 year old fixed male cat yoshi was recently diagnosed hyperthyroid. He's been prescribed 2.5 felimazole 2x daily. He's only been on it a week but he's already gained back weight and is back to his old self! This stuff is magic. My only problem is I'm a flight attendant and I have to occasionally leave him alone for 24 hours. It won't be for the next few weeks so he'll have a month of straight treatment followed by a blood test again to make sure it's working. I can possibly get my landlord to feed him the pills while I'm gone but I'm thinking with my crazy schedule there will probably be a few days a month where a dose will not be able to be given for 12-30 hours max. With all these tests and medication I barely have money to pay rent let alone pay a pet sitter. How serious is it to miss a day every few weeks? He has no other issues and his blood work besides this was completely normal and actually the vet said besides this issue he's remarkably healthy. I'm just a worried mom. He's responded so well to the medication and has no side effects except possibly abnormal vocalizations haha. Thanks for this thread you're awesome to do this!
--kiki

Dr. Mark E. Peterson said...

First of all, if your cat's T4 is 3.5 ug/dL, that is normal. And you are correct, you diagnose hyperthyroidism on more than just a single lab test. Even if your cat is borderline hyperthyroid, you still need to figure out what is wrong and treat that condition. I'd get another opinion from a different vet. Treating a cat that isn't eating with methimazole may not be malpractice, but it's getting pretty close.

Dr. Mark E. Peterson said...

Kiki, if you can't pill your cat daily, then you should talk to your vet about the other treatment options.

Jess Cade said...

Dr. Peterson,
Thank you for your feedback. I was told by the second vet, who ran the bloodwork, that 3.5 is in the "gray zone" or "danger zone" for an older cat that's symptomatic. It's good to know that you disagree. I'll try to get another vet's opinion. Thank you so much.

Niki Milligan said...

Dear Dr Peterson

We've been battling with our 16 year old's treatment since diagnosis.

First signs were over-grooming, nausea and loose bowels, but the vet missed it (several times) for a few years :(
Only when she went into severe weight loss with a clearly swollen thyroid was the problem picked up.

Since treatment started we've been trying to get her built up a bit before the surgery as she was underweight and a bit frail.

We've had to up the dose once already (following bloods), so I've two questions:

(1) Thyroid Treatment in the Face of Vomiting
When the thyroid gets out of control again she gets vomiting and diarrhea.
Do we have to get the vomiting fully under control before methiamazole treatment can recommence, as it causes stomach problems (not easy if it is a runaway thyroid causing the problem to start with)?
Or would it be better to give something like lansoprazole (or whatever the vet recommends) an hour or two before the methimazole to minimize problems and get the T4 levels down?
The last time he gave her an anti-vomiting shot which certainly stopped the vomiting but we actually thought she was going to die she was so poorly after it (the vet also blew the jugular during bloods, but there was no sign of bleeding out).

(2) Finding the Right Vet
Surgery is definitely on the cards, even though we are concerned because of her age and weight, as the medication is causing her too many problems.
However, I'm not convinced our local country vet is the best person for it and have been thinking of going to one of the more expensive city practices.
What questions would you suggest for ascertaining the best vet for the job?

Thank you!

Irina said...

Dr. Peterson, where exactly in the GI tract is oral methimazole (felimazole) absorbed? could the absorbtion be hampered by placing the pill into a gelatin capsule? My cat has a concurrent malabsorption with mostly small intestine involved (B12=229pg/ml(290-1500 ref), folate is in the normal range. he is on felimazole for about a year and his T4 got elevated recently to 5.7 from 1.4 in the fall (0.8-4.0 ref)right about the time i stated using gelatin capsules bc he is also on prednisolone for malabsorbtion.
could it be he is getting used to felimazole, or his absorbtion is getting worse or the gelatin capsule is preventing absorbtion?
thank you so much for your time and for this extremely informative blog!
Irina

Dr. Mark E. Peterson said...

This is a progressive disease, caused by one or two hyperfunctional thyroid tumors. With time, we expect that the tumors will slow enlarge and that we need to increase the methimazole dose to overcome/block the effects of the larger tumor volume.

Botton line, you need to raise the dose.

Dr. Mark E. Peterson said...

Niki,

Most hyperthyroid cats vomit after eating because they eat too much too fast. In your cat, it sounds much more complicated. Talk to your vet and follow his/her advise.

Niki Milligan said...

Hi Dr Peterson.

Amber lost her fight on Saturday.

Thank you for all your help during her last, long, struggle with hyperthyroidism. Dreadful disease and dreadful medication, but at least she had a tiny breathing space towards the end with the transdermal gel.

Thanks again.

Niki

Dr. Mark E. Peterson said...

Dear Niki,

So sorry for your loss. I'm glad that I was of some help to you and Amber.

Dr. Peterson

Petra said...

Dear Dr. Peterson,

I am writing from Germany so I want to apologize for any grammar, spelling or other mistakes in advance.

My cat has taken Felimazole now for about three years. We started with 2.5mg/day,then the dosage went up to 10mg/day, now we are down again to 2.5mg/day. The T4 goes up and down, up and down.
Along with that problems with my cat's kidneys began. Right now she gets treated with Semintra and SUC.
The T4 currently is 6.0, CREA is 2.0. (four weeks ago her T4 was 0.9 and CREA 3.4; the dosage then was lowered from 5 mg/day of Felimazole to 2.5mg/day).
My vet now wants me to give her 2.5mg of Felimazole on one day and 3.75mg of Felimazole on the other day since she thinks 6.0 is too high. I am worried about the kidneys if I increase the dosage again.
Plus: Is it a good thing not to give the same dosage every day?

Thanks in advance for your help!

Petra

Dr. Mark E. Peterson said...

You need to worry about the kidneys and should treat and manage CKD as in any cat with the disease. The hyperthyroidism needs to be controlled as well as you can. Unfortunately, the long-term prognosis for your cat is guarded and there is not a good solution.

This is the problem with methimazole. It allows the hyperthyroidism (thyroid tumor) to progress so we need to continue to increase the dose of the medication. If you had treated with surgery or radioiodine 3 years ago when the tumor was smaller, then we'd only have to worry about CKD now.

TheMsWrong said...

Hello Dr. Peterson,

My 12y/o cat was diagnosed with hyperthyroidism last Wednesday, June 21st, and was put on 5mg methimazole twice daily. However, recently he has been not his normal self. He sleeps a lot, and isn't as social as he usually is. He is still eating and is not vomiting but I'm just worried because instead of staying around me or my mom, he is isolating himself to a different room. Additionally, he was not excited for his morning treats anymore (it's normally a very big deal for him!)

I contacted our vet and he suggested lowering the dosage to 2.5mg twice daily, but after researching for hours online, we decided that 1.25mg twice daily might be better to start with. Since doing this, he's seemed a little bit better and more active, but is still not his usual self.

Is this a side effect of methimazole? Is it common for a cat to become lethargic so quickly after taking the medication? We were also thinking that we had become used to him being overactive due to the hyperthyroidism (before we knew he had it.)

Any insight is appreciated - I just want some peace of mind that my boy will be okay.

Thank you,

Kelsey

Dr. Mark E. Peterson said...

You want the lowest dose possible that will control the hyperthyroidism. This drug has many potential side effects.

Remember that this is a progressive disease (the thyroid tumor will continue to grow) so if your 12-year old cat is healthy, long-term methimazole is probably not the best choice. In young, healthy cats, definitive treatment (surgery or radioiodine) are best to remove or ablate the tumor.

Dulce24 said...

Hi Dr Peterson,
This is a great informative blog and it's amazing that you continue to answer questions. I wish I'd done research before I gave my cat this med but I was too paranoid to search google after losing a cat last year to lymphoma, I didn't want to read horror stories... It's alarming that local vets are giving this med out like candy when it can have terrible effects. My cat was diagnosed 4 weeks ago and I started him on the med which was prescribed 5mg BID, which I'm understanding now is high. This past weekend I took him to the ER because he has pruitis and was vomiting every 20-60 minutes for about 36hrs. Clearly he is in the percentile that can't tolerate the med, but who knows if he was titrated up, the results might've been different. I was hesitant to go the radioiodine route due to the cost, but after the ER bill, it's just a little more. Plus he can't tolerate the medication at all.

I don't think general vets should be prescribing this medication, and should be referred to endcrinologists or specialists. Luckily my cat was only on it consistently for 2 weeks and is returning to normal after sleeping for 2 days. He is still scratching and throwing up once per day, but his energy, mood and litter habits are back to normal. The radioiodine appt is Wed so my fingers are crossed. I just wanted to share my story and say thank you for the free information.