Methimazole For Cats: Overview, Dosage & Ancillary Effects

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Methimazole, for which accepted brands accommodate Felimazole and Tapazole, is an articulate medication frequently acclimated in veterinary anesthetic to amusement bodies with hyperthyroidism. In this article, you’ll apprentice what methimazole is, how it works, ancillary furnishings to adviser for, and some frequently asked questions.

Methimazole For Bodies Overview

Medication Type:
Antithyroid medication
Oral tablets
Prescription Required?:
FDA Approved?:
Brand Names:
Felimazole, Tapazole
Common Names:
Available Dosages:
Felimazole tablets: 2.5 and 5mg; generic: 5mg and 10mg
Expiration Range:
Products should be acclimated afore the cessation on the package. Tablets should be stored at allowance temperature and adequate from moisture.

About Methimazole For Cats

Methimazole is classified as an antithyroid medication. Essentially, it inhibits the amalgam or assembly of thyroid hormone buried by the thyroid glands. 

Methimazole is a actual accepted and admired medication for use in alleviative bodies with hyperthyroidism area the thyroid glands aftermath too abundant thyroid hormone (T4), arch to chronically debilitating furnishings on the body. 

One cast of methimazole, alleged Felimazole and bogus by Dechra Pharmaceuticals, is FDA accustomed for use in cats. Tapazole, a animal brand, is frequently acclimated off-label in cats. There are additionally abounding all-encompassing forms of methimazole. 

Although a brace of analysis methods for hyperthyroidism are available, including methimazole, iodine 131 treatment, surgical abatement of the overactive thyroid gland, and a decree diet, methimazole’s greatest account is that it is a actual bargain way to get adequately quick ascendancy over the disease. 

What Does Methimazole Do For Cats?

Feline hyperthyroidism is a actual accepted ache in earlier cats. It is advised the best accepted endocrine ache that afflicts earlier cats. 

An endocrine ache is one that apropos glands aural the anatomy that aftermath hormones. In the case of hyperthyroidism in cats, the glands are bearing too abundant T4 and T3 thyroid hormones. 

The aftereffect that too abundant thyroid hormone places on the anatomy can alter from cat to cat, but is about boundless throughout the body. Aerial thyroid levels access metabolism, about putting a kitty’s anatomy on overdrive.

Classically hyperthyroid bodies are usually at atomic 10 years of age or older, and advance a avaricious appetence accompanying with weight loss.

Hyperthyroidism in bodies is best generally acquired by a amiable bump of the thyroid glands alleged an adenoma. Absolutely annihilative cancerous tumors of the thyroid gland are auspiciously advised actual attenuate in cats, authoritative medical administration actual feasible. 

Methimazole blocks the amalgam and absolution of thyroid hormone from the thyroid gland, thereby authoritative overproduction and the furnishings of the disease. The bulk of hormone assembly that is blocked depends on the dosage acclimated and anniversary alone cat. In this way however, methimazole is a medication that can alone manage, but not cure, hyperthyroidism.

Side Furnishings Of Methimazole For Cats

If bodies acquaintance ancillary furnishings from methimazole they usually arise aural the aboriginal three months of use.

Many bodies abide methimazole well, but there are some added accepted and aberrant adverse furnishings to be acquainted of, which best generally action aural the aboriginal three months of methimazole use. The best accepted ancillary furnishings accommodate vomiting, accident of appetite/anorexia, and boundless tiredness/lethargy. 

Gastrointestinal upset furnishings action in about 10% of cats. In some cats, this may be accompanying to the absinthian aftertaste of the book form, and ambuscade it in aliment or a amusement may help. 

Other, added austere ancillary aftereffect abnormalities auspiciously action beneath commonly. Claret disorders, like clotting/platelet issues, bleeding, and low corpuscle counts action in about 4% of cats. 

Intense facial itching, termed pruritis, may action in 2% to 3% of kitties, and is generally apparent added quickly, aural the aboriginal three weeks of treatment.

Other attenuate but austere furnishings can accommodate alarmist ache and an immune-mediated disorder, both of which action in beneath than 2% of cases. 

Even added rarely, a neuromuscular ataxia alleged myasthenia gravis has been seen.

Because earlier bodies are added generally afflicted by hyperthyroidism, added diseases may sometimes already be present that can complicate treatment. The best accepted is kidney disease. Back hyperthyroidism is present, it acutely increases the filtration amount in the kidneys, which can affectation basal branch dysfunction.

This dysfunction may alone be acclaimed afterwards starting the medication, as the filtration amount in the kidneys slows bottomward with the normalizing metabolic rate. This is one primary acumen that lab assignment rechecks are frequently bare to adviser agency function. 

In bodies with abstinent or astringent branch ache already present, any analysis for hyperthyroidism should be anxiously discussed, as analysis is acceptable to accomplish branch ache worse. In the author’s experience, analysis alike in bodies with balmy branch ache who are anew diagnosed as hyperthyroid, should be approached cautiously. 

Methimazole may account complications in kitties with added disorders that may be present, including diabetes mellitus, alarmist disease, or any above-mentioned blood-related disorders or disorders of the allowed system. 

Several types of medications may collaborate with methimazole. It’s actual important to accomplish your veterinarian acquainted of any added medications or supplements your backing is currently demography back discussing use of methimazole.

Overdoses of methimazole are best acceptable to briefly abet the best accepted adverse effects, like GI agitated and lethargy. Furnishings on the alarmist may additionally occur, as able-bodied as claret disorders like anemia and low platelet count. Furnishings may abundantly depend on the dosage of methimazole and any added diseases a backing may additionally have.

If you are anytime anxious that your backing may accept developed ancillary furnishings while application methimazole, or accustomed an balance of the medication, accomplish abiding to acquaintance your veterinarian, the ASPCA Beastly Adulteration Ascendancy Center (1-888-426-4435), or Pet Adulteration Helpline (1-855-764-7661) for added advice. 

In humans, methimazole is advised a teratogen, acceptation that it can account bearing defects in a developing fetus. Abundant women or women who may become pregnant, should abrasion gloves back administration methimazole, as able-bodied as the clutter or any actual fluids (i.e., urine or vomit) from advised cats.

In bodies that do advance digestive agitated ancillary furnishings from methimazole, alteration to the contemporary transdermal anatomy may be an option, as abdomen affliction by the book is bypassed. In bodies that advance any of the added austere ancillary furnishings area methimazole needs to be discontinued, added analysis modalities for hyperthyroidism, like iodine-131, surgery, or a decree diet would be bare to administer the disease. 

Methimazole For Bodies Dosage

The FDA-approved cast name artefact Felimazole (methimazole) is a book accustomed by mouth.

Per the characterization for the FDA-approved artefact Felimazole for cats, the dosage for methimazole starts at 2.5 milligrams per cat every 12 hours. The dosage is again titrated/adjusted depending on amend lab assignment afterwards three weeks. The dosage should alone be adapted in 2.5-milligram increments.

The best absolute dosage for Felimazole is 20 milligrams per day disconnected in two 12-hours doses of not added than 10 milligrams each. 

Many bodies with hyperthyroidism may be adapted able-bodied at 2.5 milligrams alert a day, admitting adjustments may be needed, which is why amend lab assignment is so important, abnormally back starting out.

In bodies that don’t abide the book anatomy of methimazole, either because of digestive agitated effects, or a backing ancestor accepting adversity at home giving a book to their cat, articulate aqueous and contemporary transdermal affairs can be formulated through compounding pharmacies. 

While easier to administer, it is important to agenda that the transdermal alertness generally does not assignment as calmly or as able-bodied as the articulate form, generally demography best to ability levels of control, and acute college doses. 

The transdermal anatomy should additionally consistently be activated by a animal with gloves so as to anticipate assimilation of the medication through their own bark by accident. 

For pet parents because one of the added alleviative analysis options for hyperthyroidism, like surgical abatement of the overactive gland or iodine-131 treatment, a short-course of methimazole is still generally assigned to both get affection beneath ascendancy until the procedure, as able-bodied as to ensure ascendancy is accessible and that accessory branch ache does not advance afterwards thyroid action allotment added to normal. 


Methimazole is a actual frequently assigned medication for bodies with hyperthyroidism. Its allowances are that it works aural alone a brace of weeks to ascendancy signs of the disorder, and it is actual inexpensive. 

Side furnishings are about beneath common, but are well-documented and possible. However, back added analysis methods may be beneath reliable (prescription diet) or acutely big-ticket (surgery and iodine-131 treatment), methimazole still charcoal the go-to medication for administration of this accepted condition. 

Frequently Asked Questions

What are the ancillary furnishings of methimazole for cats?

There are several abeyant ancillary furnishings that can be apparent with methimazole in cats. The best accepted accommodate digestive agitated like airsickness and poor appetite, as able-bodied as lethargy. 

Sometimes GI agitated is due to the absinthian aftertaste of the articulate tablet, and may be remedied by award a way to adumbrate the aftertaste or accepting the medication circuitous into a flavored form. 

More austere ancillary furnishings that are beneath frequently apparent with methimazole accommodate claret disorders like low platelets counts and anemia, alarmist disease, astringent facial itching, and autoimmune disease. 

Fortunately, in abounding cases, these furnishings will about-face back the medication is discontinued. But abominably in these cases, use of methimazole in any anatomy charge about be absolutely discontinued, and added analysis methods for hyperthyroidism, like a decree diet, surgery, or iodine-131 treatment, charge be pursued instead. 

How continued can a cat alive on methimazole?

This varies, but on boilerplate a cat may be accepted to alive anywhere from two to bristles years with hyperthyroidism actuality managed with methimazole. 

Variability in this appraisal can be due to several factors. Adolescent bodies diagnosed beforehand with the ache may accept a best activity assumption than a abundant earlier cat who is diagnosed afterwards furnishings of the ache (such as weight loss, claret pressure, affection dysfunction, branch disease) accept already taken their assessment on the body.

Cats with above-mentioned diseases, like branch disease, diabetes, or primary affection ache may additionally book beneath able-bodied with analysis of hyperthyroidism in general. 

According to a abstraction appear in 2006 in the Account of the American Veterinary Medical Affiliation that followed about 170 cats, about one-third advised with methimazole abandoned lived for two years on average. Addition one-third advised with iodine-131 lived for four years on average, while the actual one-third who were advised aboriginal with methimazole and again with iodine-131 lived the longest, at about bristles years. 

Keep in apperception that with best bodies actuality diagnosed about amid 10 and 15 years of age (12 years on average), these activity assumption estimates are still attractive at a cat authoritative it added into after years with a appropriate affection of life. 

How continued does methimazole booty to assignment in cats?

Oral methimazole has been begin to ability a aiguille absorption in the anatomy in alone 30 to 60 minutes. However, its aftereffect on signs and affection of hyperthyroidism do booty best to realize. 

The apprehension is that three to four weeks afterwards aboriginal starting therapy, the aftereffect the starting dosage has on thyroid action will be well-represented on bloodwork. If thyroid levels are still high, it is again adapted to access the dosage. The dosage should never be added or decreased based alone on the actualization or signs a cat is assuming (unless we’re seeing an adverse aftereffect or toxicity concern).

The furnishings of hyperthyroidism on the anatomy of a kitty, like weight loss, beef loss, accessory affection disease, aerial claret pressure, etc., while about reversible, will usually booty several weeks to change noticeably. 

What is the activity assumption of a cat with hyperthyroidism?

Again, as analogously answered above, the aftereffect can be variable, but is generally at atomic two years in bodies with no added diseases complicating things, like branch disease.

Overall, based on the 2006 abstraction referred to above, the best aftereffect came from bodies aboriginal adapted with methimazole who were again advised with iodine-131, who on boilerplate lived for bristles years. 

Iodine-131 is advised the ultimate analysis of best for bodies with hyperthyroidism back the analysis alone targets the overactive thyroid tissue while attention accustomed tissue. While anaplasty can additionally be effective, the absolute gland is removed, and if any devious overactive thyroid tissue is present (which is alleged ectopic thyroid tissue), connected signs of hyperthyroidism may continue.

While I-131 and anaplasty are both advised cures for hyperthyroidism and ceremony of the ache is actual rare, they are both added big-ticket options, abrogation abiding administration of the ache with methimazole the best applicable advantage for abounding folks. 

It is doubtable that administration of hyperthyroidism with methimazole over the continued appellation still agency that the ache progresses, aloof at a decidedly slower rate, acceptable attributable to the aberration in aftereffect application medical administration abandoned against advancing a cure with I-131.

About Dr. Chris Vanderhoof, DVM, MPH

Dr. Chris Vanderhoof is a 2013 alum of the Virginia-Maryland Academy of Veterinary Anesthetic (VMCVM) at Virginia Tech, breadth he additionally becoming a Masters in Accessible Health. He completed a alternating internship with Red Coffer Veterinary Hospital in New Jersey and now works as a accepted practitioner in the Washington D.C. area. Dr. Vanderhoof is additionally a copywriter specializing in the beastly bloom acreage and architect of Ascendant Beastly Bloom Autograph Solutions, which can be begin at Dr. Vanderhoof lives in the Arctic Virginia breadth with his family, including 3 cats.

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