By Dr. Michelle Frye, DVM · Published 2026-05-01
TL;DR. Methimazole (Felimazole) treats feline hyperthyroidism by blocking thyroid hormone production. Dosed 1.25–5 mg twice daily. Comes as a tablet, compounded liquid, or transdermal ear gel. Recheck T4, kidney values, and CBC at 2–4 weeks after starting. It controls the disease, it doesn't cure it.
The Cat Who Eats Like a Linebacker and Wastes Away
The classic story: a 14-year-old cat starts hollering at 5 a.m. for breakfast. She eats two cans. Then begs for a third. The food bowl never seems to register. She's drinking more water, prowling the kitchen at midnight, and somehow getting thinner. The thyroid panel comes back: total T4 of 7.5 (reference 0.8–4.0). Hyperthyroidism. The most common endocrine disease of senior cats, and one of the most rewarding to treat.
Methimazole is, for most owners, the place we start.
What Methimazole Actually Does
Methimazole blocks the enzyme thyroid peroxidase, slowing the thyroid gland's overproduction of T4 and T3. It does not destroy the abnormal tissue or cure the disease — it manages it for as long as the cat takes the drug.
- Starting dose: 1.25–2.5 mg twice daily, adjusted by recheck T4.
- Onset: T4 starts dropping within days, full normalization in 2–4 weeks.
- Forms: oral tablet (Felimazole), pharmacy-compounded liquid, transdermal ear gel for cats who refuse pills.
- Definitive cures: radioactive iodine (I-131) or surgical thyroidectomy. Methimazole keeps the cat comfortable, including before and after I-131.
Things People Are Wrong About
Myth 1: "It cures hyperthyroidism." No. It controls it. Stop the drug, the thyroid resumes overproducing within days.
Myth 2: "My cat looks better, so we can skip the bloodwork." The most important reason for the 2–4 week recheck is not the thyroid — it's the kidneys. Untreated hyperthyroidism falsely props up kidney values; lowering the T4 unmasks underlying chronic kidney disease in roughly 30% of cats. Catching that early matters.
Myth 3: "Transdermal works as well as the pill." Mostly true, but absorption is variable. Transdermal cats need slightly higher doses and stricter monitoring — a fair trade for owners whose cats refuse to be pilled.
Myth 4: "It's safe to handle anywhere." Wear gloves, especially if you are pregnant or could be. Methimazole crosses skin.
When NOT to Use Methimazole (and What to Do Instead)
Methimazole is contraindicated in cats with serious blood-cell suppression or autoimmune reactions to a previous dose. About 5–10% of cats develop facial pruritus, GI upset, or rare hepatic or hematologic effects in the first month — call your veterinarian if you see scratching at the face, lethargy, jaundice, or unexpected bleeding. For lifetime control without daily pills, radioactive iodine (I-131) at a referral center is the gold standard cure.
What I Tell Owners After 30 Years
A treated hyperthyroid cat is one of the most dramatic transformations you'll see in feline medicine. Within a month, she's gaining weight, sleeping through the night, and her coat finally looks like a coat again. Methimazole gets us there cheaply and reliably. But it is a daily medication forever — if your cat is going to live another 5+ years, talk to your vet about whether I-131 is the better long-term answer for your household.
Frequently Asked Questions
What is methimazole used for in cats?
Methimazole treats feline hyperthyroidism — the overproduction of thyroid hormone — by blocking the enzyme thyroid peroxidase. It is the most commonly prescribed first-line treatment in the U.S.
How long does methimazole take to work?
Total T4 begins dropping within days, with most cats reaching the normal range in 2–4 weeks. A recheck T4 and chemistry panel are recommended at the 2–4 week mark.
What is the difference between methimazole and Felimazole?
Felimazole is the FDA-approved veterinary brand of methimazole, formulated specifically for cats. The active ingredient is identical to generic methimazole.
Can methimazole be given as a transdermal gel?
Yes. Pharmacy-compounded transdermal methimazole is applied to the inner ear flap and is widely used for cats who resist pills. Wear gloves to apply and rotate ears.
What are the side effects of methimazole?
About 5–10% of cats develop facial pruritus, GI upset, or lethargy in the first month. Rare but serious effects include blood-cell suppression and liver effects — bloodwork at 2–4 weeks is essential.
Does methimazole cure hyperthyroidism?
No. It controls the disease for as long as the cat takes it. Definitive cures are radioactive iodine therapy (I-131) and surgical thyroidectomy.
Why does my cat need bloodwork after starting methimazole?
Lowering elevated T4 can unmask underlying chronic kidney disease in roughly 30% of hyperthyroid cats. The 2–4 week recheck monitors thyroid, kidney, and blood-cell values.
Where can I buy methimazole for my cat?
Methimazole is prescription-only. Smarty Vets dispenses Felimazole tablets, generic methimazole, and pharmacy-compounded liquid and transdermal options.
This article is for educational purposes and is not a substitute for veterinary advice. Always consult your veterinarian before starting, stopping, or changing any medication for your pet.