Hyperthyroidism in cats
An overactive thyroid is the commonest hormone disease of older cats. It can be controlled for life with methimazole, cured with radioactive iodine or surgery, or managed with an iodine-restricted diet. Methimazole is a cheap human generic; the choice between tablets, a compounded ear gel and a one-off cure is the decision most owners face.
What it is
A benign overgrowth of the thyroid gland pumps out excess hormone, and the cat loses weight despite a ravenous appetite, becomes restless and vocal, vomits, has a fast heart and may develop heart disease and high blood pressure. It appears mostly in cats over ten and is confirmed by a blood test, with kidney and heart checks alongside because the high hormone level can hide kidney disease.
Four treatments exist and all work. Medical management with methimazole controls the disease but does not cure it and needs lifelong dosing and monitoring. Radioactive iodine, given once at a licensed facility with a few days’ isolation, cures most cats and is often the best long-term value. Surgery to remove the gland is used less now. A prescription diet very low in iodine controls the disease if the cat eats nothing else.
How it is treated
Methimazole is given once or twice daily, starting low, with thyroid levels, blood counts, liver and kidney values checked after a few weeks and the dose adjusted. Felimazole is the veterinary tablet; the human generic is the same drug. For cats that cannot be pilled, compounding pharmacies make a transdermal gel rubbed into the inner ear, which works for most cats with slightly less predictable absorption. Vomiting, facial itching and, rarely, serious blood or liver effects in the first months are the things watched for.
Beta blockers such as atenolol are sometimes added briefly for the fast heart while the thyroid comes under control. If kidney disease emerges as the thyroid is treated, the dose is balanced between the two. A cat stabilised on methimazole can then have radioactive iodine as a cure if the owner chooses.
What an owner pays cash for
On medical management the owner pays for methimazole every month for life plus periodic blood tests. Tablets are cheap: the human generic at a pharmacy is mostly dispensing fee, and the veterinary brand costs more; the transdermal gel is a compounded product with pharmacist time in it and is a convenience rather than a saving. Over several years the lifetime cost of tablets, gel and monitoring can approach the one-off cost of radioactive iodine, which is why vets raise the cure early.
The medicines, one guide each
- MethimazolePharmacy can fill
Methimazole controls an overactive thyroid in cats, one of the commonest diseases of older cats. Felimazole is the veterinary tablet; the human generic is the same drug and a pharmacy can fill a vet’s prescription for it. Compounders also make a transdermal gel applied to the ear for cats that will not take tablets.
- Benazepril and enalaprilPharmacy can fill
Benazepril and enalapril are ACE inhibitors used in dogs with heart failure and in dogs and cats with kidney disease and protein loss in the urine. Veterinary versions exist, but the human generics are the same drugs and are what most pharmacies dispense on a vet’s prescription.
- Famotidine and omeprazolePharmacy can fill
Famotidine and omeprazole reduce stomach acid in dogs and cats with vomiting, reflux, ulcers or kidney disease, and omeprazole treats gastric ulcers in horses. Both are human generics also sold over the counter, so for most pets they are among the cheapest prescriptions there are.
Questions for your vet
- Has my cat’s kidney function and blood pressure been checked alongside?
- Tablets, transdermal gel, radioactive iodine or diet for my cat, and why?
- When are the thyroid, blood count, liver and kidney tests?
- Can the prescription be written for the human generic tablet?
- How do I handle methimazole safely?
How Script Unlock gets you a cash price for your pet’s prescription
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Sources
- Merck Veterinary Manual
- FDA Approved Animal Drug Products (Green Book)
- American Veterinary Medical Association
Written by the Script Unlock editorial team from the sources above. Not yet reviewed by a veterinarian. Last checked 2026-10-03. Your vet\u2019s diagnosis and plan come first; nothing here replaces them.
Script Unlock is a marketplace that lets licensed pharmacies quote a cash price on your prescription. It does not prescribe, dispense or sell medication, does not process patient payments, and does not give medical advice. Always follow the instructions of your prescriber and pharmacist and read the medication guide supplied with your medicine.