Hyperthyroidism in cats: methimazole, carbimazole and other treatments
How hyperthyroidism shows up in older cats, how vets confirm it, and how methimazole, carbimazole (Vidalta), radioiodine, surgery and iodine-restricted food compare.
Cats get an overactive thyroid too, and much more often than most owners expect. The medicines vets use are the same two molecules people take, methimazole and carbimazole, but in veterinary products with cat doses. This section explains the disease and the options. Your vet makes the diagnosis and sets every dose.
Key points
- Common in older cats: the FDA calls it the most common hormone disease in cats over eight.
- Classic sign: weight loss even though the cat is eating more.
- Four treatments: daily medicine, radioactive iodine, surgery, or iodine-restricted food.
- Kidneys need watching: treating the thyroid can reveal kidney disease the high hormone levels were hiding.
- Use cat products: human tablets are not proven safe in cats, and the FDA warns about unapproved “generic” tablets sold online.
What causes it
In almost all cats it is a benign growth in the thyroid gland that makes too much hormone; the FDA puts that at almost 99% of cases. Thyroid cancer is rare. Too much hormone pushes the whole body into overdrive.
Signs to look for
The FDA lists weight loss despite a bigger appetite as the most common sign. Others are vomiting, diarrhoea, restlessness, drinking and urinating more, and an unkempt coat. It creeps up slowly, so changes are easy to put down to age. Left untreated it causes high blood pressure, which can detach the retina and blind the cat suddenly, and it thickens and eventually weakens the heart. The FDA describes untreated hyperthyroidism as almost always fatal.
How vets confirm it
Your vet may feel an enlarged thyroid lobe in the neck, then confirms the diagnosis with a blood test for thyroid hormone (total T4). Before starting medicine they usually also check a full blood count, kidney and liver values, and blood pressure.
The treatment options
| Medicine | Radioactive iodine | Surgery | Iodine-restricted food | |
|---|---|---|---|---|
| What it does | Blocks hormone production | Destroys the overactive tissue | Removes the affected lobe(s) | Starves the gland of iodine |
| How long | Daily, for life, unless another option follows | One treatment; 3 days to 4 weeks in hospital, then about 2 weeks of restricted contact at home | One operation; hospital stay | For life, only that food |
| Cure | No: hormone rises within days of stopping | 95% or more | 90% or more if both lobes are removed; 35% to 60% if one | No |
| Main drawbacks | Drug reactions in up to 25% of cats; daily dosing; regular blood tests | Licensed facility; you cannot visit during the stay | General anaesthesia; low calcium if the parathyroid glands are damaged (6% to 82% depending on technique), voice nerve damage | Cat must eat nothing else |
Medicine is where most cats start, either for life or to stabilise them before radioiodine or surgery. The two choices are methimazole (Felimazole, Felanorm, Thyronorm and generics) and carbimazole (Vidalta).
Radioiodine is what the 2016 AAFP guideline calls the treatment of choice for most cats, because it cures with one treatment and no anaesthetic. It also has strong results in practice. In a series of 524 cats treated with a single injection, 94.2% had a good response; only 1.5% stayed hyperthyroid at six months, 2.1% needed thyroid hormone afterwards, and 2.5% relapsed one to six years later.
Iodine-restricted food works only if the cat eats nothing else, including treats and hunting. In a small study of eight cats fed such a diet for six months, thyroid levels fell into the normal range within 8 to 16 weeks in all of them.
The kidney question
High thyroid hormone pushes more blood through the kidneys, which can mask kidney disease. Once any treatment brings hormone down, kidney values can rise. Overtreating makes it worse: in one study, cats that became underactive after treatment developed raised kidney values more often (16 of 28) than cats kept in the normal range (14 of 47), and those that did had shorter survival. This is why a trial of medicine often comes before a permanent treatment, and why vets check kidney values at every visit.
Human tablets and online “generics”
Protecting people at home
Methimazole can cause birth defects in people. The FDA advises washing hands after giving a dose, wearing gloves to handle tablets, the litter tray, vomit or urine, and extra care for anyone pregnant or breastfeeding.
Cat medicines in detail
Sources
Checked 15 October 2026.
- Hyperthyroidism in cats, two FDA-approved drugs available to treat it — US Food and Drug Administration
- FDA approves first generic methimazole coated tablets for treating hyperthyroidism in cats (July 2025) — US Food and Drug Administration
- 2016 AAFP guidelines for the management of feline hyperthyroidism (Carney et al.) — Journal of Feline Medicine and Surgery
- Radioiodine treatment of 524 cats with hyperthyroidism (Peterson and Becker, 1995) — Journal of the American Veterinary Medical Association
- Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats (Williams et al., 2010) — Journal of Veterinary Internal Medicine
- Effect of a limited iodine diet on iodine uptake by thyroid glands in hyperthyroid cats (2015) — Journal of Veterinary Internal Medicine