Hyperthyroidism in cats: treatment options compared
Checked against veterinary references: October 2026
Short answer
Hyperthyroidism is the most common hormone disorder in older cats, and it is very treatable. Methimazole tablets or ear gel control it daily and are inexpensive; radioactive iodine usually cures it with a single treatment; an iodine-restricted diet or surgery are other options. Because treatment can unmask kidney disease, vets usually start with methimazole before a permanent cure.
Hyperthyroidism treatments for cats at a glance
- Felimazole, or human genericHow it works: Blocks thyroid hormone productionUpside: Inexpensive, reversible, easy to adjustDownside: Lifelong daily tablets; side effects in some cats; regular blood testsWhere to get it: Regular pharmacy for the human generic, or vet
- Methimazole gelCompounded, applied to the earHow it works: Same medicine, absorbed through the skinUpside: No pillingDownside: Not FDA-approved; absorption varies; wear glovesWhere to get it: Compounding pharmacy
- Radioactive iodineI-131 treatmentHow it works: Destroys the overactive thyroid tissueUpside: Usually a cure with one treatmentDownside: Higher upfront cost; specialist center; days in the hospitalWhere to get it: Specialist referral
- Iodine-restricted dietPrescription foodHow it works: Starves the thyroid of iodineUpside: No medicineDownside: Must be the only food, with no treats or huntingWhere to get it: Vet
- SurgeryThyroidectomyHow it works: Removes the affected glandUpside: Can cureDownside: Anesthesia risk; possible low calcium afterwardWhere to get it: Vet or surgical specialist
| Treatment | How it works | Upside | Downside | Where to get it |
|---|---|---|---|---|
| Methimazole tabletsFelimazole, or human generic | Blocks thyroid hormone production | Inexpensive, reversible, easy to adjust | Lifelong daily tablets; side effects in some cats; regular blood tests | Regular pharmacy for the human generic, or vet |
| Methimazole gelCompounded, applied to the ear | Same medicine, absorbed through the skin | No pilling | Not FDA-approved; absorption varies; wear gloves | Compounding pharmacy |
| Radioactive iodineI-131 treatment | Destroys the overactive thyroid tissue | Usually a cure with one treatment | Higher upfront cost; specialist center; days in the hospital | Specialist referral |
| Iodine-restricted dietPrescription food | Starves the thyroid of iodine | No medicine | Must be the only food, with no treats or hunting | Vet |
| SurgeryThyroidectomy | Removes the affected gland | Can cure | Anesthesia risk; possible low calcium afterward | Vet or surgical specialist |
Signs and diagnosis
The classic sign is weight loss despite a big appetite, often with increased thirst, vomiting, restlessness, night-time yowling and an unkempt coat. A blood test for the thyroid hormone T4 usually confirms it, sometimes with further tests when results are borderline.
Why vets often start with methimazole
An overactive thyroid increases blood flow to the kidneys, which can hide kidney disease. Once the thyroid is controlled, kidney values may rise. Starting with methimazole, which can be adjusted or stopped, lets your vet see how the kidneys cope before a permanent treatment such as radioactive iodine. See kidney disease in cats.
Living with methimazole
Methimazole is usually given once or twice a day for life. Blood tests check thyroid levels, kidney values and blood counts a few weeks after starting and after dose changes, then periodically. Report vomiting, poor appetite, lethargy, scratching at the face, yellowing of the gums or eyes, or unusual bruising, which can signal side effects that need attention.
Wash your hands after giving tablets or gel, wear gloves with the gel, and if you are pregnant, avoid handling it where possible.
Choosing a cure
Radioactive iodine is considered the treatment of choice for many cats because one injection usually cures the disease without anesthesia and ends daily medicine. It requires a specialist center and a hospital stay of several days because of radiation rules, and costs more upfront, though it can cost less over a lifetime than years of tablets and blood tests.
Why treatment matters
Untreated hyperthyroidism strains the heart and raises blood pressure, which can damage the eyes, kidneys and brain, and the weight loss continues. Treated cats usually regain weight and energy within weeks.
Frequently asked questions
Can hyperthyroidism in cats be cured?
Yes. Radioactive iodine usually cures it with one treatment, and surgery can too. Methimazole and the iodine-restricted diet control it rather than cure it.
Can a regular pharmacy fill methimazole for my cat?
Yes. Human generic methimazole tablets are the same medicine, and a pharmacy can fill your vet’s prescription. Compounding pharmacies make the ear gel and flavored liquids.
What happens if hyperthyroidism is not treated?
It continues to cause weight loss and strains the heart, and the high blood pressure it causes can damage the eyes, kidneys and brain.
Is radioactive iodine worth it?
For many cats, yes: it is usually a one-time cure without anesthesia. Your vet will check kidney function first, often with a trial of methimazole, to make sure it is a good choice.