Hyperthyroidism is the most common endocrine disease of middle-aged and older cats, typically appearing from about eight years of age onward. It occurs when the thyroid glands in the neck produce too much thyroxine (T4), a hormone that sets the speed of metabolism. In roughly 95 to 98 percent of cases the cause is a benign enlargement of the gland, not cancer. Because thyroid hormone affects nearly every organ, an untreated cat’s body runs permanently in overdrive, eventually damaging the heart, raising blood pressure, and in the worst cases causing sudden blindness or heart failure. The good news: with treatment, most hyperthyroid cats do very well.
Symptoms of Hyperthyroidism in Cats
The signs come on slowly and are often mistaken for “getting old with character”. The most typical pattern is a cat that eats more yet loses weight:
- Weight loss despite a ravenous appetite — the hallmark sign.
- Increased thirst and urination.
- Restlessness, hyperactivity, and loud night-time vocalising.
- Vomiting or diarrhoea, sometimes with unusually large stools.
- Unkempt coat — greasy, matted, or patchy fur from poor grooming.
- Rapid heart rate or a heart murmur heard on examination.
- A small lump in the neck — an enlarged thyroid gland your vet can feel.
| Sign | Why it happens |
|---|---|
| Weight loss with big appetite | Metabolism burns calories far faster than normal |
| Drinking and peeing more | Hormone excess affects kidneys and fluid balance |
| Hyperactivity and yowling | Thyroid hormone over-stimulates the nervous system |
| Poor coat and muscle wasting | Body resources diverted to the revved-up metabolism |
When to See a Vet
Any cat over seven years old that is losing weight while eating well should have a thyroid check. Diagnosis is confirmed with a blood T4 measurement, together with a general health screen. That screen matters: hyperthyroidism can mask underlying kidney disease, and many affected cats also develop high blood pressure, so your vet will want to assess the heart, kidneys, and eyes before choosing a treatment. Do not delay — the earlier treatment starts, the less damage the excess hormone does.
Common Vet-Directed Treatments
There are four established options, and the right one depends on your cat’s health, age, and your circumstances:
- Anti-thyroid medication (methimazole or carbimazole), given as a pill twice daily or as a gel rubbed into the ear. It controls the disease but is not a cure, so it must be given for life with periodic blood monitoring.
- Radioactive iodine therapy — a single injection destroys the overactive thyroid tissue and cures about 95 percent of cats. It requires a stay of several days in a specialised isolation facility.
- Surgery to remove the affected thyroid gland can be curative, but anaesthesia carries risk in older cats and the nearby parathyroid glands can be accidentally damaged.
- Prescription iodine-restricted diet, which controls hormone levels as long as the cat eats nothing else at all — a challenge in multi-cat homes.
In this video, emergency veterinarian Dr. Justine Lee explains feline hyperthyroidism and the treatment choices in plain language:
Home Care for a Hyperthyroid Cat
Whatever option you choose, follow-up is where owners make the real difference. Give medication at consistent times and ask your vet about transdermal gel if pill-giving turns into a wrestling match. Weigh your cat monthly and track appetite, thirst, and litter-box output. Attend every scheduled recheck so hormone levels, blood pressure, and kidney values stay on target. Keep treats, table scraps, and other pets’ food away from a cat on the iodine-restricted diet, since even small amounts break the therapy.
Prevention
There is no known way to prevent hyperthyroidism, so early detection is the best protection: schedule wellness exams every six months for senior cats, with bloodwork that includes a T4 level. Caught early, before heart and blood-pressure damage develops, hyperthyroidism is one of the most treatable diseases of ageing cats.
Disclaimer: This article is for educational content only and is not a substitute for professional veterinary advice. Always consult a licensed veterinarian for diagnosis and treatment of your pet.