Hyperthyroidism, or an overactive thyroid, is a common condition in middle-aged and older cats. The signs can look like a healthy appetite and a bit of weight loss, so it is easy to miss. The good news is that it is usually treatable.
What is it?
The thyroid gland sits in the neck and controls metabolism. In hyperthyroidism the gland becomes enlarged and makes too much thyroid hormone (T3 and T4). According to the Cornell Feline Health Center, most cases are caused by a non-cancerous growth called an adenoma. The exact cause is unknown, and the condition mostly affects cats who are middle-aged and older.
Signs to watch for
- Weight loss, often despite eating more
- Increased appetite
- More thirst and urination
- Vomiting or diarrhoea
- Restlessness or hyperactivity
- A matted, greasy or unkempt coat
Signs are often subtle at first and get worse over time. A cat that eats more and still loses weight should always see a vet. Because thirst and weight loss also appear in kidney disease and diabetes, only tests can tell them apart. See kidney disease in cats.
How it is diagnosed
- A physical exam, including feeling the neck for an enlarged thyroid, and checking heart rate and blood pressure.
- Bloodwork and a thyroid hormone test. Most affected cats have a high T4, though a small number have a normal T4 and need further testing.
- Checks of the heart and kidneys, because hyperthyroidism can affect both.
Treatment options
| Option | How it works | Things to know |
|---|---|---|
| Medication | Anti-thyroid drugs reduce hormone production | Controls but does not cure. Usually lifelong, often twice a day. A skin gel is also available. Regular blood tests needed. |
| Radioactive iodine | Destroys the overactive thyroid tissue | Often the treatment of choice where available. Cornell says it cures about 95% of cats within three months. Usually a short hospital stay. |
| Surgery | Removes the thyroid gland | Can give a long-term cure but needs anaesthesia and carries some risk. Less commonly chosen. |
| Low-iodine diet | Limits the iodine the thyroid needs | May suit some cats who cannot have other treatments. Discuss with your vet. |
Cornell notes that up to 50% of cats treated with radioactive iodine may develop some degree of hypothyroidism, which may need supplements.
Effects on the heart and kidneys
Hyperthyroidism can cause high blood pressure and thickening of the heart muscle. These problems often improve once the thyroid is controlled, though some cats need extra treatment. Treating hyperthyroidism can also reveal hidden kidney disease, which is one reason vets monitor kidney values during treatment.
Outlook
Cornell's guidance states that the outlook for cats with hyperthyroidism is generally good with appropriate treatment. Complications in other organs can change that, which is why finding it early helps.
What you can do
- Weigh your cat monthly and note any loss. See healthy cat weight.
- Keep up twice-yearly vet visits from age 10. See senior cat health.
- Mention increased appetite, thirst or restlessness to your vet early.
Learn more about when a cat is considered senior.
Frequently asked questions
What age do cats get hyperthyroidism?
It mostly affects cats who are middle-aged and older.
Why is my cat losing weight but eating more?
That pattern is a classic sign of hyperthyroidism, though other conditions such as diabetes can cause it. A vet visit and blood tests will tell.
Is hyperthyroidism in cats curable?
Radioactive iodine and surgery can cure it. Medication and diet control it but do not cure it.
Is hyperthyroidism painful?
Pain is not the main sign, but the condition can make a cat feel unwell and put strain on the heart and kidneys, so it needs treatment.
This article is general information, not veterinary advice. If you think your cat has hyperthyroidism, see a vet.