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The important points, first
- In an older cat, weight loss with more appetite, increased thirst and urination, vomiting, restlessness or an unkempt coat should be assessed.
- Measuring thyroxine (T4) in senior cat blood tests allows early detection.
- Treatment options are medication, radioactive iodine, surgery and diet. The choice is made with your veterinarian.
- During treatment, regular checks of T4, blood, urine and blood pressure are needed, partly to keep an eye on the kidneys.
What it is and which cats it affects
According to the American Association of Feline Practitioners (AAFP) guidelines, hyperthyroidism is the most common endocrine disorder in middle-aged and older cats in the United States, where it affects up to 10% of cats over 10 years. Worldwide it is diagnosed in 1.5–11.4% of older cats, and it has become steadily more common since it was first described in the late 1970s and early 1980s.
The Cornell Feline Health Center explains that in most cases the thyroid enlarges because of a benign tumour, an adenoma; malignant forms are rare. The cause is unknown. Possible factors include deficiencies or excesses of some substances in the diet and long-term exposure to chemicals in food or the environment that disrupt the thyroid.12
Signs not to put down to old age
The classic signs listed by AAFP are weight loss, increased appetite, increased thirst and urination, more vocalising, agitation and hyperactivity, fast breathing and heart rate, vomiting, diarrhoea and an unkempt coat. Cornell adds that the coat may look matted or greasy and that signs, mild at first, get worse as the disease progresses.
Many cats are now diagnosed earlier, thanks to T4 being included in senior check-up blood tests: according to AAFP this allows diagnosis long before the cat becomes thin, unkempt and agitated. The AAHA guidelines strongly recommend measuring T4 every year in senior cats. At the visit the veterinarian feels the neck for the thyroid and checks heart rate and blood pressure, and uses blood and urine tests to assess the heart and kidneys too.123
Available treatments
Cornell describes four options. Antithyroid medications, also available as a gel applied to the skin, control the disease but do not cure it, must be given for life and can have side effects. Radioactive iodine, where available, is considered the treatment of choice: within three months it cures about 95% of cats, but it requires a few days in hospital at a licensed facility. Surgery works but is rarely chosen today, because medication and radioactive iodine are just as effective and less invasive. Low-iodine diets can help in some cases but are still debated.
Hyperthyroidism can cause heart problems and high blood pressure, which often improve or disappear once the thyroid is under control. The AAFP guidelines recommend treating all hyperthyroid cats, even when they have other diseases. With good management cats often live for a long time: in cats without kidney disease, more recent studies report a median survival of up to 5.3 years.21
Kidneys and long-term monitoring
Chronic kidney disease is very common in older cats, and according to Cornell it affects up to 40% of cats over 10. It often coexists with hyperthyroidism and may only become apparent after treatment starts; when that happens, survival is shorter. AAFP notes that many veterinarians believe a high T4 supports the kidneys, but more recent evidence indicates that the thyroid should be treated even in cats with kidney insufficiency. The aim is to bring it back to normal, without leaving the cat slightly hyperthyroid and without making it hypothyroid.
According to AAFP, the first checks are done 2–4 weeks after starting treatment and 2–4 weeks after any change. In stable cats without complications they are repeated every 4–6 months, with T4, complete blood count, chemistry and urinalysis, and blood pressure should ideally be measured at every visit. At home, note weight, appetite, thirst and behaviour so you can report a change early.14
Sources and checks
Numbers in the text lead to the exact source. Commercial blogs and social posts are not used as evidence.
- 1Veterinary guideline
2016 AAFP Guidelines for the Management of Feline Hyperthyroidism
Journal of Feline Medicine and Surgery (2016) · PubMed PMID 27143042 · Full text on PubMed Central: prevalence, signs, T4 in senior panels, treat all cats, kidneys and survival, checks at 2–4 weeks then every 4–6 months, blood pressure at every visit.
Link checked on 16 Sept 2026 - 2University veterinary centre
Hyperthyroidism in Cats
Cornell Feline Health Center · Causes, signs, diagnosis, four treatment options, heart complications and high blood pressure, prognosis.
Link checked on 16 Sept 2026 - 3Veterinary guideline
2023 AAHA Senior Care Guidelines for Dogs and Cats: Diagnostic Tests and Recommended Frequencies
American Animal Hospital Association · T4 strongly recommended annually in senior cats.
Link checked on 16 Sept 2026 - 4University veterinary centre
Chronic Kidney Disease
Cornell Feline Health Center · Kidney disease in up to 40% of cats over 10, importance of monitoring senior cats.
Link checked on 16 Sept 2026
This guide does not recommend medicines or doses. Diagnosis, choice of treatment and monitoring frequency are for your veterinarian.