Hormonal disease · Updated 2026-10-25
Feline hyperthyroidism
Excess thyroid hormone in older cats can accelerate metabolism and strain the heart, kidneys and other organs.
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Signs to watch for
- Weight loss despite appetite
- Restlessness
- Vomiting or diarrhoea
- Drinking more
- Fast heartbeat
- Poor coat
Causes and risk factors
Most cases result from benign overactive thyroid tissue; malignant thyroid tumours are uncommon.
How veterinarians diagnose it
Examination, blood pressure and thyroid hormone testing are combined with kidney, liver and heart assessment.
Treatment and management
Options include radioactive iodine, medication, surgery or a strictly controlled iodine-restricted diet; suitability varies by cat.
Prevention
There is no proven prevention. Routine senior examinations and weight tracking support early detection.
Outlook
Often excellent with definitive treatment or careful lifelong control, though concurrent disease influences outcome.
Common questions
- What are the signs of feline hyperthyroidism?
- Weight loss despite appetite; Restlessness; Vomiting or diarrhoea; Drinking more; Fast heartbeat; Poor coat.
- What causes feline hyperthyroidism?
- Most cases result from benign overactive thyroid tissue; malignant thyroid tumours are uncommon.
- How is feline hyperthyroidism diagnosed?
- Examination, blood pressure and thyroid hormone testing are combined with kidney, liver and heart assessment.
- How is feline hyperthyroidism treated?
- Options include radioactive iodine, medication, surgery or a strictly controlled iodine-restricted diet; suitability varies by cat.
- Can feline hyperthyroidism be prevented?
- There is no proven prevention. Routine senior examinations and weight tracking support early detection.
- When should I contact a veterinarian?
- Book promptly for unexplained weight loss or marked appetite change; breathing distress, collapse or profound weakness is urgent.
Hyperthyroid cat: decisions at each stage
| Situation | Next step |
|---|---|
| Older cat eating well but losing weight | Book a routine vet visit and ask about a thyroid blood test. |
| Diagnosis confirmed | Discuss medication, diet, surgery and radioactive iodine with the vet. |
| On daily medication | Give doses consistently and keep monitoring blood-test appointments. |
| Monthly home check | Weigh the cat and note appetite, thirst and behaviour changes. |
| Cat on treatment goes off food or vomits | Contact the clinic; the dose or another condition may need attention. |
Untreated hyperthyroidism damages the heart and kidneys. Never restrict food in an older cat without a diagnosis.
A hungry, restless, slimming older cat needs a thyroid check
Hyperthyroidism is one of the most common illnesses of older cats. The thyroid gland produces too much hormone, speeding up the whole body: the cat eats eagerly yet loses weight, becomes restless or vocal, drinks more, and may vomit or develop a poor coat. Because the signs creep in gradually and mimic normal ageing, they are easy to dismiss, so any of this pattern in a cat over about eight years old justifies a blood test at a routine appointment.
Left untreated, the condition strains the heart and kidneys, so earlier diagnosis genuinely changes outcomes. Diagnosis is usually a straightforward blood test, sometimes repeated or combined with other checks, and your vet will also look for the kidney and heart problems that commonly travel with it. Do not put an older cat on a diet or restrict food at home because it seems to eat too much; that instinct punishes a sick cat.
Several good treatments exist, and the choice is individual
The main options are daily anti-thyroid medication, prescription iodine-restricted feeding, surgical removal of the gland, and radioactive iodine treatment, which is often considered the most definitive. Each has trade-offs in cost, availability, monitoring and suitability for cats with other illnesses, and your vet will walk through which fit your cat. Medication controls rather than cures, so it means tablets or liquid every day plus periodic blood tests to check the dose and watch the kidneys.
Whichever route you choose, keep review appointments even when the cat looks well, because doses drift and related conditions emerge quietly. Weigh the cat monthly at home and note appetite, thirst and behaviour; bring those notes to reviews. If a cat on treatment suddenly refuses food, vomits persistently or becomes withdrawn, contact the clinic, since that can signal the dose is wrong or a second problem has appeared. With treatment, most cats return to a comfortable, stable life for years.
Sources for this reference
AI-assisted educational guidance, updated 25 October 2026. Not professionally reviewed; sources do not imply endorsement of this page.
Preparing for a feline hyperthyroidism consultation
- Record when these signs began and how they changed: Weight loss despite appetite; Restlessness; Vomiting or diarrhoea. Record appetite, water intake, toileting and any medicines or supplements.
- The listed urgency is prompt; worsening breathing, collapse, seizures or severe pain can make any situation an emergency. Ask the clinic rather than relying only on this category.
- Ask what else could explain the signs, which tests would change the plan and how discomfort will be managed. Symptoms alone cannot confirm this condition.
- Before going home, agree what to monitor, when to recheck and which signs mean calling sooner. Follow the individual treatment plan; do not copy another pet's prescription.
Educational reference, not an individual diagnosis or a verified professional review. Read our editorial policy.
Guidance libraries and further reading
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