Cat Health
Hyperthyroidism in Cats: Symptoms, Diagnosis & Treatment Options
Published Last updated 3 min read
Quick answer
Hyperthyroidism — overproduction of thyroid hormone — is common in cats over eight years old. Classic signs include weight loss with a ravenous appetite, hyperactivity, vomiting, and poor coat. It is treatable with medication, diet, surgery, or radioactive iodine. Untreated hyperthyroidism damages the heart and kidneys — senior cats should have thyroid levels checked at annual exams.
What is feline hyperthyroidism?
The thyroid glands in the neck produce hormones that regulate metabolism. In hyperthyroidism, one or both glands become overactive — usually from benign adenomas (tumours) — and release excess thyroid hormone.
The result is a revved-up metabolism: calories burn faster than the cat can eat, organs work under strain, and the cardiovascular system faces increased demand.
Hyperthyroidism is the most common endocrine disease in cats, affecting up to ten percent of senior cats.
Symptoms of hyperthyroidism
Signs develop gradually and may be mistaken for normal ageing:
- Weight loss despite increased appetite — sometimes ravenous eating
- Hyperactivity — restlessness, vocalisation, pacing
- Vomiting and diarrhoea
- Increased thirst and urination
- Poor, greasy, or matted coat
- Behaviour changes — irritability or aggression
- Rapid heart rate and heart murmur
- Heat intolerance
Some cats show only subtle signs early. Bloodwork at senior wellness exams catches hyperthyroidism before dramatic weight loss.
Diagnosis
Your vet diagnoses hyperthyroidism with a blood test measuring total T4 (thyroxine). Some cats with early or concurrent illness need additional testing — free T4 or T3 suppression tests.
Blood pressure measurement and kidney values are checked at diagnosis because hyperthyroidism often coexists with hypertension and chronic kidney disease. Treatment plans must balance thyroid control with kidney function.
Treatment options
Oral medication (methimazole)
Daily tablets or transdermal gel block thyroid hormone production. It is effective and reversible but requires lifelong administration and periodic blood monitoring. Side effects — vomiting, facial itching, liver changes — occur in a minority of cats.
Prescription iodine-restricted diet (y/d)
A controlled iodine diet reduces hormone production when fed exclusively. No other food or treats can be given. Suitable for single-cat households willing to commit fully.
Radioactive iodine (I-131) therapy
A single injection destroys overactive thyroid tissue. Cure rates exceed ninety-five percent. Requires specialised facilities and brief hospitalisation. Often the treatment of choice for otherwise healthy cats.
Surgical thyroidectomy
Removal of affected thyroid tissue cures the disease but carries anaesthesia risk in older cats and may affect parathyroid glands. Less common now that I-131 is widely available.
Living with a treated hyperthyroid cat
Regular rechecks of thyroid levels, kidney values, and blood pressure ensure stable management. Weight should gradually normalise and activity levels often settle. Heart changes may partially reverse with successful treatment.
Sources & further reading
Facts in this guide are rewritten in plain English from publicly available UK advice. We name the organisation where a specific point comes from their guidance. Links below go to the original pages — use them to read the source material directly.
PETHEALTH+ is independent. These organisations do not sponsor, approve, or partner with this website. Guidance checked against sources listed below (last updated 2026-06-24).
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Frequently Asked Questions
- What are the signs of hyperthyroidism in cats?
- Weight loss despite increased appetite, hyperactivity, vomiting, diarrhoea, increased thirst and urination, and poor coat condition are classic signs. Some cats become irritable or develop a heart murmur.
- What age do cats get hyperthyroidism?
- It almost exclusively affects cats over eight years old, with peak incidence between ten and thirteen years. Routine senior bloodwork helps catch it early.
- Can hyperthyroidism in cats be cured?
- Radioactive iodine therapy and surgical thyroidectomy can cure the disease. Daily medication and prescription diets manage it long-term but do not cure it.
- What happens if feline hyperthyroidism is untreated?
- Untreated hyperthyroidism leads to heart disease, high blood pressure, kidney damage, and weight loss to emaciation. It is eventually fatal without treatment.