Cat Hyperthyroidism Symptoms: Signs, Diagnosis and Treatment

Typical symptoms of feline hyperthyroidism

Cats Protection and PDSA list weight loss despite a good or increased appetite, increased thirst and urination, restlessness or vocal behaviour, vomiting, diarrhoea, a poor coat and a fast heart rate. Some cats have a small enlargement in the neck where a thyroid gland can be felt, but an owner cannot reliably find or interpret this at home.

Not every cat has the classic hungry-and-thin pattern. Some have a reduced appetite, low energy or no obvious change in food intake, particularly when another illness is present. A symptom list can raise suspicion but cannot diagnose hyperthyroidism.

• Weight loss or muscle loss while eating normally or more than usual.
• Increased drinking, urination, activity, anxiety, vocalisation or inability to settle.
• Vomiting, loose stools or a dry, unkempt coat.
• A fast heartbeat, panting or weakness.

Why early veterinary assessment matters

Untreated excess thyroid hormone places extra demand on the heart and can contribute to high blood pressure and other organ problems. Hyperthyroidism can also make kidney function look better than it is by increasing blood flow through the kidneys. Once thyroid levels are controlled, hidden kidney disease may become more apparent.

Other conditions, including kidney disease, diabetes and intestinal disease, can produce similar weight and appetite changes. Arrange an appointment as soon as you notice a pattern rather than trying to distinguish them at home.

How a vet confirms the diagnosis

Your vet will take a history, examine body condition and heart rate, and may feel the neck for an enlarged thyroid gland. A blood sample measuring thyroxine (usually total T4) is central to diagnosis; extra tests may be needed when the result is borderline or another disease is suspected.

Blood chemistry, blood pressure and urine tests help assess kidney function, hydration and complications. RVC notes that cats with concurrent kidney disease need careful assessment because thyroid and renal results can influence one another. Do not interpret a single home observation or online symptom checker as a diagnosis.

Treatment options your vet may discuss

Treatment is individual. Options include daily anti-thyroid medicine, surgery, radioactive iodine at a specialist centre or a veterinary low-iodine diet. Each has advantages, limitations, monitoring needs and suitability considerations, especially if heart, kidney or liver disease is also present.

Medication requires the exact prescribed dose and repeat checks; side effects can include poor appetite, vomiting, skin reactions or liver and blood abnormalities. A low-iodine diet only works if it is the cat's sole food, so do not start one without a vet and do not add supplements or treats that undermine the plan.

Supporting a treated cat at home

Give medication exactly as directed, keep follow-up appointments and report appetite, vomiting, diarrhoea, behaviour, weight and drinking changes. A quiet routine, easy access to water and a complete diet support wellbeing, but they do not replace thyroid treatment.

Weigh your cat regularly if this can be done without stress and keep a written record for the practice. Never stop anti-thyroid medicine, double a missed dose or change diets without asking the prescribing team.