Hyperthyroidism is a common condition in older cats in which the thyroid glands produce too much thyroid hormone. These hormones help regulate the body's metabolism, controlling how quickly energy is used. When too much thyroid hormone is produced, the body works harder than normal, often causing weight loss despite an increased appetite.
In most cases, hyperthyroidism is caused by a benign enlargement of the thyroid tissue. Rarely, it can be caused by a malignant (cancerous) thyroid tumour.
Common signs of hyperthyroidism include:
Weight loss despite an increased appetite
Increased thirst and urination
Vomiting or diarrhoea
Hyperactivity or increased vocalisation
Poor coat condition
Faster heart rate
An enlarged thyroid gland, which may be felt as a lump in the neck
Contact your vet if you notice any of the symptoms described above, particularly weight loss despite an increased appetite.
Hyperthyroidism is more common in older cats and can worsen if left untreated. Early diagnosis and treatment can help manage the condition and reduce the risk of complications.
Your vet will examine your cat and assess their symptoms. A blood test measuring thyroid hormone levels is used to help confirm the diagnosis.
Further tests, including blood and urine tests and blood pressure measurement, may be recommended to check for other conditions, particularly kidney disease and high blood pressure, and to assess your cat's overall health.
Hyperthyroidism can be managed in several ways. The best option depends on your cat's general health and individual circumstances.
Medication: Medication controls thyroid hormone production and is often used for lifelong management. Regular veterinary check-ups and blood tests are needed to monitor hormone levels and check for side effects.
Surgery: Removal of the affected thyroid gland can be curative. However, surgery requires a general anaesthetic and carries risks, including damage to the nearby parathyroid glands, which regulate calcium levels. Other complications are possible, and hyperthyroidism can recur if the remaining thyroid gland becomes overactive.
Radioactive iodine therapy: This treatment destroys the overactive thyroid tissue and can provide a permanent cure. It does not require surgery but involves a stay at a specialist centre.
Specialist diet: A prescription diet restricted in iodine can help control thyroid hormone production. It must be fed exclusively to be effective.
There is no known way to prevent hyperthyroidism. Regular veterinary check-ups, particularly in older cats, can help identify the condition early and allow treatment to start before complications develop.
Most cats with hyperthyroidism respond well to treatment and can enjoy a good quality of life for many years.
The outlook depends partly on whether other conditions, particularly kidney or heart disease, are also present. The effect of these conditions varies depending on their severity and how well they can be managed.
Kidney disease may become more apparent after treatment as thyroid hormone levels return to normal. This is because hyperthyroidism can affect kidney function and mask underlying kidney disease. Continued monitoring is therefore important, particularly after treatment begins.
Some cats require lifelong medication and monitoring, while others can be cured with surgery or radioactive iodine therapy. In rare cases where hyperthyroidism is caused by a malignant thyroid tumour, treatment may be more difficult and the outlook less favourable.
Written by Gillian Burkhill, Veterinary Surgeon, Animal Trust Vets CIC | Published July 2026 | Review date July 2029 | This advice is for UK pets only and is not a replacement for seeing a vet
