Hyperthyroidism in cats
Also known as Overactive thyroid.
Too much thyroid hormone because of a benign tumour of the gland. It is very typical from age 10: the cat eats a great deal and loses weight. It responds well to treatment.
- System
- Hormones and metabolism
- Is it urgent?
- Not usually
- Spreads to other cats
- No
- Spreads to people
- No
- Typical cost in Spain
- Blood test with T4, €60–120; medication, €20–50 a month; radioactive iodine, over €1,500.
Symptoms
- Loses weight while eating more than ever
- Drinks and urinates more
- Restless, yowls at night
- Vomiting or diarrhoea
- Unkempt coat
- Racing heart
Causes
A benign adenoma of the thyroid. Environmental factors are being investigated, but the exact cause is unknown.
How it is diagnosed
Total T4 measured in the blood and, in unclear cases, free T4 or scintigraphy.
Treatment
Daily medication, a low-iodine diet, radioactive iodine (curative, available at few centres in Spain) or surgery.
No doses, on purpose: any medicine must be prescribed by your vet for your cat.
Prognosis
Good. Treating it may reveal kidney disease that had been masked.
Prevention
It cannot be prevented: it is caught early through the annual blood test for older cats.
Frequently asked questions
What causes hyperthyroidism?
A benign adenoma of the thyroid. Environmental factors are being investigated, but the exact cause is unknown.
Can hyperthyroidism be cured?
Good. Treating it may reveal kidney disease that had been masked. Daily medication, a low-iodine diet, radioactive iodine (curative, available at few centres in Spain) or surgery.
How much does it cost to treat hyperthyroidism?
As a guide for Spain in 2026: Blood test with T4, €60–120; medication, €20–50 a month; radioactive iodine, over €1,500. Prices vary a lot between clinics and cities, and out-of-hours emergencies cost more.
How can hyperthyroidism be prevented?
It cannot be prevented: it is caught early through the annual blood test for older cats.
More in hormones and metabolism
Educational content from Michipedia. It does not replace diagnosis or treatment by a vet: if you have any doubt about your cat's health, ask. Costs are a guide for Spain in 2026.