Glossary

Hyperthyroidism

A common disease of older cats in which an overactive thyroid speeds up the body, causing weight loss despite a big appetite.

Also called: feline hyperthyroidism, overactive thyroid, thyroid disease in cats

Hyperthyroidism is the most common hormone disorder of older cats. One or both thyroid glands in the neck enlarge, nearly always from a benign growth, and make too much thyroid hormone. This speeds up the metabolism. Most affected cats are over ten years of age.

The classic picture is a cat that eats more than ever yet loses weight. Other signs are drinking and urinating more, restlessness, vocalizing at night, vomiting or loose stools, a poor coat and a fast heartbeat. Some cats instead become quiet and stop eating. The disease can lead to high blood pressure and thicken the heart muscle, and it can hide chronic kidney disease by increasing blood flow through the kidneys.

A vet diagnoses it from the exam, which may include feeling an enlarged gland, and a blood test for thyroid hormone. Treatment choices include daily medicine, a prescription diet, radioactive iodine treatment at a specialist center and surgery. Each has benefits and drawbacks, and the best choice depends on kidney function and the cat's health. Do not use a supplement or an iodine-free food without direction. Ask your vet to explain the options, and to recheck kidney values and blood pressure during treatment.

Last reviewed November 7, 2026

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Related terms

Chronic kidney disease A gradual loss of kidney function, common in older cats, that can be managed for months or years but not cured. Blood pressure The force of blood against artery walls; high readings in cats usually come from another disease and can damage the eyes, brain, heart and kidneys. Hypertrophic cardiomyopathy The most common feline heart disease, in which the wall of the main pumping chamber thickens and the heart works less well. Blood panel A set of blood tests that counts cells and measures chemicals to show how a cat's organs are working. Obesity Excess body fat that harms a cat's health, raising the risk of diabetes, arthritis and urinary disease.

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