Feline Hyperthyroidism: Managing a Common Disease of Older Cats

Hyperthyroidism is the most common endocrine disorder in older cats, causing significant suffering when untreated. Effective treatments exist, but early detection requires proactive veterinary screening.

Key Facts

Feline hyperthyroidism affects up to 10% of cats over 10 years of age, making it among the most common senior cat diseases
It is caused by benign adenomatous hyperplasia of the thyroid gland, producing excess thyroid hormone
Clinical signs include weight loss despite increased appetite, increased thirst, hyperactivity, vomiting, and poor coat condition
Untreated hyperthyroidism causes hypertension, cardiac hypertrophy, kidney damage, and death within months to years
Four treatment options exist: daily oral medication (methimazole), radioactive iodine (curative), surgery, and iodine-restricted diet
Radioactive iodine is the only curative treatment and resolves hyperthyroidism in over 95% of cases with a single treatment
The disease can mask concurrent chronic kidney disease; treatment may unmask kidney compromise requiring concurrent management

Welfare Considerations

Feline hyperthyroidism causes chronic suffering through the metabolic effects of excess thyroid hormone: persistent hunger without weight gain, chronic gastrointestinal upset, hypertension causing headache-equivalent neurological symptoms, and progressive cardiac disease. The disease is frequently detected late because early signs are subtle and owners may interpret weight loss and increased activity in older cats as normal. Welfare-centred senior cat care includes annual or biannual thyroid hormone screening from age 9–10, enabling treatment before secondary organ damage occurs. The welfare hierarchy of treatment options favours radioactive iodine for its curative effect, followed by oral medication for cats with complicating factors. Untreated hyperthyroidism is a preventable source of significant chronic suffering.

What You Can Do

Request thyroid hormone screening (T4) for cats over 9 years at annual or biannual health checks
Seek prompt veterinary assessment if your older cat shows weight loss with maintained or increased appetite
Discuss radioactive iodine treatment with your vet — it offers a permanent cure and eliminates daily medication stress
If using oral methimazole, monitor for side effects including facial itching, vomiting, and bone marrow suppression
Ensure concurrent blood pressure monitoring and kidney function assessment when managing hyperthyroid cats