Feline Hyperthyroidism: Managing a Common Senior Cat Disease

Hyperthyroidism is the most common endocrine disorder in cats over age 10. Early recognition and treatment dramatically improves quality of life and extends healthy lifespan.

Key Facts

Hyperthyroidism affects approximately 10% of cats over age 10 and up to 20% of cats over 15
The condition is caused by benign thyroid adenoma producing excess thyroid hormone
Classic signs include weight loss despite increased appetite, hyperactivity, vomiting, and unkempt coat
Untreated hyperthyroidism causes progressive weight loss, heart disease, hypertension, and eventually death
Four treatment options exist: daily medication, radioactive iodine, surgery, and dietary iodine restriction
Radioactive iodine is considered the gold standard treatment with a 95% cure rate after a single treatment
Methimazole (Felimazole) twice daily controls disease effectively but requires lifelong compliance
Hypertension occurs in up to 87% of hyperthyroid cats and requires separate monitoring and treatment

Welfare Considerations

Hyperthyroidism causes significant welfare impairment through multiple mechanisms: weight loss produces weakness and muscle wasting, concurrent hypertension risks retinal detachment and sudden blindness, and the cardiovascular strain of chronically elevated thyroid hormone causes heart remodeling that can lead to congestive heart failure. Cats with untreated hyperthyroidism often appear active and alert initially, masking the progressive welfare decline underway. Treatment produces dramatic quality of life improvement in most cats, making this one of the most rewarding senior cat conditions to manage. The welfare risk of masking concurrent renal disease when thyroid function normalizes requires careful monitoring after treatment initiation.

What You Can Do

Schedule annual blood and urine testing for cats over age 8 to catch hyperthyroidism before clinical signs appear
Recognize the classic signs: weight loss with increased appetite, excessive vocalization, and restlessness
Discuss treatment options including radioactive iodine with your veterinarian to find the best fit for your cat
Monitor blood pressure in hyperthyroid cats and treat hypertension to prevent vision loss
After treatment begins, recheck kidney values within 4-6 weeks as renal disease may be unmasked
Ensure twice-daily medication compliance if using methimazole; a transdermal gel formulation aids compliance