Feline Hyperthyroidism: The Most Common Hormonal Disease in Older Cats
Hyperthyroidism causes the thyroid to produce excess hormones. It's very common in older cats and very treatable.
Hyperthyroidism is the most common endocrine disorder in cats, caused by a benign thyroid gland tumor that produces excess thyroid hormones. It speeds up the body's metabolism, affecting nearly every organ. The good news: it's very treatable, and most cats make a full recovery.
Key Points
- Most common in cats over 10 years old
- Caused by a benign thyroid tumor in 98% of cases
- Weight loss despite increased appetite is the classic sign
- Can cause heart damage if untreated
- Multiple treatment options: medication, diet, surgery, radioactive iodine
- Radioactive iodine (I-131) is the closest thing to a cure
Understanding Hyperthyroidism
The thyroid glands in the neck produce hormones that regulate metabolism. In hyperthyroidism, a benign tumor (adenoma) produces excessive thyroid hormones, speeding up the body's metabolism. This affects the heart, kidneys, digestive system, and behavior. Without treatment, it can lead to heart failure and dangerously high blood pressure.
Signs to Watch For
Weight loss despite a ravenous appetite is the most common sign. Others include: increased thirst and urination, restlessness or hyperactivity, vomiting, diarrhea, unkempt coat, rapid heart rate, heart murmur, and behavioral changes (yowling at night, aggression). Some cats develop 'thyroid storm' — a dangerous crisis with extremely high heart rate and temperature.
Diagnosis
A simple blood test measuring total T4 (thyroid hormone) level is usually diagnostic. In early or mild cases, a free T4 by equilibrium dialysis may be needed. Blood pressure measurement and cardiac evaluation (echocardiogram) are also important, as hyperthyroidism often masks underlying kidney disease and causes heart changes.
Treatment Options
Methimazole (Felimazole): daily medication that blocks thyroid hormone production. Effective but lifelong. Hills y/d diet: iodine-restricted diet that starves the thyroid of raw material. Strict feeding protocol required. Radioactive iodine (I-131): single injection that destroys the abnormal thyroid tissue. Cures the disease in 95%+ of cases. Surgery: removal of the affected thyroid lobe. Less commonly performed today.
When to See a Vet Immediately
- Your older cat is losing weight despite eating well
- Increased thirst and urination
- Restlessness, hyperactivity, or yowling at night
- Rapid heart rate or heart murmur detected
- Vomiting or diarrhea
- Unkempt coat
How RVC Can Help
RVC diagnoses and treats feline hyperthyroidism with medical management, dietary therapy, and referral for radioactive iodine treatment. We monitor kidney function and heart health throughout treatment. Call +853 6677 6611.
This article is for educational purposes only and does not replace professional veterinary advice. If your pet is showing any symptoms, please contact Royal Veterinary Center immediately at +853 6677 6611.