The Most Common Hormonal Disease in Older Cats — And the Signs Most Owners Miss

Your older cat seems more active than they have in years. They're eating well — better than ever, actually. Maybe they've lost a little weight, but they seem to have so much energy. That seems positive, right?

Not necessarily. These exact signs — increased appetite, apparent energy, weight loss — are classic symptoms of hyperthyroidism, the most common hormonal disease in cats over 10 years old. And if it's not treated, it quietly destroys the heart and kidneys.

What Is Hyperthyroidism?

The thyroid gland produces hormones that regulate metabolism. In hyperthyroid cats, the gland becomes overactive — usually due to a benign tumor — and floods the body with excess thyroid hormone. Everything speeds up: metabolism, heart rate, organ function. It feels like revving an engine too high for too long.

The Signs (Many Look Like "Normal Aging")

• Weight loss despite eating well or eating more — the classic presentation
• Increased appetite — sometimes ravenous hunger
• Increased thirst and urination
• Restlessness, hyperactivity, or irritability — especially at night
• Vomiting or diarrhea
• Poor coat quality — greasy, unkempt, matted
• Rapid or irregular heartbeat (detected by vet on exam)
• Difficulty breathing in advanced cases

Why It Gets Missed

Many of these signs are easy to attribute to old age. "She's always hungry but staying thin — just an old cat metabolism." "He's more vocal at night — old cats do that." The key red flag that should always prompt a vet visit: weight loss in a cat that is eating normally or eating more than usual. That combination is almost never normal aging.

Diagnosis and Treatment

Diagnosis is simple — a blood test measuring T4 levels. Treatment is highly effective and comes in several forms:

• Medication (methimazole) — daily pill or ear gel; controls but doesn't cure; needs lifelong treatment and monitoring
• Radioactive iodine (I-131) — single injection, curative in over 95% of cases; the gold standard
• Surgery — removal of the affected thyroid tissue; less commonly used now
• Prescription diet — iodine-restricted food; works only if strictly followed

With treatment, most cats do extremely well and return to normal health. Without treatment, the heart enlarges, blood pressure rises, and kidneys deteriorate — often irreversibly.

Any senior cat showing weight loss should have a thyroid test. Ask your vet to include T4 in their next bloodwork panel.