Hyperthyroidism is the most common endocrine disorder in cats over 10 years old, causing significant metabolic suffering that is highly treatable when caught early.
Cats with uncontrolled hyperthyroidism experience significant metabolic suffering. Chronically elevated thyroid hormone causes a racing metabolism — cats feel perpetually hungry, restless, and unable to maintain body weight despite eating voraciously. The associated hypertension causes retinal damage and sudden blindness. Cardiac changes — hypertrophic cardiomyopathy from thyroid-driven heart enlargement — cause arrhythmias and eventually heart failure. Gastrointestinal effects include chronic vomiting and diarrhea. What owners sometimes interpret as a "senior cat becoming more active" is actually metabolic distress. Early diagnosis through annual bloodwork allows treatment before irreversible organ damage occurs. Methimazole management, while effective, requires daily pilling that some cats find stressful — radioactive iodine offers a one-time cure for cats who can access it.