Hyperthyroidism is the most common endocrine disorder in cats, causing distressing symptoms that are highly treatable with appropriate intervention.
Hyperthyroidism causes significant ongoing welfare impairment: affected cats lose weight despite voracious appetite, may experience cardiac changes including hypertension and hypertrophic cardiomyopathy, and suffer from gastrointestinal distress. The hyperactive state may seem like energy but reflects physiological stress. Treatment is highly effective — radioactive iodine is curative with a single treatment and minimal stress if the cat copes reasonably with a 2-3 day hospitalization. Medication is effective but requires daily administration. Quality of life typically improves dramatically with treatment. Untreated hyperthyroidism causes progressive deterioration and is not a humane option once diagnosed.