Hyperthyroidism is the most common hormonal disease of older cats. Untreated, it causes significant suffering; with appropriate treatment it can be well managed, often restoring good quality of life.
Untreated hyperthyroidism causes cats to feel chronically unwell — persistently hungry yet losing weight, anxious, restless, and often nauseous or uncomfortable. The concurrent hypertension can cause sudden blindness from retinal detachment — a distressing acute event that is entirely preventable with timely diagnosis and treatment. The cardiac effects of untreated hyperthyroidism include hypertrophic cardiomyopathy and arrhythmias. Treatment itself has welfare implications: daily pilling is stressful for some cats; the isolation required after radioiodine therapy is distressing for sociable cats; and the dietary monotony of the prescription iodine-restricted diet is a quality-of-life consideration. Monitoring for CKD emergence after treatment is critical — masked renal disease that deteriorates post-treatment requires careful management to optimise both thyroid and renal welfare simultaneously.