Hyperthyroidism is the most common endocrine disease in older cats, affecting approximately 10% of cats over 10 years. Multiple treatment options exist, each with distinct welfare profiles that help guide individualised care decisions.
Untreated hyperthyroidism causes progressive weight loss despite ravenous appetite, muscle wasting, restlessness, vomiting, diarrhoea, polydipsia, and cardiac complications including hypertrophic cardiomyopathy. Hypertension causes retinal detachment, blindness, and neurological damage. The welfare impact is significant—cats experience chronic metabolic hyperactivity, cardiovascular stress, and progressive physical deterioration. Recognition of hyperthyroidism as a treatable condition rather than "normal ageing" dramatically improves welfare outcomes.
Methimazole (oral) and carbimazole (oral/transdermal) inhibit thyroid hormone synthesis. Transdermal gel applied to the inner pinna is often better tolerated than tablets in difficult cats. Monitoring every 3-6 months (T4 levels, renal function, blood pressure) adjusts dosing. Side effects—GI signs, facial pruritus, blood dyscrasia—occur in a minority and require medication change. Lifelong daily medication is a compliance challenge; owner education about signs of undertreated or overtreated hyperthyroidism enables proactive home monitoring.
Radioactive iodine therapy provides definitive, single-treatment cure in most cats. I-131 selectively destroys overactive thyroid tissue while sparing normal tissue. The welfare consideration is mandatory hospitalisation in specialist facilities for 2-4 weeks (radiation safety) which causes significant stress in some cats, particularly those with concurrent anxiety. Post-treatment, approximately 95% of cats achieve permanent remission. For cats that tolerate hospitalisation, this represents the optimal long-term welfare outcome.
Prescription iodine-restricted diets (Hill's y/d) reduce thyroid hormone production by limiting iodine substrate. Efficacy depends on strict diet compliance—no iodine from other food sources. For multi-cat households or cats with poor diet compliance, this option is less practical. Welfare considerations include palatability (some cats accept the diet poorly) and the strict feeding management required.
Treating hyperthyroidism reveals pre-existing CKD previously masked by hyperthyroid-elevated GFR. Progressive renal decline after treatment requires monitoring and management. In severe CKD cases, deliberately maintaining mild hyperthyroidism may be considered to support renal perfusion—a complex welfare trade-off requiring specialist guidance. Regular renal monitoring after initiating hyperthyroid treatment is mandatory.