Feline Hyperthyroidism: Welfare Considerations
Hyperthyroidism is the most common endocrine disorder of older cats, affecting approximately 10% of cats over 10 years old. Caused by functional adenomas of the thyroid gland producing excess thyroid hormones, it causes a multisystem disease with significant welfare implications if untreated.
Clinical Signs and Welfare Impact
Excess thyroid hormones accelerate metabolism, causing weight loss despite increased appetite, muscle wasting, restlessness, hyperactivity, aggression, vomiting, diarrhoea, and increased water consumption. Many cats experience significant anxiety and distress — the constant physiological state of accelerated metabolism is uncomfortable.
Cardiac effects include hypertrophic cardiomyopathy, hypertension, and arrhythmias. Hypertension causes retinal detachment and blindness — often the presenting sign. Without treatment, these progressive changes cause significant suffering and eventually death.
Treatment Options
Medical management (methimazole/carbimazole): Daily oral medication that inhibits thyroid hormone synthesis. Effective but requires lifelong medication and regular monitoring. Side effects (facial pruritus, vomiting, bone marrow suppression) occur in a minority but require monitoring. Some cats resist pilling — transdermal gel application to ear pinnae may improve compliance.
Radioactive iodine (I-131): Considered the gold standard treatment — a single injection selectively destroys overactive thyroid tissue. Highly effective, with minimal side effects. Requires a period of hospitalisation in specialist facilities until radioactivity decreases to safe levels. Definitive cure in most cases.
Surgical thyroidectomy: Effective but carries anaesthetic and surgical risks in older cats, particularly those with concurrent cardiac disease. Risk of inadvertent parathyroid damage causing hypocalcaemia.
Dietary management (iodine-restricted diet): Effectively reduces thyroid hormone production if fed exclusively. Palatability and compliance can be challenging in cats with concurrent illness.
Concurrent Renal Disease
Hyperthyroidism artificially increases GFR (glomerular filtration rate), potentially masking concurrent chronic kidney disease (CKD). Treatment of hyperthyroidism can unmask previously hidden CKD — requiring careful renal function monitoring before committing to permanent treatment. Trial medical therapy before radioiodine allows assessment of renal response.
Monitoring and Quality of Life
Regular monitoring (weight, clinical signs, blood pressure, thyroid levels, renal function) allows timely adjustment of treatment. Quality of life assessment should encompass comfort, activity, appetite, social interaction, and pain. Older hyperthyroid cats often improve dramatically with treatment — increased comfort, weight gain, reduced restlessness — demonstrating clear welfare benefit of intervention.