Feline hyperthyroidism is the most common endocrine disorder of older cats; effective treatment dramatically improves quality of life and enables years of comfortable living.
Hyperthyroidism causes significant suffering in affected cats — the experience of chronic increased metabolism, weight loss, hypertension, and progressive cardiac and renal disease impairs wellbeing significantly. Because affected cats are often still active and appetite is increased, owners sometimes misread the signs as the cat being well. Diagnosis through blood testing is straightforward. Treatment is highly effective regardless of method — methimazole controls the condition in most cats; radioiodine cures it. An important consideration is concurrent kidney disease, which hyperthyroidism masks by increasing blood flow to the kidneys; treating hyperthyroidism may unmask underlying CKD. Post-treatment kidney monitoring is therefore essential. Early detection through annual blood testing from age 8 is the key to optimal outcomes.