Hyperthyroidism is the most common hormonal disorder of older cats, causing weight loss, cardiovascular strain, and discomfort. Effective treatment restores quality of life and is available in multiple forms.
Untreated hyperthyroidism causes progressive, multisystem suffering. The hypermetabolic state produces constant hunger despite eating more, creating a distressing paradox. Cardiovascular changes — elevated heart rate, hypertension, cardiac hypertrophy — cause physiological stress and risk of heart failure and stroke. Gastrointestinal signs cause chronic discomfort. Many owners initially interpret the increased energy as a positive sign, delaying diagnosis. Effective treatment dramatically improves quality of life — cats return to normal weight, calm behavior, and coat condition. Methimazole side effects (vomiting, facial itching) occur in a minority but require monitoring. Renal function must be assessed before and after treatment, as hyperthyroidism can mask renal disease that becomes apparent once thyroid function is corrected.