Hyperthyroidism is the most commonly diagnosed endocrinopathy in the cat. Overproduction of the thyroid hormones, triiodothyronine [T3] and thyroxine [T4], is attributable to thyroid adenoma in more than 95% of cases. Locally an innocuous tumour, its “malignancy” stems from the enhanced tissue catabolism that is initiated by high levels of these hormones. The result is a disease state that is multisystemic in nature. Although no breed or sex predilection exists, the disorder characteristically affects the age groups between six and twenty years. The cause of tumor development in these cats has yet to be discovered.