A diagnosis of feline hyperthyroidism has increased sharply since 1976. There is no sex predeliction, but Siamese and Himilayans are less likely to develop it. It is most often found in cats over 8 years of age (>95%), but it can be found in almost any age cat. Histologically, adenomatous hyperplasias are most often diagnosed followed by benign adenomas, and rarely (<5%) carcinomas. The cause remains elusive. Clinical signs are caused by excessive production of thyroid hormone and owners most often complain of weight loss, polyphagia, vomiting, and restlessness or hyperactivity. Other clinical signs include polydipsia, polyuria, diarrhea. Since these signs overlap with many other medical conditions found in older cats, it is necessary to differentiate them from other common diseases such as diabetes mellitus, chronic renal disease, inflammatory bowel disease, and neoplasia. Physical examination often reveals a palpable thyroid nodule, poor body condition with muscle wasting, tachycardia with or without a heart murmur, or arrhythmia. They can become easily stressed, very aggressive to handle, or in rare cases apathetic (10-20%). A complete work-up of cats presenting with the above signs includes: CBC, chemistry profile, urine analysis, and a thyroid hormone panel. Due to the high prevalence of cardiovascular changes thoracic radiographs, and ECG, and blood pressure measurements are also recommended. A Technicium scan is necessary to detect ectopic tissue, but may not be available in all clinics. Common treatment options include medical management (radioactive iodine, chronic antithyroid drug therapy) or surgery. All treatment choices have both advantages and disadvantages and must be made on a case by case basis with the owner. Consequences of not treating can be fatal. Feline hyperthyroidism can be a very serious disease that can present in many different ways and it is up to the clinician to be able to recognize, diagnose, and determine the best course of therapy for this very treatable disease.