Hyperthyroidism is a common disorder in older cats which leads to weight loss, rapid heart rate, anxiety, and increased appetite. Methimazole is administered as an oral tablet, and is effective in lowering thyroid hormone levels and resolving the clinical signs of hyperthyroidism in most cats. However, 10 to 20% of cats experience vomiting and loss of appetite during treatment, and some cats cannot be treated with oral methimazole because of poor appetite, a fractious nature, or concurrent problems with intestinal absorption. Recently, custom veterinary pharmacies have offered methimazole in a gel formulation for topical application in cats. When applied as a small amount of gel to the inner ear, this route of administration is much more convenient for owners and appears to successfully control hyperthyroidism in a small number of cats that have been treated. The purpose of this study is to determine the degree of absorption of topical methimazole in cats, and to determine whether topical methimazole is as effective as oral methimazole for the control of naturally occurring hyperthyroidism in cats.