Hyperthyroidism is the most common hormonal disease affecting middle-aged to older cats. Molecular abnormalities in the thyroid gland are the likely culprits that play a part in this malady. The cause of the changes in gland function is unknown. Some recent studies have implicated diet as playing a role in the development of the disease. Cats evolved as meat-eating animals and canned cat food contains ingredients, other than meat. High concentrations of some of these same ingredients cause thyroid disease in people. Since feline hyperthyroidism usually occurs in older animals, it is possible that exposure to these substances over many years results in changes in the thyroid gland. This study will attempt to identify potentially harmful substances that can be targeted for future investigation.