Feline hyperthyroidism was first reported in 1979 and has become the most common endocrine disease of older cats. In previous studies, recent diets of commercial canned cat foods, exposure to flea control products and lawn chemicals, indoor housing, and cat litter have been found to be associated with increased risk. The purpose of this work was to determine whether the increased prevalence of feline hyperthyroidism is due to an aging of the cat population and to identify risk factors for clinical hyperthyroidism in cats based on lifetime histories of diets and exposures to certain endocrine disrupters. The Veterinary Medical Database was analyzed to determine the twenty-year prevalence of feline hyperthyroidism. The Feline Lifetime Nutrition and Health Survey was designed to test hypotheses regarding lifetime exposures and hyperthyroidism. Questionnaire responses from owners of cats presented to Purdue University Veterinary Teaching Hospital were used to conduct a case-control study to identify risk factors. The hospital prevalence of feline hyperthyroidism has increased significantly in the past 20 years and is not solely attributable to the aging of the cat population. Cats consuming foods packaged in cans, especially those with easy-open lids, were nearly four times as likely to develop hyperthyroidism as those eating dry foods. Female gender was associated with a three-fold increased risk; consumption of food predominantly without explicit iodine supplementation in any life stage increased the risk four times. Increased risk was associated with each additional year of well water consumption (12%), gas fireplace exposure (16%), and age (15%). Based on these findings, owners of cats should reduce or avoid feeding foods from cans with easy-open lids. Consumption of well water, particularly for male cats, and exposure to gas fireplaces should be reduced or avoided, and cat food should be supplemented with iodine. Pet food companies should determine the optimal level of iodine in cat food and supplement foods accordingly. The environmental risk factors (pop-top can linings, gas fireplaces, and well water) suggest that endocrine disrupters may play a role in the pathogenesis of feline hyperthyroidism.
