This site also discusses some of these studies in a bit more detail:http://www.vin.com/VINDBPub/SearchPB...00/PR00107.htm
"Two recent large studies have looked at possible environmental or dietary factors involved in the patho-genesis of hyperthyroidism. One of the studies with a case controlled design looked at 100 cats with hyper-thyroidism and 163 control cats. The cats medical records were reviewed and the owners were asked to complete a mailed questionnaire. Data included demographic variables, environmental exposures, and diet (including preferred flavors of canned cat food). In this study, housing, exposure to fertilizers, herbicides, regular use of flea products, and the presence of a smoker in the house were not associated with an increased risk, but cats that preferred fish or liver and giblets flavors of canned cat food had an increased risk. The results suggested that cats that prefer to eat certain flavors of canned cat food might have a significantly increased risk of hyperthyroidism.
In the second case controlled study owners of 379 hyperthyroid and 351 control cats were questioned about their cats' exposure to potential risk factors including breed, demographic factors, medical history, indoor environment, chemicals applied to the cat and environment, and diet. The association between these hypothesized risk factors and outcome of disease was evaluated by conditional logistic regression. Two genetically related cat breeds (Siamese and Himalayan) were found to have diminished risk of developing hyperthyroidism. Cats that used litter had higher risk of developing hyperthyroidism than those that did not. Use of topical ectoparasite preparations was associated with increased risk of developing hyperthyroidism. Compared with cats that did not eat canned food, those that ate commercially prepared canned food had an approximate two-fold increase in risk of disease. When these four variables (breed, use of cat litter, consumption of canned cat food, and use of topical ectoparasite preparations) from the univariate analysis were selected for further study as candidate risk factors and analyzed by multivariate conditional logistic regression, a persistent protective effect of breed (Siamese or Himalayan) was found. In addition, results suggested a two- to three-fold increase in risk of developing hyperthyroidism among cats eating a diet composed mostly of canned cat food and a three-fold increase in risk among those using cat litter. In contrast, the use of commercial flea products did not retain a strong association. The results of this study indicate that further research into dietary and other potentially important environmental factors (cat litter) is warranted."