An isolated DNA molecule comprising a fragment of the gene encoding the feline NIS is disclosed as well as methods of use thereof. Also provided are methods for rational diet design of food composition suitable for administration to feline companion animals afflicted with hyperthyroidism, comprising (a) accessing at least one database that comprises a first data set relating functional gene profile of a biofluid or tissue sample from an animal to physiological condition of the animal, where the functional gene profile is that of the feline NIS gene (b) accessing at least one database that comprises a second data set relating effects of bioactive dietary components on the functional gene profile (c) using an algorithm drawing on these data sets, processing input data defining physiological condition to derive a nutritional formula promoting wellness of a feline companion animal afflicted with hyperthyroidism and (d) preparing a food composition based on the nutritional formula.
