Hyperthyroidism is the most common endocrine disease of the cat, and therefore responsible for a substantial proportion of feline morbidity. Thyroid hormone exerts broad metabolic effects; however, a comprehensive assessment of metabolic pathways affected in hyperthyroidism has yet to be undertaken. Additionally, factors predisposing cats to hyperthyroidism are not well defined and all current treatments for the disease have drawbacks. While radioactive iodine (I131) treatment is expensive and has limited availability, it is a definitive treatment with a success rate of over 95%. Not all pathophysiologic changes associated with hyperthyroidism resolve with effective treatment. For example, some changes in metabolomic profiles in hyperthyroid people persist and after treatment and the renin-angiotensin-aldosterone system remains activated in cats with hyperthyroidism. Therefore, we hypothesize that hyperthyroidism is characterized by unique metabolic signatures and an altered fecal microbiome and that many of these signatures persist even after achieving euthyroid status. Our secondary hypothesis is that distinct metabolomic markers or microbiome patterns will align with common intercurrent diseases (e.g. chronic kidney disease). Cat’s presenting to the Cornell University Hospital for Animals for I131 treatment will constitute the cases to test our hypothesis through the following specific aims: 1) to define the metabolic signatures and fecal microbiome patterns of hyperthyroid cats, 2) to serially evaluate the metabolic signatures and fecal microbiome patterns of hyperthyroid cats following treatment with I131, and 3) to compare metabolomic and microbiome profiles with salient clinical features of hyperthyroidism. We will leverage high throughput metabolomic profiling by mass spectrometry and 16S sequencing techniques to resolve the metabolic pathways and enteric microbial populations, respectively, that represent the feline hyperthyroid state. We anticipate this study will substantially advance our knowledge of feline hyperthyroidism.