Hyperthyroidism is the most common feline endocrine disease, causing serious abnormalities in many organs. Most cats with hyperthyroidism have elevated liver enzymes for unknown reasons. So far, the association between hyperthyroidism and liver disease has been poorly documented. Liver enzymes will often return to normal once cats are treated for their hyperthyroidism. However close to 50% of cats will still have moderate to severe elevations in the liver enzyme alanine aminotransferase (ALT). In humans with hyperthyroidism, liver changes have included fat infiltrates and areas of focal necrosis. The prevalence of hepatic disease in cats with hyperthyroidism and the type of liver changes present have not yet been determined. This study will be looking at the relationship between feline hyperthyroidism and the functional and structural damage that may be present in their livers. It will be looking at those cats with persistently high liver-specific enzymes, despite treatment with radioiodine.