Objective: To determine the efficacy and safety of iopanoic acid for the treatment of experimentally-induced hyperthyroidism in cats. Animals: 15 healthy adult domestic short hair cats Procedures: Hyperthyroidism was induced by daily subcutaneous administration of levothyroxine for 42 days. On day 28, cats were randomized to a control group receiving a placebo PO every 12 hours, a low dose group receiving 50 mg iopanoic acid PO every 12 hours, and a high dose group receiving 100 mg iopanoic acid PO every 12 hours. Cats were treated for 14 days. Weight and heart rates were obtained on days -8, 0, 28, 35, and 42. Blood was collected for CBC and biochemical analysis and for T4, T3, and rT3 measurement on days -8, 28, 35, and 42. Results: Two cats were removed prior to day 28 due to prolonged anorexia and another on day 36 because of heart failure. The low dose and high dose groups had significantly lower T3 concentrations on days 35 and 42 compared to the control group and to their own T3 concentrations on day 28. The T3 concentrations in cats administered iopanoic acid were not different from those obtained prior to induction of hyperthyroidism. Body weight and food consumption were not altered by iopanoic acid treatment, while heart rate was decreased in the low dose group on day 35 when compared to day 28. Conclusions and clinical relevance: Iopanoic acid was effective in decreasing T3 concentrations, but its effect on clinical signs of hyperthyroidism was less apparent. Studies evaluating the long-term efficacy in cats with naturally-occurring hyperthyroidism are warranted. / Master of Science