Hyperthyroidism has been recognized as a debilitating and potentially fatal disorder of middle to old aged cats for approximately thirty years. This disease is a multisystemic metabolic disorder caused by an excess of circulating thyroid hormone. Fourteen cats with confirmed feline hyperthyroidism were selected for this study. Eleven cats were treated with the herbal formula Hai Zao Yu Hu Tang (Sargassum Jadepot) 0.2 gm BID and 3 cats were treated with methimazole (2.5 mg BID), the most commonly prescribed allopathic medication. Over an 8 week treatment period, nine parameters were assessed: heart rate, respiratory rate, general behavior, urinations and thirst, appetite, vomiting, aggressive behavior, quality of life and serum tetraiodothyronine (T4) levels. All cats receiving Hai Zao Yu Hu Tang (Sargassum Jadepot) improved their clinical signs during the 8 week period, but some continued to have elevated serum T4 levels. When the results obtained with Hai Zao Yu Hu Tang (Sargassum Jadepot) were compared to methimazole, the herbal formula produced statistically significant (P<0.05) greater improvement in every parameter measured except the circulating serum T4 levels.