Thyroid storm is a syndrome described in human medicine to define a multi systemic disorder resulting from organ exposure to excessive levels of thyroid hormone. This form of acute thyrotoxicosis can be life-threatening and is a significant cause of mortality in human emergency rooms. Thyrotoxicosis is a term used to describe any condition in which there is an excessive amount of circulating thyroid hormone whether from excess production and secretion from an overactive thyroid gland, leakage from a damaged thyroid gland, or from an exogenous source. In contrast, hyperthyroidism describes thyroid gland hyperfunction. Therefore, acute thyrotoxicosis may occur in the absence of thyroid gland hyperfunction, although this is rare in veterinary medicine. In humans, thyroid storm can occur at any age. It can be present in euthyroid patients as well as treated and partially-treated hyperthyroid patients. Although this is a well recognized syndrome in human medicine, it has not been described as a clinical entity in veterinary medicine. Hyperthyroidism is a common disease in older feline patients. It is also rarely seen in dogs with thyroid carcinoma. Hyperthyroid cats that present with acute exacerbation of thyrotoxicosis may be described to have thyroid storm. This chapter will discuss the human syndrome and define a similar syndrome in hyperthyroid veterinary patients. The clinical signs of and treatment modalities for feline thyroid storm will also be presented.