• Thyroid storm is a well recognized syndrome in humans but has not previously been described as a clinical entity in veterinary medicine.
• Diagnosis of thyroid storm is based on the presence of thyrotoxicosis, appropriate clinical signs, and evidence of precipitating events.
• Early recognition and aggressive treatment of feline thyroid storm should improve survival.

Thyrotoxicosis in cats is most commonly recognized in middle-aged to older cats presenting with hyperthyroidism. Humans have a unique presentation of thyrotoxicosis called "thyroid storm," which causes significant mortality because it can occur at any age and can present in euthyroid patients as well as treated and partially treated hyperthyroid patients. The exact pathogenesis is unknown and most likely multifactorial. What is key is not the level of thyroid hormone but a large, rapid change in circulating thyroid hormones. Thyroid storm in humans can be precipitated by such events as infection, thyroidal and nonthyroidal surgery, radioactive iodine therapy, administration of iodinated contrast dyes, withdrawal of antithyroid hormones, ingestion of exogenous amounts of thyroid hormone, emotional stress, and acute nonthyroidal illnesses. In cats, thyroid storm could occur as a result of radioactive iodine therapy for hyperthyroidism, thyroidal surgery, or vigorous palpation of the thyroid causing cell destruction and release of hormone. Sudden withdrawal of antithyroid hormones could also cause an acute increase in thyroid hormones. Stress, illness, and infection are most likely important triggers for feline thyroid storm in hyperthyroid cats. The clinical signs of thyroid storm are an acute exacerbation of signs of thyrotoxicosis. In cats, these include any combination of tachypnea, tachycardia, hyperthermia, respiratory distress, cardiac murmur, cardiac arrhythmia, auscultatory crackles, sudden blindness, severe muscle weakness, ventroflexion of the neck, absent motor limb function, neurologic signs, and sudden death. Cats may be hypertensive and may have retinal changes, hemorrhage, edema, or degeneration and acute retinal detachment. Diagnosis is made by identification of thyrotoxicosis, clinical signs, and a precipitating event.

Feline thyroid storm. Ward CR. Vet Clin North Am 37:745-751, 2007.