Thyroid storm or thyrotoxic crisis is a rare but life-threatening condition requiring immediate treatment, preferably in an intensive care unit. Its incidence is about 1-2% among patients with overt hyperthyroidism. A thyrotoxic crisis occurs predominantly in the elderly and is three to five times more common in women than in men. The overall mortality is 10-20%. Even though the pathogenesis is still not fully understood, an increased sensitivity to catecholamines appears to be an important mechanism, and a number of endogenous and exogenous stress factors that can provoke the onset of a thyrotoxic storm have been identified. The diagnosis of a thyrotoxic crisis is made entirely on the clinical findings. Most importantly, there is no difference in thyroid hormone levels between patients with "uncomplicated" thyrotoxicosis and those undergoing a thyroid storm. Any delay in therapy, e.g. by awaiting additional laboratory results, must be strictly avoided, because the mortality rate may rise to 75%. Thus early thyroidectomy should be considered as the treatment of choice, if medical treatment fails to result in clinical improvement. Medical treatment is based on three principles: 1) counteracting the peripheral effects of thyroid hormones; 2) inhibition of thyroid hormone synthesis; and 3) treatment of systemic complications. These measures should bring about clinical improvement within 12-24 hours. If death occurs it is most likely to be cardiopulmonary failure, particularly in the elderly.