It is rare to meet an uncontrolled hyperthyroid patient in elective surgery. But, in emergency surgery, recognition of hyperthyroidism may be difficult, as trauma or pain may mask hyperthyroidism. The patient may be unaware of its existence or incapable of transmitting the information. During or after surgery, untreated hyperthyroidism can cause thyrotoxic crisis (thyroid storm), which may be a highly fatal complication. Thyrotoxic crisis is an extreme accentuation of signs and symptoms of throtoxicosis. Clinical manifestations are sinus tachycardia or atrial fibrillation, marked increase in systolic pressure and pulse pressure, high temperature, profuse sweating, tremor, dehydration, tachypnea, extreme restlessness and agitation, delirium, and frank psychosis. In this case, atrial fibrillation and hypertension developed after induction, and severe thyrotoxic symptoms appeared after emergence, which were caused by undetected hyperthyroidism.