A 61-year-old man presented with lower extremity paralysis and severe hypokalemia. His thyroid function test showed thyrotoxicosis. Despite attempts to correct his hypokalemia, he developed pulseless polymorphic ventricular tachycardia two hours later. He was successfully resuscitated after defibrillation. We performed continuous venovenous hemodiafiltration for 10 days due to acute kidney injury and rhabdomyolysis. We observed life-threatening polymorphic ventricular tachycardia requiring urgent defibrillation, as well as rhabdomyolysis requiring dialysis during the transient thyrotoxic phase of painless thyroiditis. Pay attention to the possibility of the development of life-threatening ventricular tachycardia associated with hypokalemia in the setting of thyroiditis and thyrotoxic paralysis.