A 47-year-old man presented to our emergency department (ED) with limbs weakness for 2 h. His heart rate was 127 beats per minute and blood pressure was 95/49 mm Hg. He found weakness of limbs after 4-h sleep. Physical examinations revealed that the muscle strength of upper limbs is 3/5, and lower limbs are 2/5. Electrocardiogram (ECG) revealed wide QRS complex, monomorphic ventricular tachycardia (VT) with ST-segment depression and long QT interval. Serum potassium level was extremely low as 1.0 mEq/L. This led to periodic hypokalemic paralysis. Due to severe hypokalemia with possible atrioventricular block, the patient was admitted to the intensive care unit. During hospitalization, his potassium level returned to 5.1 mEq/L on the first day. He had a low level of thyroid stimulating hormone (TSH) of <0.03 micro-IU/mL (normal range: 0.25-4.00) and a high free thyroxine (T4) level of 2.43 ng/dL (normal range: 0.89-1.79 ng/dL). Therefore, hyperthyroidism was diagnosed, and 5 mg of methimazole was administered twice a day. The patient was discharged on the seventh day after admission. The final diagnosis is thyrotoxic periodic paralysis (TPP), also as known as nocturnal paralysis or night palsy.