Thyrotoxic periodic paralysis is characterized by acute hypokalemia, muscle weakness and thyroxoxicosis triad without total body potassium deficiency. A case of thyrotoxic periodic paralysis is presented in the paper. A 21-year-old male patient with an Afghan nationality admitted to the emergency room with fatigue and muscle weakness. Neurological examination revealed lower extremity motor muscle strength, proximal 2/5 and distal 4/5, upper extremity muscle strength proximal 3/5 and distal 4/5. The patient was followed up with intravenous (IV) KCL replacement therapy because potassium level was 2.6 mmol/L and the patient was symptomatic. Thyroid function tests for the differential diagnosis of hypokalemia revealed obvious hypethyroidism (TSH: 0.01 IU/mL sT4: 4.32 ng/dL sT3: 17.4 pg/mL). Prolonged QT, diffuse ST depression, pathologic and U wave was detected on ECG. Potassium measurements were performed at the 1st, 4th, and 12th hours while patient was receiving IV and oral potassium replacement. Potassium infusion was stopped because at the 4th hour of treatment potassium level was at the desired level. Metimazole 45 mg and propranolol 40 mg was started for hyperthyroidism. Thyrotoxic periodic paralysis is a rare disease but it can be fatal if it is not treated.