A 42-year-old man of Chinese descent, known to have Graves' disease, presented with muscle weakness. Neurological examination showed paralysis of the arms and legs, with intact sensory function, while blood-test revealed hypokalaemia and thyrotoxicosis. The neurological symptoms resolved quickly after correction of the serum potassium level. Thyrotoxic periodic paralysis is a rare, reversible complication of hyperthyroidism is characterised by hypokalaemia, hyperthyroidism and paralysis.