A 26-year-old Hispanic man with no significant medical history presented to our emergency room with gradual onset weakness of his lower extremities. He was haemodynamically stable and examination revealed loss of motor function in his lower limbs up to the level of hips. Laboratory data revealed hypokalaemia. The patient was started on potassium supplementation and he recovered his muscle strength. Differential diagnosis included familial hypokalaemic periodic paralysis and thyrotoxic periodic paralysis (TPP). Further investigations revealed a low thyroid-stimulating hormone and high free thyroxine levels. Radio iodine 123 scan revealed an enhanced homogeneous uptake in the thyroid suggesting Graves' disease. Thyroid stimulating antibodies were also found to be elevated. The patient was started on methimazole and propranolol and he never had another attack of TPP even at 1 year follow-up.