Diabetic ketoacidosis (DKA) is a common cause for admission in Emergency Department. Its treatment is well defined. Nevertheless, in some cases, type I diabetes combines with auto-immune polyendocrine syndrome, which can carry life-threatening consequences. Here we report the case of a young man with inaugural DKA who exhibited ventricular fibrillation and cardiac arrest due to significant hypokalaemia, following undiagnosed thyrotoxicosis with periodic paralysis.