Myxedema coma is a life-threatening condition; it is a complication of untreated hypothyroidism and an endocrine emergency. Most patients are elderly women with a previous history of long-standing hypothyroidism which presents during the winter. The myxedema coma has an insidious onset and it is very rare; its recognition can be quite difficult. Once suspected, treatment can be lifesaving and should be start promptly in anticipation of confirmation of the diagnosis by laboratory test. The mortality rate is high. I presented a case of an old woman with myxedema coma with an undiagnosed hypothyroidism, with altered mental status, normal temperature, pneumonia, hyponatremia and high level of creatine phosphokinase, who presented in the emergency room.