Chronic deterioration of the glucocorticoid function may result from a primary disorder of the suprarenal glands; from a deficit of the corticotropic hormone (ACTH) due in most cases to an extended hypophyseal affection with reduced secretion of other hypophyseal hormones and from the lack of corticotropin releasing hormone (CRH) at the hypothalamic level. Isolated ACTH deficiency is a rare clinical entity. A subgroup within this pathology is the limited reserve of ACTH which would constitute a mild form of dysfunction in the corticotropic cells, with enough secretion of ACTH to maintain a normal adrenal function in the absence of stress-inducing situations, but unable to show an appropriate reaction in such situations. The difference between these two entities lies in the normal levels of 17-OH-corticosteroids in the limited reserve of ACTH. We describe the case of a patient with acute severe hyponatremia in whom this rare affection was diagnosed and we make some considerations regarding this entity.