Corticotropin deficiency: a rare cause of hyponatremia mimicking SIADH.

We describe 2 patients presenting with severe chronic hyponatremia in whom clinical and biochemical features strongly suggested the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Both, however, were proven to have a primary pituitary deficiency of corticotropin. Their short synacthen tests were only mildly abnormal but associated with low basal ACTH levels. The diagnosis of ACTH deficiency was made more convincingly by their dramatic response to glucocorticoid replacement therapy. In patients in whom no cause for SIADH can be found, a trial of maintenance cortisol therapy is warranted to exclude this eminently treatable condition.