[Hyponatremia caused by pituitary metastasis of lung cancer].

A 57-year-old man was admitted with headache, vomiting, and bloody sputum. We diagnosed large cell lung cancer T4N2M1 (pituitary metastasis), Stage IV. When hospitalized, low values of cortisol and hyponatremia were found. A hormone stimulation test was performed, because we suspected hypopituitarism. The reaction of adrenocorticotropic hormone (ACTH) to the corticotropin-releasing hormone (CRH) loading test was good, but the reaction of serum cortisol was minimal. After corticosteroid administration, his serum sodium normalized. Limited ACTH reserve according to insufficient pituitary function was suggested as a cause of the hyponatremia. He received gamma-knife therapy, however his pituitary gland tumor did not decrease in size. Clinical symptoms such as visual field disturbance, oculomotor paresis, and visual impairment progressed, and he died about 5 months later. We report a case of hyponatremia in a patient with pituitary metastasis of lung cancer, as it is comparatively rare.