Inappropriate antidiuretic hormone complicating histiocytic lymphoma

The syndrome of inappropriate antidiuretic hormone (IADH) often causes the hyponatremia that may be seen in patients with malignant disorders. Most physicians correctly associate IADH with small cell carcinoma of the lung. We describe two patients in whom IADH was caused by histiocytic lymphoma. One patient was thought to have small cell carcinoma of the lung on the basis of marrow infiltration and the IADH. When the proper diagnosis was made and therapy instituted, both patients responded, with rapid resolution of their disease and the IADH. The identification of the neoplasm that produces the IADH is important, since histiocytic lymphoma may mimic small cell carcinoma of the lung, yet may be very responsive with newer treatment regimens.