RESUMEN
The causes of hypo-osmolar hyponatremic patients without edema are mainly related to either SIADH (syndrome of inappropriate antidiuretic hormone secretion) or hypovolemia and the evaluation of extracelluar volume status can be a clue to differentiate between the two. As a diagnostic andtherapeutic tool, positive response to isotonic saline load test is recognized in favor of hypovolemic hyponatremia but there is a pitfall due to similar response in a subset of euvolemic hyponatremia, SIADH in case that combined with hypovolemia or accompanied by low sodium intake. In such case, water load test may provide more help for exact evaluation of hypovolemia and SIADH. Hereby, we report a case of a 74 year old patient with doubtful thymoma radiologically presented to emergency room with symptomatic severe hyponatremia (108 mEq/L) consistent with clinical and biochemical features of hypovolemia and positive response to initial isotonic saline infusion. However, episodes of hyponatremia recurred despite euvolemic status, which was diagnosed as SIADH by water load test. We initially considered thymoma as a cause of SIADH but its pathologic finding was a thymic cyst. And hyponatremia with hypovolemic feature recurred but the final diagnosis came out as idiopathic SIADH by repeated water load test. His hyponatremia was completely corrected by strict water restriction (<500 cc/day).