SYNDROME OF INAPPROPRIATE SECRETION OF ADH (SIADH) DUE TO SMALL-CELL LUNG-CANCER WITH EXTREMELY HIGH PLASMA VASOPRESSIN LEVEL

A 76-year-old man with small cell lung cancer associated with the synd rome of inappropriate secretion of ADH (SIADH) visited our hospital. T he serum Na level was normal on the first visit, but 2 weeks later it decreased to 114 mEq/L with an extremely high plasma vasopressin (VP) level of 1520 pg/ml. Serum Na was normalized after the reduction of th e tumor size by chemotherapy, but the plasma VP level remained between 150 to 600 pg/ml. On gel filtration of plasma VP two peaks of immunor eactive VP were eluted at the positions of a larger molecule than auth entic VP and authentic VP, and VP in urine gave only one peak compared to that of authentic VP. The dilution curve of plasma VP was almost p arallel and that of urine was completely parallel to the standard curv e. These findings suggest that a larger VP with low physiological acti vity was predominantly secreted in the present patient and manifested relatively mild symptoms despite the extremely high plasma VP level.