Inappropriate antidiuresis associated with pituitary adenoma - mechanisms not involving inappropriate secretion of vasopressin

We report an unusual case of inappropriate antidiuresis with undetectable v asopressin in an elderly man presenting with confusion due to severe hypona tremia. Further investigations led to the diagnosis of non-functional pitui tary macroadenoma. The patient had normal thyroid and adrenal function. The abnormal water balance resolved promptly after transsphenoidal removal of the tumor, confirmed by a repeat water loading test. We conclude that inapp ropriate antidiuresis in the absence of excess vasopressin secretion may im plicate mass effect from an underlying pituitary tumor.