Hormone-inactive pituitary macroadenoma : an uncommon cause for the syndrome of inappropriate antidiuresis

A 76-year-old patient was admitted with dizzy spells and fainting. Laboratory analysis indicated the syndrome of inappropriate antidiuresis, which was further characterized as type C ("reset osmostat"). Extended workup revealed a hormone - inactive pituitary macroadenoma. After complete transsphenoidal resection, serum sodium levels and plasma and urinary osmolality promptly normalized. Except for the gonadal axis, the anterior pituitary functions remained intact. There was no adenoma recurrence over a five-year follow-up, and repetitive testing on ad libitum fluid intake showed normal sodium concentrations. Thus, the pituitary macroadenoma represents the sole detectable cause for the syndrome of inappropriate antidiuresis in this patient.