Severe Hyponatremia Caused by Secondary Adrenal Insufficiency in a Patient with Giant Pituitary Prolactinoma

A 55-year-old-man was admitted to Saiseikai Central Hospital, Tokyo, Japan, complaining of nausea and appetite loss, and was found to have severe hyponatremia. Despite severe hyponatremia and plasma hypo-osmolarity, urinary sodium excretion was not reduced. A brain magnetic resonance imaging (MRI) scan revealed a giant pituitary prolactinoma, and endocrinological tests showed a markedly increased prolactin level. Despite the observation that the basal plasma ACTH level was normal, serum cortisol and urinary cortisol excretion levels were low. Rapid ACTH loading sufficiently stimulated an increase in serum cortisol levels, suggesting secondary adrenal insufficiency. Notably, loading of CRH induced a good ACTH response; however, the serum cortisol response remained low. In contrast, the continuous daily administration of exogenous ACTH dramatically increased serum cortisol levels. These discrepant responses may have been caused by the low biological activity of innate ACTH. Following partial resection of the prolactinoma, postoperative adjuvant therapy with cabergoline effectively reduced prolactin levels, but did not improve the hyponatremia. In contrast, hydrocortisone replacement therapy recovered the serum sodium level to the normal range. The present case is the first report describing a link between severe hyponatremia and biologically inactive circulating ACTH as a likely result of giant prolactinoma.