Macroprolactinoma-Induced Syndrome of Inappropriate Antidiuresis and Its Reversal with Dopamine Agonist Therapy.

Hyponatremia is an uncommon manifestation of pituitary adenomas. Herein, I report a case of syndrome of inappropriate antidiuresis (SIAD) caused by a macroprolactinoma that rapidly resolved with dopamine agonist therapy. A 29-year-old White woman presented with euvolemic, hypotonic hyponatremia, normal thyroid and glucocorticoid axes, and inappropriately concentrated urine. She was found to have a 1.2-cm sellar mass. Investigation of additional pituitary axes revealed an elevated prolactin level of 193.7 ng/mL. The SIAD experienced by the patient corrected rapidly with initiation of cabergoline. The patient could not tolerate dopamine agonist therapy, and after 1 year, she underwent transsphenoidal resection of the mass after the prolactin began to increase. Pathological examination confirmed the diagnosis of macroprolactinoma. There was no recurrence of the tumor, and the patient continued to have normonatremia and normoprolactinemia 7 years after her operation. To my knowledge, this is the first report in the literature of pathology-confirmed macroprolactinoma marked by SIAD that showed rapid normalization of water metabolism with dopamine agonist therapy.