SIADH is only an atypical clinical feature in a patient with prolactinoma.

A 50-year-old man was admitted to determine the pathogenesis of hyponatremia. He had a poor appetite and was easily fatigued. Physical findings showed that he was conscious and alert. He had neither dry skin or tongue, nor pretibial edema. Laboratory data revealed that the serum sodium level was 110 mmol/l; plasma osmolality, 238 mmol/kg; and urinary osmolality, 417 mmol/kg. Plasma arginine vasopressin was 0.5 pg/ml despite plasma osmolality of 242 mmol/kg. An acute water load showed impaired water excretion, as percent excretion of water load was 30% and minimal urinary osmolality was 642 mmol/kg. Serum prolactin was 254 ng/ml, and anterior pituitary hormones of ACTH, TSH and GH were in the normal ranges. Brain magnetic resonance imaging (MRI) showed a pituitary tumor with a size of 20 x 22 x 21 mm and it pushed a pituitary stalk upward. Immunohistochemistry revealed prolactinoma. After the adenomectomy, serum sodium level has been kept normal with free access to water intake. The present study indicates that syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is manifested in association with pituitary macroadenoma of prolactinoma.