RESUMO
We present a case of severe hyponatremia in a 64-year-old man who had a pituitary tumor. He had nausea and recurrent vomiting with a severe headache, and was admitted to Chungnam National University Hospital for further evaluation. On physical examination, he was alert, and had bitemporal hemianopsia. There was no indication of dehydration or edema. Laboratory data showed a serum sodium level of 126 mEq/L, plasma osmolality of 259 mOsm/kg, and urinary osmolality of 544 mOsm/kg. The plasma argipressin level was 2.88 pg/mL, despite marked hyposmolality. Otherwise, pituitary function was normal. Brain magnetic resonance imaging showed a pituitary macroadenoma measuring 25x16x13 mm and no visible normal pituitary stalk or gland in the sella turcica. After the adenomectomy, the serum sodium level maintained normal without treatment. Histology showed the presence of a pituitary adenoma. These findings indicate that a non-functioning pituitary tumor may cause the exaggerated secretion of argipressin, resulting in inappropriate antidiuretic hormone syndrome.