Essential hypernatremia.

Described is a patient who presented with hypernatremia in the absence of dehydration. Further investigation revealed a tumor in the hypothalamic area, and evidence of anterior pituitary hypofunction. Water loading did not correct hypernatremia, and the results of the water-loading test suggested that hypernatremia had resulted from an elevated "osmotic set point" for the release of antidiuretic hormone, ie, "essential hypernatremia."