A deficiency in the release of vasopressin (antidiuretic hormone) by the posterior pituitary gland. Why?

Possibly infarction. Possibly a tumor or something else. Lack of blood supply to the pituitary gland or a pituitary tumor may change the secretions of adh (vasopressin). However, there are other causes also, such as pregnancy that can cause this area to be infarcted (loss of blood supply).