Initially, an insulin tolerance test was used to induce hypoglycemia in patients 3 months after transsphenoidal pituitary surgery, which showed low copeptin levels in patients with AVP deficiency of 3.7 ± 0.7 pmol/l, with hypoglycemic copeptin levels of <4.75 pmol/l having the best diagnostic accuracy of 100%. This evidence concerns the gene AVP and neurohypophyseal diabetes insipidus.