Central diabetes insipidus (DI) can be differentiated from nephrogenic DI by the response to desmopressin, a synthetic vasopressin. After water deprivation, urine osmolality is greatly concentrated and greater than serum osmolality. In central DI, administration of desmopressin results in an increase in urine osmolality since the response to ADH is preserved. In contrast, in nephrogenic DI, urine osmolality does not change since there is no physiologic response to ADH administration. Measuring blood glucose levels after an oral glucose load is the oral glucose tolerance test used to diagnose diabetes mellitus (DM). Serial measurements of serum ACTH and serum cortisol after administration of corticotropin releasing hormone (CRH) is the CRH stimulation test used to differentiate pituitary tumors from an ectopic or adrenal source. Serum cortisol levels are measured before and serially after ACTH is injected describes the ACTH stimulation test used to diagnose primary versus secondary adrenal insufficiency.