In order to test posterior pituitary function it is important to assess and replace corticotroph function before assessing posterior pituitary hormone production because ACTH deficiency leads to a reduced GFR and the inability to excrete a water load which may therefore mask diabetes insipidus.
With diabetes insipidus the urine output is usually >3 l a day. Other causes of osmotic diuresis need to be excluded.
The fluid deprivation test assesses the ability of the kidney to concentrate urine under the influence of ADH. Occasionally further investigations are required particularly when only partial forms of the condition are present.
The patient is allowed fluids overnight. The patient is deprived of fluids for 8 hours or until 5% of the body mass has been lost. The patient needs to be weighed hourly. Plasma osmolality is measured 4 hourly and urine volume and osmolality every 2 h. At the end of 8 h the patient is given 2 mcg of intramuscular desmopressin and urine and plasma osmolality checked over the next 4 h.
If serum osmolality rises to >305 mmol/kg the patient has diabetes insipidus and the test is stopped.
With cranial DI the urine osmolality remains below 300 osmols/kg and rises to >800 after desmopressin. With nephrogenetic diabetes insipidus the urine osmolality is <300 both before and after desmopressin.