Testing corticotropin-releasing hormone (CRH) levels involves measuring the response to an intravenous bolus injection of synthetic ovine CRH at doses of 1 mcg (200 nmol) per kg of body weight (or total dose of 100 mcg). 
|Plasma Concentration||Peak Level||Increase From Baseline||Time to Peak Level|
|Corticotropin||10-120 pg/mL (2.2-24 pmol)||35-900%||10-30 min postinjection|
|Cortisol||13-36 mcg/dL (360-1000 nmol/L)||20-600%||30-60 min postinjection|
Collection and Panels
See the Medscape Reference topic Adrenocorticotropin (ACTH).
Patients should fast for a minimum of 4 hours prior to the test,  following which ovine or human (human corticotropin-releasing hormone [CRH] is not approved by US Food and Drug Administration [FDA]) is injected intravenously over 30 seconds. Blood for specimens is collected at 15 minutes and 1 minute before CRH administration and at 30, 45, 60, 90, and 120 minutes after CRH administration for measuring corticotropin and cortisol levels. Recent studies have shown that ovine CRH and human CRH perform comparably. 
The test can be performed at any time of the day. Corticotropin increments are similar in morning and evening, but peak values are higher in the morning; cortisol values peak similarly in the morning and evening. Details are below:
Specimen: Plasma, frozen
Volume: 0.8 mL
Minimum volume: 0.3 mL (Note: This volume does not allow for repeat testing.)
In patients with pituitary Cushing syndrome, cortisol and corticotropin levels are normal or increased. These levels are suppressed in the presence of other conditions, such as ectopic Cushing syndrome, pseudo-Cushing syndrome, and adrenal syndrome. For patients with pseudo-Cushing syndrome, low-dose dexamethasone suppression testing alongside corticotropin-releasing hormone (CRH) testing aids in diagnosis. CRH testing is also used to differentiate hypothalamic adrenal insufficiency, in which patients have subnormal cortisol response and a prolonged and delayed corticotropin response, and pituitary adrenal insufficiency, in which patients demonstrate no response.
|Diagnosis||Corticotropin Response||Cortisol Response|
|Pituitary Cushing syndrome||Normal or increased||Normal or increased|
|Adrenal Cushing syndrome||Suppressed||Suppressed|
|Ectopic Cushing syndrome||Suppressed||Suppressed|
|Pituitary Adrenal insufficiency||Suppressed||Suppressed|
|Hypothalamic adrenal insufficiency||Normal||Low|
Although CRH testing is both difficult and costly, it is mainly used in certain settings such as finding the cause of corticotropin-dependent Cushing syndrome, distinguishing between pseudo-Cushing syndrome and Cushing syndrome, and identifying the difference between central and primary adrenal insufficiency.
CRH testing has a sensitivity of 86% and a specificity of 90% in assessing corticotropin-dependent Cushing syndrome.  The test best suited to distinguish between ectopic corticotropin secretion and Cushing disease is the inferior petrosal sinus sampling test, which helps localize the site within the pituitary for partial surgical resection, if desired. Combining desmopressin with CRH testing has helped with the differential diagnosis of Cushing syndrome. 
CRH is useful in differentiating corticotropin-dependent and independent Cushing syndrome, and it also helps in the investigation of pediatric Cushing disease. It helps enable localization of the side of pituitary gradient when used with bilateral inferior petrosal sinus sampling (IPSS). 
Diagnostic accuracy of the CRH test in patients with Cushing syndrome is comparable to the high-dose dexamethasone test. 
A low-dose dexamethasone suppression test prior to CRH testing further helps identify a corticotropin-secreting tumor. This combined test helps to distinguish true Cushing syndrome from pseudo-Cushing syndrome (increased cortisol level related to a non–hypothalamic-pituitary-adrenal (HPA) disorder, such as depression).
Corticotropin-releasing hormone (CRH) is a 41-amino acid peptide with noted sequence homology among species, specifically in the region required for biologic activity (amino-terminal region), and is a major secretagogue of pituitary corticotropin.  Because of homology among species, human and ovine sequences are both used to stimulate pituitary production and adrenal cortisol production.  Synthetic ovine CRH is equipotent to human CRH and has a more prolonged duration of action than human CRH, which makes it useful for clinical testing.  Human CRH is not approved by FDA.
1. To diagnose primary, secondary (pituitary), and tertiary (hypothalamic) adrenal insufficiency conditions  :
In primary adrenal insufficiency, there are high baseline corticotropin levels, which increase in response to CRH; cortisol levels remain low before and after CRH
In secondary adrenal insufficiency, there are low baseline corticotropin levels, which do not respond to CRH; cortisol levels are not affected by CRH
In tertiary disease, there are low baseline corticotropin levels, which show exaggerated, prolonged responses to CRH; serum cortisol levels do not exceed 20 mcg/dL
2. To differentiate pituitary Cushing disease (which responds by causing a >20% rise in cortisol and a >35% increase in corticotropin to CRH) from other conditions such as primary adrenal hypercortisolism or ectopic corticotropin syndrome, which does not respond to CRH 
Because CRH testing is rare and complex, diagnostic cut points are less quantitative than qualitative. Before a definitive treatment decision can be made, results from CRH testing must be considered alongside various other clinical data.
At the recommended dose level, ovine CRH has no adverse effects; however, some have reported mild brief facial flushing immediately after injection.  Allergic reactions have not been reported.