INTRODUCTION — Corticotropin-releasing hormone (CRH) is a 41-amino acid peptide that is the major hypothalamic, physiologic corticotropin (ACTH) secretagogue [1]. There is considerable sequence homology of CRH among species, particularly in the amino-terminal region, which is required for biologic activity. As a result, both ovine and human sequences can be used for testing.
The CRH stimulation test is typically performed to:
●Evaluate the etiology of ACTH-dependent Cushing's syndrome (pituitary [Cushing's disease] versus ectopic ACTH production). (See 'Cushing's disease versus ectopic ACTH secretion' below.)
●Discriminate between pseudo-Cushing's (physiological/non-neoplastic hypercortisolism) and Cushing's syndrome [2,3]. (See 'Cushing's disease versus "pseudo-Cushing's syndrome"' below.)
●Assess pituitary corticotroph suppression in patients suspected of mild primary adrenal Cushing's syndrome when basal ACTH levels are incompletely suppressed. (See 'Dysregulated mild cortisol excess from adrenal incidentalomas and mild Cushing's disease' below.)
Most investigators use an increase in either plasma ACTH or plasma cortisol concentration as the criterion for response, although some use them in combination. Multiple blood samples must be obtained (at least one basal sample and several sequential post-CRH samples) [4]. The test should be performed by specialists in carefully selected patients. In the past, ovine CRH was available in the United States and approved for the differential diagnosis of Cushing's syndrome. As of July 2020, ovine CRH for injection is not available in the United States. Human CRH for injection is available outside the United States.
The CRH test is expensive and not available in all countries; the cost for CRH may be as much as USD $1000, and the charge to the patient is considerably higher. Desmopressin is much less expensive than CRH, more widely available, and can be used alternatively for Cushing's disease investigation and petrosal sinus sampling, but not to evaluate normal pituitary function or for mild primary adrenal Cushing's syndrome. (See "Desmopressin (DDAVP) stimulation test".)
This topic will review the technique and interpretation of the CRH stimulation test. Other tests used to determine the cause of Cushing's syndrome are described separately. (See "Establishing the cause of Cushing's syndrome", section on 'Noninvasive biochemical testing' and "Desmopressin (DDAVP) stimulation test".)
CRH PHYSIOLOGY — The corticotropin-releasing hormone (CRH) that controls corticotropin (ACTH) secretion is synthesized in the paraventricular nuclei (PVN) of the hypothalamus and is the neurohumoral interface between the brain and the corticotrophs of the anterior pituitary gland that secrete ACTH. Parvocellular CRH neurons in the PVN have short axons that terminate on capillaries in the median eminence that drain into the anterior pituitary via short portal veins. Once in the anterior pituitary, CRH stimulates ACTH release, which, when in the systemic circulation, stimulates cortisol synthesis and release from the adrenal cortex. Cortisol exerts inhibitory effects on the PVN CRH neurons, as well as the pituitary corticotrophs, thereby closing a negative feedback loop such that an excessive cortisol response is limited.
The main two types of neural inputs into the PVN CRH neurons are "stress" inputs (activated by, for example, social stress, hypoglycemia, pain, hypotension) and basal circadian rhythm input from the suprachiasmatic nuclei of the hypothalamus. Injection of exogenous CRH will stimulate the pituitary corticotrophs to release ACTH under normal conditions and can be used to evaluate anterior pituitary function.
TEST PROCEDURE — The patient usually fasts for four hours or more, after which an intravenous access line is established and synthetic ovine corticotropin-releasing hormone (CRH; 1 mcg [200 nmoles] per kg body weight or 100 mcg total dose) is injected as an intravenous bolus. Blood samples for corticotropin (ACTH) and cortisol are drawn 15 (or 5) and 0 minutes before and as often as 15, 30, 45, 60, 90, and 120 minutes after CRH injection, although a shorter sampling protocol may be effective [4]. Normally both hormones should be measured since the criteria for a positive response may include increases in either plasma ACTH or serum cortisol concentrations [5].
Some patients have mild, brief facial flushing immediately after injection, and/or a metallic taste in the mouth, but there are no other side effects at this dose level [6]. Allergic reactions have not been reported.
INTERPRETATION OF RESULTS
Normal responses — Responses to corticotropin-releasing hormone (CRH) are quite variable among subjects and from one time to another in the same subject. Also, different hormone assay methods are likely to add to the variability. Furthermore, the cause of this variability cannot be explained by an effect of variable basal plasma cortisol concentrations to inhibit corticotropin (ACTH) response [7].
●Plasma ACTH concentrations increase 35 to 900 percent (mean 400 percent) in normal subjects and reach a peak of around 20 to 120 pg/mL (2.2 to 24 pmol/L) 30 minutes after CRH injection [8].
●Serum cortisol concentrations increase 20 to 600 percent (mean 250 percent) to 13 to 36 mcg/dL (mean 25 mcg/dL; 360 to 1000 nmol/L [mean 690 nmol/L]), reaching a peak 30 to 60 minutes after CRH injection [8-11].
●The increment in plasma ACTH is similar in the morning and evening. However, the peak value is greater in the morning in normal subjects when the basal plasma ACTH concentration is higher; in one report, there was no statistically significant difference in 20 men tested at each time [8].
●In contrast, the peak serum cortisol value is similar at both times of day, but the increment is smaller in the morning when the basal value is higher.
●Hyperglycemia does not appear to influence the results, as judged by response during a hyperglycemic clamp study [12,13].
In patients with Cushing's syndrome, in whom the normal circadian rhythm in ACTH secretion is absent, the CRH test can be performed at any time of day with similar results.
Cushing's disease versus ectopic ACTH secretion — Experienced endocrinologists differ on the diagnostic strategy to determine the cause of corticotropin (ACTH)-dependent Cushing's syndrome. (See "Establishing the cause of Cushing's syndrome".)
In all cases of ACTH-dependent Cushing's syndrome, inferior sinus sampling (IPSS) with CRH administration is the best test for distinguishing between Cushing's disease and ectopic ACTH secretion. However, it is expensive, CRH is not available in all countries, and the test requires an experienced team to execute. As a result, some endocrinologists recommend immediate referral to a neurosurgeon if a tumor >6 mm is seen on pituitary magnetic resonance imaging (MRI), while others substitute use of the 8 mg dexamethasone suppression test and the CRH stimulation test. The advantages and disadvantages of these approaches are considered elsewhere. (See "Establishing the cause of Cushing's syndrome".)
The concept underlying the use of the CRH test in the differential diagnosis of ACTH-dependent Cushing's syndrome is that pituitary tumors causing Cushing's disease usually respond to CRH, whereas ectopic ACTH-secreting tumors usually do not [4,14-18]. (See "Establishing the cause of Cushing's syndrome", section on 'CRH stimulation test'.)
Cushing's disease versus "pseudo-Cushing's syndrome" — The use of the CRH test can theoretically distinguish between Cushing's disease and so-called pseudo-Cushing's disease (physiological [non-neoplastic] hypercortisolism) [19]. One of the earliest described causes was alcohol-induced hypercortisolism in that the ACTH response to ovine CRH was attenuated in actively drinking alcoholics [20].
Other causes of pseudo-Cushing's syndrome include depression, neuropsychiatric disorders, chronic kidney disease, and syndromes of insulin resistance [19]. Although the CRH (human) stimulation testing may be useful if both the ACTH and cortisol responses are considered [16], the dexamethasone-CRH and desmopressin stimulation tests are superior [19,21]. (See "Desmopressin (DDAVP) stimulation test".)
Most patients with Cushing's disease respond with ACTH and cortisol increases within 45 minutes after CRH [5,14,15,22-24]. However, the criteria for interpretation have varied at different centers, depending on whether ovine or human CRH is used and the type of ACTH and cortisol assays employed [15,16,25].
Other important points are as follows:
●The ACTH and cortisol responses to ovine CRH in Cushing's disease are traditionally defined as a 35 percent increase from the basal plasma ACTH concentration(s) or a 20 percent increase from the basal serum cortisol concentration(s) [5].
●An increase in serum cortisol >14 percent in response to human CRH discriminated between Cushing's disease and ectopic ACTH [16].
●A shorter and simplified CRH test showed that an increase in plasma ACTH of greater than 43 percent at 15 minutes had a sensitivity of 83 percent and specificity of 94 percent for Cushing's disease [4]. Combining this result with an 8 mg high-dose dexamethasone suppression test (HDDST) result was very effective in excluding ectopic ACTH.
●In conjunction with the HDDST and desmopressin stimulation tests, the CRH stimulation test may be useful to avoid IPSS sampling in selected patients with negative pituitary MRI results when all three tests are concordant [17]. However, it is common for neurosurgeons to request IPSS before performing pituitary neurosurgery in patients with negative or equivocal pituitary-directed MRI.
●Human CRH for injection is available outside the United States.
Ovine versus human CRH — As noted, ovine CRH for injection is not available in the United States, and it is unclear if and when it will become available. Human CRH for injection is available outside the United States. (See 'Introduction' above.)
Cushing's disease versus ectopic ACTH secretion was shown to have similar diagnostic accuracy (using the ACTH response) [15]. Ovine CRH has a more prolonged duration of action than human CRH [10,26,27], which may make it more superior for clinical testing [15,28].
Dysregulated mild cortisol excess from adrenal incidentalomas and mild Cushing's disease — Another challenging and emerging field is the recognition of mild dysregulated hypercortisolism (previously termed "subclinical" Cushing's syndrome), typically in patients with adrenal "incidentalomas." These adenomas produce subtle increases in cortisol secretion that may have significant systemic effects, including weight gain, osteoporosis, lipid abnormalities, glucose intolerance, and hypertension [29,30]. The CRH test may be useful in identifying these patients compared with normal subjects and compared with mild Cushing's disease. (See "Evaluation and management of the adrenal incidentaloma", section on 'Subclinical Cushing syndrome'.)
●Typically, patients with overt ACTH-independent (adrenal) Cushing's syndrome will have baseline plasma ACTH <5 pg/mL (<1.1 pmol/L) and minimal increase in plasma ACTH or cortisol after human CRH administration [31].
●Patients with primary mild adrenal cortisol overproduction may have baseline plasma ACTH levels in a "gray" area (eg, 5 to 35 pg/mL [1.1 to 7.7 pmol/L]) [32] and generate a small increase in plasma ACTH and serum cortisol with human CRH stimulation. However, additional studies are needed to establish cutoff points for interpretation of the test to distinguish these patients from either normal individuals or those with mild Cushing's disease.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Diagnosis and treatment of Cushing's syndrome".)
SUMMARY
●The hypothalamic hormone corticotropin-releasing hormone (CRH) is a major physiologic stimulator of pituitary corticotropin (ACTH) secretion in humans. Both human and ovine synthetic CRH are used in diagnostic tests of the hypothalamic-pituitary-adrenal axis.
•As of July 2020, ovine CRH for injection is not available in the United States, and it is unclear if and when it will become available. Human CRH for injection is available outside the United States.
●The CRH stimulation test is typically performed to:
•Evaluate the etiology of ACTH-dependent Cushing's syndrome (pituitary [Cushing's disease] versus ectopic ACTH production). Data support the use of the CRH stimulation test for the differential diagnosis between patients with an ectopic versus a pituitary ACTH-secreting tumor. In general, approximately 90 percent of each group is correctly diagnosed by the test. However, inferior petrosal sinus sampling (IPSS) using CRH or desmopressin stimulation is still considered the test of choice. (See 'Cushing's disease versus ectopic ACTH secretion' above.)
•Discriminate between pseudo-Cushing's (physiological/non-neoplastic hypercortisolism) and Cushing's syndrome [2,3]. (See 'Cushing's disease versus "pseudo-Cushing's syndrome"' above.)
•Assess pituitary corticotroph suppression in patients suspected of mild primary adrenal Cushing's syndrome when basal ACTH levels are incompletely suppressed. When conventional tests for the diagnosis of pathologic hypercortisolism give equivocal or conflicting results in patients with an adrenal incidentaloma, the CRH test may be a useful adjunctive test to differentiate between mild Cushing's disease and mild primary adrenal Cushing's syndrome. Because this test has not been widely validated, it should be interpreted by an endocrinologist with extensive personal experience in evaluation of these patients. (See 'Dysregulated mild cortisol excess from adrenal incidentalomas and mild Cushing's disease' above.)
DISCLOSURE — The views expressed in this topic are those of the author(s) and do not reflect the official views or policy of the United States Government or its components.
