The dexamethasone suppression test (DST) is used in the evaluation of endogenous Cushing syndrome (CS), by assessing for the lack of suppression of the hypothalamic-pituitary-adrenal (HPA) axis by exogenous corticosteroids. Dexamethasone is a potent synthetic corticosteroid (dexamethasone 0.75 mg = prednisone 5 mg = methylprednisolone 4 mg = hydrocortisone 20 mg) with high affinity for the glucocorticoid receptors and long duration of action (biological half-life 36 to 54 hours; plasma half-life 4 to 5 hours). It possesses minimal mineralocorticoid activity and unlike other glucocorticoids, it does not interfere with cortisol measurement in the plasma, urine or saliva. It is these characteristics which make dexamethasone, the steroid of choice for the evaluation of the HPA axis.
The HPA axis, a primary neuroendocrine system, helps maintain the body’s homeostatic function and stress response. The neurons in the paraventricular nucleus of hypothalamus synthesize corticotropin-releasing hormone (CRH) which via the hypophysial portal blood is transported to the anterior pituitary, wherein it stimulates the production of adrenocorticotrophic hormone (ACTH). ACTH is then transported by the bloodstream to the adrenal glands and stimulates the synthesis and secretion of cortisol by zona fasciculata of the adrenal cortex. Serum cortisol, also called the stress hormone, in turn, exerts negative feedback on both the hypothalamus and the anterior pituitary; thus, inhibiting the secretion of CRH and ACTH, respectively. This positive/negative feedback mechanism helps regulate the body’s serum cortisol levels and stress response.When HPA axis is intact, exogenously administered corticosteroids exert feedback inhibition on the production of serum CRH and ACTH by binding to the hypothalamic and pituitary glucocorticoid receptors, respectively, that subsequently causes suppression of the synthesis and secretion of serum cortisol. However, in pathological hypercortisolism, the HPA axis becomes partially or entirely resistant to feedback inhibition by exogenous steroids.
Types of dexamethasone suppression test (DST):
1. Low-dose DST
2. High-dose DST
3. Intravenous DST
4. Dexamethasone-CRH test
1. Low-dose DST (LDDST)
LDDST helps in the initial diagnosis of Cushing syndrome, as a screening or a confirmatory test. It can be performed either as an overnight or a two-day test.
2. High-dose DST (HDDST)
Once the diagnosis of Cushing syndrome is confirmed, the next step is to categorize ACTH-independent vs. ACTH-dependent Cushing syndrome, by checking the plasma ACTH levels. In ACTH-independent Cushing syndrome, the plasma ACTH is low or undetectable, indicating an adrenal etiology (causing pituitary suppression of ACTH). While, in ACTH-dependent Cushing syndrome, the plasma ACTH is inappropriately normal or high, suggesting either pituitary or an ectopic source.
In ACTH-dependent Cushing syndrome, HDDST can help distinguish pituitary (i.e., Cushing disease) from an ectopic source of ACTH overproduction. The principle behind the high-dose test is that overproduction of ACTH in Cushing disease (but not ectopic tumors) can undergo partial or full suppression by high doses of dexamethasone (approximately 8 mg). Like LDDST, it utilizes oral dexamethasone, either as an overnight or two-day test.
3. Intravenous DST
This test helps in the initial diagnosis of Cushing syndrome while overcoming concerns of drug compliance and malabsorption. Additionally, it is also useful to differentiate Cushing disease (CD) from ACTH-dependent ectopic tumor and ACTH-independent adrenal etiology. After a baseline morning serum cortisol (8 to 9 AM) is obtained, an infusion of intravenous dexamethasone at 1 mg/hour for 4 to 7 hours is administered. Repeat serum cortisol levels are measured at the end of the infusion (Day-1) and 23 to 24 hours later (Day-2).
4. Dexamethasone – CRH Test
Based on the rationale that glucocorticoid suppression of the HPA axis can be overcome by CRH stimulation in Cushing disease and not in pseudo-Cushing syndrome (physiologic hypercortisolism), this test helps distinguish the two entities. Dexamethasone 0.5 mg every 6 hours (12 PM, 6 PM, 12 AM, 6 AM) is given orally for 48 hours. Two hours after the last dose of dexamethasone, an intravenous CRH dose of 1 mcg/kg is administered (8 AM) and serum cortisol is drawn 15 minutes later .
1. Iatrogenic hypercortisolism can cause exogenous Cushing syndrome. Biochemical testing shows elevated serum cortisol levels (due to cross-reactivity of most exogenous steroids with cortisol immunoassays) and depressed ACTH level as it is usually seen in ACTH-independent Cushing syndrome. It is crucial that the individuals receiving exogenous corticosteroids (inhaled/topical/parenteral/intraarticular) are identified, before pursuing a work-up for pathological hypercortisolemia.
2. Pseudo-Cushing syndrome, also called physiological or non-neoplastic hypercortisolism is seen in conditions like alcoholism, obesity, insulin resistance and neuropsychiatric disorders due to HPA axis stimulation. A thorough history and physical examination can play a pivotal role in recognizing many subjects with pseudo-Cushing syndrome. In equivocal cases, midnight serum cortisol, late-night salivary cortisol, dexamethasone-CRH, or desmopressin test can assist in the distinction.
3. In any acute illness (emotional or physical) a stress response can be presented via the HPA axis, resulting in elevated ACTH and cortisol levels, that's why the evaluation for Cushing syndrome should take place after the resolution of acute stress.
4. Corticosteroid-binding globulin (CBG): About 90% of the circulating cortisol is protein-bound and currently available assays measure the total cortisol (free and protein-bound); therefore, conditions resulting in elevated levels of CBG (pregnancy, estrogen-pill, etc.), or reduced levels of CBG (nephrotic syndrome, malnutrition, etc.) may lead to spurious results on DSTs. In these scenarios, late-night salivary cortisol, or UFC are the preferred tests. Patients receiving estrogen therapy should stop treatment at least for six-weeks before DST.
5. Dexamethasone bioavailability: Malabsorption, altered metabolism, or non-compliance with taking the medication can result in variable bioavailability of dexamethasone confounding the results. Dexamethasone gets metabolized in the liver via CYP3A4. Thus, CYP3A4 inducers (phenytoin, carbamazepine, etc.), or CYP3A4 inhibitors (itraconazole, fluoxetine, ritonavir) may result in decreased or increased clearance of the dexamethasone, risking false positive or negative results, respectively. This error can be overcome by measuring serum dexamethasone levels at the same time as serum cortisol levels. Most laboratories that conduct this test, provide reference ranges based on dexamethasone dose and interval of blood drawn.
6. Improper urine collection: For UFC, an inadequately collected urine sample leads to diagnostic errors. For this reason, the urine sample should include testing for 24-hour urinary creatinine excretion, in addition to cortisol. In adults less than 50 years old, 24-hour urinary creatinine excretion is approximately 15-20 mg/kg/day in women and 20 to 25 mg/kg/day in men. In people older than 50 years of age, there is a progressive decline in muscle mass, hence, creatinine excretion can be as low as 10 mg/kg/day.
In either of the LDDST (overnight, or two-day), serum cortisol level of 1.8 mcg/dl (50 nmol/L) is the recommended cut-off value that increases the diagnostic sensitivity of the test to approximately 95%. However, at this cut-off value, the specificity of the two-day test is better compared to that of the overnight test (97 to 100% versus 86%). A recent meta-analysis showed that for the 1 mg overnight test, abnormal and normal results possessed a positive and negative likelihood ratio (LR) of 11.6 and 0.09, respectively. On the other hand, for the two-day 2 mg test, abnormal and normal results had a positive and negative LR of 7.3 and 0.8, respectively. Serum cortisol level under 1.8 mcg/dl suggests adequate HPA axis suppression by dexamethasone and excludes CS. Levels over 1.8 mcg/dl should be verified with a second test (24-hour urinary free cortisol, or late-night salivary cortisol), before establishing a confirmed diagnosis of CS.
Reduction in UFC, or serum cortisol greater than 50% in the overnight, or two-day HDDST makes Cushing disease (CD) the likely source of ACTH-dependent Cushing syndrome. At a cut-off value of 50% suppression, HDDST provides a sensitivity and specificity of 60 to 100%. Increasing the cut-off to above 90% cortisol suppression increases the specificity of diagnosing Cushing disease to almost 100%, albeit at a much-reduced sensitivity. Because of this limitation, HDDST is not the recommendation, unless, both the pituitary MRI and bilateral inferior petrosal venous sampling are negative or logistically challenging. Even in this scenario, HDDST is performed in conjunction with a CRH stimulation test, to enhance diagnostic accuracy.
3. Intravenous DST
The diagnosis of Cushing syndrome is made if the day-2 serum cortisol level is above 20% of baseline [or greater than 4.7 mcg/dl (130 nmol/L)] with sensitivity 100% and specificity 96%. Additionally, in Cushing disease as opposed to other etiologies of Cushing syndrome, the day-1 (i.e., the end of infusion) serum cortisol level shows greater than 70% suppression from the baseline, followed by rebound hypercortisolism in 24 hours.
4. Dexamethasone - CRH
Serum cortisol levels over 1.4 mcg/dl (39 nmol/L) at 15 minutes suggest CD with a 90 to 100% sensitivity and 50 to 100% specificity. Raising the cut-off value to over 3.8 mcg/dl (87 nmol/L) increases the specificity to 100%, at the cost of slightly reduced sensitivity (94%). The cumbersomeness of this test limits its extensive application in the ambulatory setting.
Iatrogenic hypercortisolism is the most common cause of Cushing syndrome; and it should be recognized before these individuals are subject to further diagnostic workup; instead, the focus should be on titrating down (or discontinuing, if feasible) the prescribed steroid dosages. It is crucial that the dexamethasone suppression test is performed and interpreted in the light of pretest probability that should be based on a thorough history and physical examination. Additionally, clinicians should be mindful of all the tests' diagnostic accuracy, limitations, and interfering factors. In subjects with high clinical suspicion of Cushing syndrome but equivocal or negative test results, repeat testing should take place in 3 to 6 months, as untreated hypercortisolemia has detrimental consequences.
Primary care providers initially evaluate the patients with suspected Cushing syndrome and they refer to an endocrinologist for further work-up. Clear communication and care coordination between physician, patient, and nurse are of paramount importance as the correct implementation of DST and sample collection can have a dramatic impact of these factors on subsequent results.
|||Spencer RL,Deak T, A users guide to HPA axis research. Physiology [PubMed PMID: 27871862]|
|||Wood PJ,Barth JH,Freedman DB,Perry L,Sheridan B, Evidence for the low dose dexamethasone suppression test to screen for Cushing's syndrome--recommendations for a protocol for biochemistry laboratories. Annals of clinical biochemistry. 1997 May; [PubMed PMID: 9158818]|
|||Cronin C,Igoe D,Duffy MJ,Cunningham SK,McKenna TJ, The overnight dexamethasone test is a worthwhile screening procedure. Clinical endocrinology. 1990 Jul; [PubMed PMID: 2401096]|
|||NUGENT CA,NICHOLS T,TYLER FH, DIAGNOSIS OF CUSHING'S SYNDROME; SINGLE DOSE DEXAMETHASONE SUPPRESSION TEST. Archives of internal medicine. 1965 Aug; [PubMed PMID: 14315650]|
|||LIDDLE GW, Tests of pituitary-adrenal suppressibility in the diagnosis of Cushing's syndrome. The Journal of clinical endocrinology and metabolism. 1960 Dec; [PubMed PMID: 13761950]|
|||Bruno OD,Rossi MA,Contreras LN,Gómez RM,Galparsoro G,Cazado E,Kral M,Leber B,Arias D, Nocturnal high-dose dexamethasone suppression test in the aetiological diagnosis of Cushing's syndrome. Acta endocrinologica. 1985 Jun; [PubMed PMID: 2990131]|
|||Biemond P,de Jong FH,Lamberts SW, Continuous dexamethasone infusion for seven hours in patients with the Cushing syndrome. A superior differential diagnostic test. Annals of internal medicine. 1990 May 15; [PubMed PMID: 2158760]|
|||Jung C,Alford FP,Topliss DJ,Burgess JR,Long F,Gome JJ,Stockigt JR,Inder WJ, The 4-mg intravenous dexamethasone suppression test in the diagnosis of Cushing's syndrome. Clinical endocrinology. 2010 Jul; [PubMed PMID: 20039897]|
|||Tran HA,Petrovsky N, Dexamethasone infusion testing in the diagnosis of Cushing's syndrome. Endocrine journal. 2005 Feb; [PubMed PMID: 15758565]|
|||Abou Samra AB,Dechaud H,Estour B,Chalendar D,Fevre-Montange M,Pugeat M,Tourniaire J, Beta-lipotropin and cortisol responses to an intravenous infusion dexamethasone suppression test in Cushing's syndrome and obesity. The Journal of clinical endocrinology and metabolism. 1985 Jul; [PubMed PMID: 3998073]|
|||Yanovski JA,Cutler GB Jr,Chrousos GP,Nieman LK, Corticotropin-releasing hormone stimulation following low-dose dexamethasone administration. A new test to distinguish Cushing's syndrome from pseudo-Cushing's states. JAMA. 1993 May 5; [PubMed PMID: 8386285]|
|||Chabre O, The difficulties of pseudo-Cushing's syndrome (or [PubMed PMID: 29716734]|
|||Alwani RA,Schmit Jongbloed LW,de Jong FH,van der Lely AJ,de Herder WW,Feelders RA, Differentiating between Cushing's disease and pseudo-Cushing's syndrome: comparison of four tests. European journal of endocrinology. 2014 Apr; [PubMed PMID: 24394725]|
|||Bae YJ,Kratzsch J, Corticosteroid-binding globulin: modulating mechanisms of bioavailability of cortisol and its clinical implications. Best practice [PubMed PMID: 26522460]|
|||Meikle AW, Dexamethasone suppression tests: usefulness of simultaneous measurement of plasma cortisol and dexamethasone. Clinical endocrinology. 1982 Apr; [PubMed PMID: 7094363]|
|||Rush AJ,Schlesser MA,Giles DE,Crowley GT,Fairchild C,Altshuler KZ, The effect of dosage on the dexamethasone suppression test in normal controls. Psychiatry research. 1982 Dec; [PubMed PMID: 6962436]|
|||Meinardi JR,Wolffenbuttel BH,Dullaart RP, Cyclic Cushing's syndrome: a clinical challenge. European journal of endocrinology. 2007 Sep; [PubMed PMID: 17766705]|
|||Manenschijn L,van den Akker EL,Lamberts SW,van Rossum EF, Clinical features associated with glucocorticoid receptor polymorphisms. An overview. Annals of the New York Academy of Sciences. 2009 Oct; [PubMed PMID: 19906240]|
|||Nieman LK,Biller BM,Findling JW,Newell-Price J,Savage MO,Stewart PM,Montori VM, The diagnosis of Cushing's syndrome: an Endocrine Society Clinical Practice Guideline. The Journal of clinical endocrinology and metabolism. 2008 May; [PubMed PMID: 18334580]|
|||Ceccato F,Barbot M,Zilio M,Frigo AC,Albiger N,Camozzi V,Antonelli G,Plebani M,Mantero F,Boscaro M,Scaroni C, Screening Tests for Cushing's Syndrome: Urinary Free Cortisol Role Measured by LC-MS/MS. The Journal of clinical endocrinology and metabolism. 2015 Oct; [PubMed PMID: 26274344]|
|||Elamin MB,Murad MH,Mullan R,Erickson D,Harris K,Nadeem S,Ennis R,Erwin PJ,Montori VM, Accuracy of diagnostic tests for Cushing's syndrome: a systematic review and metaanalyses. The Journal of clinical endocrinology and metabolism. 2008 May; [PubMed PMID: 18334594]|
|||Flack MR,Oldfield EH,Cutler GB Jr,Zweig MH,Malley JD,Chrousos GP,Loriaux DL,Nieman LK, Urine free cortisol in the high-dose dexamethasone suppression test for the differential diagnosis of the Cushing syndrome. Annals of internal medicine. 1992 Feb 1; [PubMed PMID: 1728204]|
|||Aron DC,Raff H,Findling JW, Effectiveness versus efficacy: the limited value in clinical practice of high dose dexamethasone suppression testing in the differential diagnosis of adrenocorticotropin-dependent Cushing's syndrome. The Journal of clinical endocrinology and metabolism. 1997 Jun; [PubMed PMID: 9177382]|
|||Pecori Giraldi F,Pivonello R,Ambrogio AG,De Martino MC,De Martin M,Scacchi M,Colao A,Toja PM,Lombardi G,Cavagnini F, The dexamethasone-suppressed corticotropin-releasing hormone stimulation test and the desmopressin test to distinguish Cushing's syndrome from pseudo-Cushing's states. Clinical endocrinology. 2007 Feb; [PubMed PMID: 17223996]|
|||Gatta B,Chabre O,Cortet C,Martinie M,Corcuff JB,Roger P,Tabarin A, Reevaluation of the combined dexamethasone suppression-corticotropin-releasing hormone test for differentiation of mild cushing's disease from pseudo-Cushing's syndrome. The Journal of clinical endocrinology and metabolism. 2007 Nov; [PubMed PMID: 17635947]|
|||Tirabassi G,Papa R,Faloia E,Boscaro M,Arnaldi G, Corticotrophin-releasing hormone and desmopressin tests in the differential diagnosis between Cushing's disease and pseudo-Cushing state: a comparative study. Clinical endocrinology. 2011 Nov; [PubMed PMID: 21554373]|
|||Findling JW,Raff H, DIAGNOSIS OF ENDOCRINE DISEASE: Differentiation of pathologic/neoplastic hypercortisolism (Cushing's syndrome) from physiologic/non-neoplastic hypercortisolism (formerly known as pseudo-Cushing's syndrome). European journal of endocrinology. 2017 May; [PubMed PMID: 28179447]|