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 1 mg = prednisone 6 mg = hydrocortisone 25 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; 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; subsequently suppressing the synthesis and secretion of serum cortisol. However, in pathological hypercortisolism, the HPA axis is partially or entirely resistant to feedback inhibition by exogenous steroids.
Types of dexamethasone suppression test:
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 and 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 to 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 over-production. The principle behind the high-dose test is that overproduction of ACTH in Cushing disease (but not ectopic tumors) can undergo partial or full suppressed 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; there is an infusion of intravenous dexamethasone at 1 mg/hour for 4 to 7 hours. 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, intravenous CRH 1 mcg/kg is administered (8 AM). Serum cortisol is drawn 15 minutes later .
1) Iatrogenic hypercortisolism: Causes 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; like ACTH-independent Cushing syndrome. It is crucial that 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; seen in condition like alcoholism, obesity, insulin resistance, neuropsychiatric disorders due to HPA axis stimulation. A thorough history and physical examination 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) Acute illness: Any illness (emotional or physical) mounts a stress response via the HPA axis, resulting in elevated ACTH and cortisol levels. 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 (pregnancy, estrogen-pill, etc.) or reduced CBG (nephrotic syndrome, malnutrition, etc.) may lead to spurious results on DSTs. In these scenarios, late-night salivary cortisol or UFC is the preferred test. Patients receiving estrogen therapy should stop treatment at least for six-weeks before DST.
5) Dexamethasone bioavailability: Malabsorption, altered metabolism or non-compliance can result in variable bioavailability of dexamethasone; confounding results. Dexamethasone gets metabolized in the liver via CYP3A4. Thus, CYP3A4 inducers (phenytoin, carbamazepine, etc.) or inhibitors (itraconazole, fluoxetine, ritonavir) result in decreased or increased clearance (respectively) of 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 which conduct this test, provide reference ranges based on dexamethasone dose and interval of blood draw.
6) Improper urine collection: For UFC, an inadequately collected urine sample leads to diagnostic errors. For this reason, the urine sample should have testing for 24-hour urinary creatinine excretion, in addition to cortisol. Adults less than 50 years, 24-hour urinary creatinine excretion is approximately 15-20 mg/kg/day in women and 20 to 25 mg/kg/day in men. After 50 years, there is a progressive decline in muscle mass; hence, creatinine excretion to 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 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 1 mg overnight tests, 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 under 1.8 mcg/dl suggests adequate HPA axis suppression by dexamethasone and exclude 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 by 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 for 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 if day-2 serum cortisol level is above 20% of baseline [or greater than 4.7 mcg/dl (130 nmol/L)]; sensitivity 100%; 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 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; these individuals should not be subjected to further diagnostic workup; instead, the focus should be on titrating down (or discontinuing, if feasible) the prescribed doses. It is crucial that the dexamethasone suppression test is performed and interpreted in the light of pretest probability; based on a thorough history and physical examination. Additionally, clinicians should be mindful of the test's 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 most patients with suspected Cushing syndrome and refer to an endocrinologist for further work-up. Clear communication and care coordination between physician, patient, and nurse are of paramount importance, for correct implementation of DST and sample collection, due to the direct impact of these factors on subsequent results.
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