Dexamethasone suppression tests

INTRODUCTION — Dexamethasone suppression tests (DSTs) are primarily used to screen for excess cortisol production (Cushing syndrome) and are useful in detecting dysregulated cortisol hypersecretion in adrenal incidentalomas. DSTs are less reliable for the evaluation of the differential diagnosis of corticotropin (ACTH)-dependent Cushing syndrome. Dexamethasone is approximately 30 to 40 times more potent than cortisol and has no appreciable mineralocorticoid activity. The DSTs assess the hypothalamic and pituitary corticotroph cell responses to glucocorticoid negative feedback inhibition of corticotropin-releasing hormone (CRH) and ACTH secretion.

This topic will review the basic principles of the DSTs. Additional information on when to choose them to determine the diagnosis and the cause of Cushing syndrome is discussed separately.

●(See "Establishing the diagnosis of Cushing syndrome".)

●(See "Establishing the cause of Cushing syndrome".)

●(See "Evaluation and management of the adrenal incidentaloma".)

LOW-DOSE DSTs

Indications and rationale — The low-dose dexamethasone suppression tests (DSTs) are standard screening tests to differentiate patients with endogenous Cushing syndrome of any cause from patients who do not have Cushing syndrome. The 1 mg overnight DST is also used to identify modest excess cortisol secretion in patients with adrenal incidentalomas, primary aldosteronism (with cortisol co-secretion), or adrenocortical carcinoma without clinical signs of overt Cushing syndrome. The high-dose test described below should not be used for these purposes. (See 'High-dose DSTs' below and "Establishing the diagnosis of Cushing syndrome".)

The binding of dexamethasone to glucocorticoid receptors in hypothalamic paraventricular nuclei and in pituitary corticotroph cells inhibits corticotropin-releasing hormone (CRH) and corticotropin (ACTH) secretion. In humans, dexamethasone does not directly inhibit adrenal steroid production [1]. If the hypothalamic-pituitary-adrenal (HPA) axis is functioning normally, any supraphysiologic dose of dexamethasone is sufficient to suppress pituitary ACTH secretion. This should reduce cortisol production, with concomitant decreases in serum, saliva, and urine concentrations.

Two main protocols are used: the overnight 1 mg screening test and the two-day, low-dose test. Side effects are extremely rare, and either test can be conducted on an outpatient basis. The test should not be conducted if the patient is receiving exogenous ACTH, any type of glucocorticoid therapy, or is acutely ill or undergoing significant stress [2].

Overnight screening test — The overnight 1 mg DST is a quick screening test [3,4]. Dexamethasone (1 mg) is taken orally between 11 PM and midnight, and a single blood sample is drawn at 8 AM the next morning for assay of serum cortisol and, if available, serum dexamethasone. A dose of 0.3 mg/m² surface area can be used in children [5].

Dose adjustments for adults with obesity — We continue to suggest the standard 1 mg dose for the overnight screening DST, including for patients with obesity. While there have been concerns that individuals with obesity may require higher dexamethasone doses, data are conflicting:

●In one study of 34 healthy weight (13 men and 21 women) and 87 adults with obesity (36 men and 51 women), pituitary sensitivity to feedback inhibition by dexamethasone in the group with obesity was preserved, even at doses lower than 1 mg [6].

●However, in a second series of 100 consecutive adults with obesity (body mass index [BMI] >30 kg/m²), a false-positive rate of morning plasma cortisol >1.8 mcg/dL was found in 8 percent of individuals receiving 1 mg overnight dexamethasone, while it was only 2 percent in those receiving 2 mg overnight dexamethasone [7].

Normal results — Using current immunoassays (which are more specific for cortisol than older assays), most individuals suppress their 8 AM cortisol value to less than 2 mcg/dL (55 nmol/L) [8-10].

An occasional patient with Cushing disease will suppress to 1.8 mcg/dL (50 nmol/L), so this cutoff point is used to maximize sensitivity [11-13]; it also appears to be valid when using liquid chromatography-tandem mass spectrometry (LC-MS/MS) to measure serum cortisol [14]. While structurally based assays such as LC-MS/MS fully discriminate between dexamethasone and cortisol, because serum dexamethasone levels are low and have limited cross-reactivity in immunoassays, structurally based assays are not required.

A meta-analysis of 50 studies including 1531 patients with Cushing syndrome and 3267 control subjects reported that the 1 mg overnight DST using a cortisol cutoff of 1.8 mcg/dL (50 nmol/L) had the highest sensitivity (98.6 percent) and the lowest specificity (90.6 percent) compared with other initial tests for the diagnosis of Cushing syndrome [15].

Falsely normal results — Despite use of a diagnostic criterion chosen to improve sensitivity, some patients with Cushing disease suppress the 8 AM serum cortisol concentration to less than 1.8 mcg/dL (50 nmol/L) after the overnight DST [9]. Because of this, it is important to obtain at least two screening tests. (See "Establishing the diagnosis of Cushing syndrome".)

●A review of a single practitioner's experience in 103 patients with Cushing syndrome showed that 6 of 80 (8 percent) suppressed to less than 2 mcg/dL (55 nmol/L) [16].

●In another study of 97 patients with Cushing syndrome using a slightly higher 1.5 mg dexamethasone dose, four suppressed to between 1.2 and 2.8 mcg/dL (34 and 77 nmol/L) [17].

Falsely abnormal results — The 2008 Endocrine Society and 2021 Pituitary Society guideline update both suggest a diagnostic cortisol criterion of 1.8 mcg/dL (50 nmol/L), recognizing that this choice decreases specificity [12,13]. The use of progressively higher cutoff points will reduce falsely abnormal results at the expense of failing to identify patients with Cushing syndrome.

●This was illustrated by an Italian investigation of 137 individuals suspected of Cushing syndrome [18]. The cutoff of 1.8 mcg/dL (50 nmol/L) identified 38 cases of Cushing syndrome with 100 percent sensitivity and a specificity of 91 percent. When using a cutoff of 5 mcg/dL (137 nmol/L), the sensitivity decreased to 87 percent, but the specificity increased to 97 percent.

●Morning serum cortisol values between 1.8 and 7.2 mcg/dL could be false positive due to a variety of factors including failure to take or absorb the dexamethasone properly [19]. Simultaneous measurement of serum dexamethasone can identify such cases [20-22]. Because of the possibility of a false result, the 1 mg low-dose dexamethasone test should not be used as the sole criterion for excluding or establishing the diagnosis of Cushing syndrome. (See "Establishing the diagnosis of Cushing syndrome", section on 'Initial testing'.)

Interpretation based upon assay used — Modern immunoassays using monoclonal antibodies that have minimal cross-reactivity with endogenous steroids are likely to yield results similar to LC-MS/MS and have a cutoff of <1.8 mcg/dL [11,23,24]. Older studies that used higher cutoff points also used less specific assays. (See "Diagnosis of adrenal insufficiency in adults".)

We do not recommend the routine use of salivary cortisol measurements for the overnight 1 mg DST, because results vary between assays [25,26]. Although most salivary cortisol immunoassays are not affected by dexamethasone, a commonly used US Food and Drug Administration (FDA)-cleared salivary cortisol enzyme immunoassay has 19 percent cross-reactivity with dexamethasone so it cannot be used for the DST [27]. If salivary cortisol is used, the assay should be validated for this purpose in patients with and without Cushing syndrome to provide an appropriate criterion for its interpretation [28,29]. (See "Measurement of cortisol in serum and saliva".)

Two-day, low-dose test — The two-day test is used to assess suppressibility in patients with an equivocal overnight test or as an alternative to the overnight test. Dexamethasone 0.5 mg is taken orally every six hours, usually at 8 AM, 2 PM, 8 PM, and 2 AM on each day, for a total of eight doses. This test is sometimes referred to as the two-day, 2 mg test, which refers to the number of days and the total daily dose (2 mg) (see "Establishing the diagnosis of Cushing syndrome"). The dose can be modified in children who weigh less than approximately 45 kg [30].

Blood is drawn two or six hours after the last dose for measurement of cortisol and dexamethasone (and ACTH, if desired).

The normal response to the low-dose, two-day test consists of the following:

●There is no single established criterion for interpretation of the two-day test. When using current assays to measure serum cortisol, the same criterion for suppression is recommended for the 1 mg and 2 mg, two-day test: a serum cortisol <1.8 mcg/dL (<50 nmol/L). Using this criterion at 24 or 48 hours [31], one retrospective study correctly identified 98 percent of 245 patients with Cushing syndrome [31]. (See "Measurement of cortisol in serum and saliva".)

●Smaller prospective studies using a serum cortisol concentration of 1.4 or 2.2 mcg/dL (38 or 60 nmol/L) at 48 hours as the exclusion criterion reported 90 to 100 percent sensitivity and 97 to 100 percent specificity in patients suspected of having Cushing syndrome [32-34].

●Therefore, the use of the two-day, low-dose test has greater specificity at high sensitivity than the 1 mg overnight test. As a result, this is the better screening test, with the caveat that it requires more patient effort than the 1 mg test to achieve excellent diagnostic results.

●Because the overnight test is easier to administer, it is often the preferred test in the United States, but the two-day test often is used elsewhere because of its better performance. Although there are no formal studies validating this, the two-day, low-dose DST may be preferable to the overnight low-dose test when evaluating patients who are night-shift workers.

●Urinary corticosteroid endpoints provide suboptimal sensitivity and specificity [9,10,32]; serum cortisol endpoints should be used instead.

●Measuring plasma ACTH, if either of the tests is abnormal, gives an indication of the etiology of the hypercortisolism; it will usually be high normal or high in patients with the ectopic ACTH syndrome, within the normal range or elevated in those with Cushing disease, and low or undetectable in those with a primary adrenal source. (See "Establishing the cause of Cushing syndrome".)

HIGH-DOSE DSTs — In theory, an 8 mg dexamethasone dose (equivalent to more than 10 times the daily production of cortisol) should suppress ACTH secretion from corticotroph tumors, which generally retain some responsiveness to high-dose glucocorticoid negative feedback inhibition [35]. In contrast, many nonpituitary tumors that produce ACTH ectopically (such as small cell lung carcinomas) do not contain active glucocorticoid receptors and are typically not responsive to glucocorticoid negative feedback.

Unfortunately, there are pituitary corticotroph adenomas that do not suppress with the HDDST (particularly those with high ACTH production), and ectopic tumors that do [36-38]. Therefore, the HDDST should not be used alone and should be interpreted in conjunction with a corticotropin-releasing hormone (CRH) stimulation test (where available) or desmopressin test and pituitary magnetic resonance imaging (MRI) if inferior petrosal sinus sampling is not available [36,39-41].

While some experts have abandoned the HDDST in favor of inferior petrosal sinus sampling alone, or the combined use of CRH and desmopressin stimulation tests [13], the technical expertise and test agents for this approach are not universally available. In such situations, the HDDST should be interpreted in conjunction with the MRI and clinical features. For example, older age, male sex, rapid onset of symptoms, severe hypertension, hypokalemia, very high urine free cortisol and high plasma ACTH combined with a normal pituitary MRI and failure to suppress on the HDDST strongly suggests ectopic ACTH secretion. A more complete discussion of diagnostic strategies is provided elsewhere. (See "Establishing the cause of Cushing syndrome", section on 'Petrosal venous sinus catheterization'.)

There are several HDDSTs that have been used:

Overnight 8 mg test — Dexamethasone (8 mg) is taken orally between 11 PM and midnight. A single blood sample is drawn at 8:30 and 9 AM the day before and the next day for measurement of serum cortisol and, if one wishes, plasma ACTH and serum dexamethasone.

Interpretation — The test is interpreted by calculating the suppression of morning serum cortisol concentration on the day before and after dexamethasone administration, and considering a 50 to 80 percent or greater suppression to indicate Cushing disease [42-45]. This approach yields 56 to 92 percent sensitivity and 57 to 100 percent specificity [42,43,45-51].

Other types of DSTs

Intravenous DSTs — Several versions of an intravenous dexamethasone suppression test (DST) have been proposed for initial confirmation and differential diagnosis of Cushing syndrome within one day, while avoiding the potential difficulties of drug compliance and absorption with oral dexamethasone. However, this approach has been evaluated in a limited number of patients, and we do not suggest its routine use.

Dexamethasone is infused at 1 mg/hour intravenously for four [52,53], five [54], or seven [55] hours. Plasma cortisol is suppressed to levels <1.4 to 3.0 mcg/dL (<38 to 83 nmol/L) in the evening and the following morning in individuals with healthy weight and obesity, and it is above these levels at 9 AM in patients with Cushing syndrome (>20 percent of baseline value or >4.7 mcg/dL [130 nmol/L]), with a sensitivity and specificity of 100 and 90 percent, respectively [53,55]. Using cortisol and ACTH values during the 4 mg intravenous DST, differentiation of Cushing disease (n = 32) from functional hypercortisolism (pseudo-Cushing n = 36) was achieved with a sensitivity of 100 percent and specificity of 83.3 percent [56].

SOURCES OF ERROR — There are several common sources of error in dexamethasone suppression tests (DSTs):

●Acute stress or illness – In general, DSTs should not be performed in patients who are critically ill and/or in the postoperative period. Acute stress, infection, and the immediate postoperative period activate the hypothalamic-pituitary-adrenal (HPA) axis so that suppression with dexamethasone may not be normal.

●Elevated corticosteroid-binding globulin – Increased corticosteroid-binding globulin (CBG) levels may confound results in DSTs that use serum cortisol as an endpoint and result in an apparent lack of suppression to the low-dose DST [57]. In this setting, salivary cortisol values, which reflect unbound cortisol, will show normal suppression [58]. Thus, it may be more practical to evaluate individuals with elevated CBG levels using late-night salivary cortisol as a screening test [59,60]. In 30 healthy women with elevated CBG levels, the two-day, low-dose DST yielded fewer false-positive results (27 percent) compared with the 1 mg DST (63 percent), whereas late-night salivary cortisol was normal in all [61]. The diagnostic evaluation for Cushing syndrome in individuals with elevated CBG levels is reviewed separately. (See "Establishing the diagnosis of Cushing syndrome", section on 'Available tests'.)

Pregnancy leads to elevated CBG levels and HPA axis activity [62-65]. In one study, CBG levels were still elevated in most women at two to three months postpartum [66]. The diagnosis of Cushing syndrome in pregnancy is covered elsewhere. (See "Diagnosis and management of Cushing syndrome during pregnancy".)

●Variation in dexamethasone intake or metabolism – Failure of the patient to take the dexamethasone or abnormal metabolism of the dexamethasone can interfere with interpretation of the test. Drugs that induce hepatic CYP3A4 enzymes, such as barbiturates, phenytoin, rifampin, and carbamazepine increase the metabolism of dexamethasone and other steroids [67,68]. Nearly one-half of the US Food and Drug Administration (FDA)-approved drugs in the United States interact with CYP3A4 and may confound the interpretation of any DST [69]. These effects can be detected by measuring serum dexamethasone at the appropriate interval after the last dose.

●Glucocorticoid receptor polymorphisms – Several glucocorticoid receptor polymorphisms have been identified that confer increased glucocorticoid sensitivity. Individuals with N363S, GR9b, or BclI polymorphisms have greater serum cortisol reduction after dexamethasone administration [70]. In contrast, the ER22/23EK and A3669G polymorphisms are associated with decreased glucocorticoid sensitivity and relative glucocorticoid resistance [71]. These glucocorticoid receptor polymorphisms are not determined on a clinical basis but may be implicated in some false-positive or negative response to dexamethasone tests [70].

●False-negative responses using the high-dose DST (HDDST) to identify patients with Cushing disease are more common with higher baseline corticotropin (ACTH) and cortisol secretion. In less than 5 percent of patients with Cushing disease, for example, higher doses of dexamethasone (16 to 100 mg/day) were required to produce significant suppression. These patients tend to have large tumors and more severe hypercortisolism [72-74].

●False-positive responses to the 8 mg DST occur in some patients with ACTH-secreting neuroendocrine tumors (usually pulmonary, and mostly benign); occasionally other tumors respond to HDDST with decreased tumor secretion of ACTH and cortisol [39,75,76]. Such tumors present a difficult diagnostic problem because they are often occult and may overlap with the clinical features of Cushing disease without the short course, severe hypercortisolism and hypokalemia considered more typical of ectopic ACTH secretion from malignant tumors such as small cell lung carcinomas. (See "Lung neuroendocrine (carcinoid) tumors: Epidemiology, risk factors, classification, histology, diagnosis, and staging".)

USE OF DEXAMETHASONE MEASUREMENTS — Measuring serum dexamethasone is suggested for all dexamethasone suppression tests (DSTs). It provides verification that the drug was taken and indicates whether the serum concentration is within the limits expected in an individual who metabolizes the drug normally. Laboratory nomograms and guidelines are available that relate serum dexamethasone concentrations to serum cortisol concentrations in normal subjects and in patients with Cushing disease [19-22,77].

Commercial laboratories often provide a range of expected values for a specific dose and interval until the blood draw. Finding an abnormally high or low serum dexamethasone concentration allows one to interpret the cause of an unusual serum or urinary cortisol response and to repeat the test, if necessary, with the same or another dexamethasone dose.

Using modern liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays, a serum dexamethasone concentration of 3.3 to 3.6 nmol/L is sufficient to get appropriate suppression of morning serum cortisol [14]. The reference range for serum dexamethasone measured at 8 AM following the 1 mg dose taken at 11 PM the night before is 140 to 295 ng/dL (3.6 to 7.5 nmol/L) [20-22,78].

USE OF DSTs IN SPECIAL POPULATIONS — The use of dexamethasone suppression tests (DSTs) in each of these populations is discussed further in their specific topics.

Mild cortisol secretion in adrenal incidentalomas and other adrenal tumors — The low-dose DST is the preferred screening test used to identify mild autonomous cortisol secretion in patients with incidentally found adrenal masses. (See "Evaluation and management of the adrenal incidentaloma", section on 'Subclinical Cushing syndrome' and "Clinical presentation and evaluation of adrenocortical tumors" and "Diagnosis of primary aldosteronism", section on 'Cortisol cosecretion'.)

Pregnant women — The low-dose 1 mg overnight DST is not recommended to diagnose Cushing syndrome during pregnancy, because of the risk for false-positive results. Instead, the initial evaluation includes a late-night salivary cortisol and a 24-hour urinary cortisol. The 8 mg high-dose DST is sometimes performed to help determine etiology. The criteria for interpretation are different in these patients. (See "Diagnosis and management of Cushing syndrome during pregnancy".)

Primary pigmented nodular adrenocortical disease (PPNAD) — A paradoxical increase in urinary free cortisol during the sequential low-dose (2 mg) and high-dose (8 mg) six-day dexamethasone suppression testing [27] may be seen in patients with Cushing syndrome due to primary pigmented nodular adrenocortical disease (PPNAD) [79]. This delayed "paradoxical" response can be useful to identify otherwise asymptomatic carriers in familial forms of PPNAD or to distinguish PPNAD from other adrenocortical tumors. (See "Cushing syndrome due to primary pigmented nodular adrenocortical 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 syndrome".)

SUMMARY AND RECOMMENDATIONS

●Low-dose dexamethasone suppression test (DST) – The low-dose dexamethasone suppression tests (DSTs; 1 mg overnight and two-day, low-dose) are used to differentiate patients with Cushing syndrome of any cause from patients who do not produce cortisol excess.

●High-dose DSTs (not for routine use) – The high-dose (8 mg) DSTs (HDDSTs) have been used for the differential diagnosis of corticotropin (ACTH)-dependent Cushing syndrome. The HDDST may be used with caution in conjunction with corticotropin-releasing hormone (CRH) test and/or desmopressin test, and pituitary magnetic resonance imaging (MRI) to assess the further requirement for inferior petrosal sinus sampling to confirm source of ACTH.

●Serum dexamethasone measurements – Simultaneous measurement of serum dexamethasone concentration may be useful to confirm that the dexamethasone was taken, absorbed, and reached sufficient blood levels to exert glucocorticoid negative feedback.

●Sources of error – Increased corticosteroid-binding globulin (CBG) levels may result in an apparent lack of suppression to the low-dose DST. (See 'Sources of error' above.)

ACKNOWLEDGMENT — 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.