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Dexamethasone suppression test

The dexamethasone suppression test (DST) is a diagnostic blood test that assesses the function of the hypothalamic-pituitary-adrenal (HPA) axis by administering dexamethasone, a synthetic glucocorticoid, and measuring whether it appropriately suppresses endogenous cortisol production by the adrenal glands.^[1]^[2] In individuals without HPA axis dysfunction, dexamethasone mimics cortisol's effects, providing negative feedback to reduce adrenocorticotropic hormone (ACTH) secretion from the pituitary gland and subsequent cortisol release; failure of this suppression indicates potential pathology such as Cushing syndrome.^[1]^[3] Developed in the 1960s, the DST remains a cornerstone for screening and confirming hypercortisolism, with high sensitivity when performed correctly.^[2] The test is available in low-dose and high-dose protocols, tailored to specific diagnostic needs.^[1] The low-dose DST, often the initial screening tool, involves administering 1 mg of dexamethasone orally at 11 p.m. (overnight variant) or 0.5 mg every 6 hours for 48 hours (standard variant), followed by cortisol measurement in blood or urine the next morning or after the final dose.^[2]^[3] Normal results show suppression to less than 1.8 µg/dL (50 nmol/L) in serum cortisol for the overnight test or below 10 µg/day in urinary free cortisol for the standard test, effectively ruling out Cushing syndrome if achieved.^[1]^[2] In contrast, the high-dose DST uses 8 mg overnight or 2 mg every 6 hours for 48 hours to evaluate suppression extent, where greater than 50% reduction in cortisol suggests pituitary-dependent Cushing disease, while lesser suppression points to ectopic ACTH production or adrenal tumors.^[1]^[2] Beyond Cushing syndrome diagnosis, the DST aids in distinguishing its etiologies—such as pituitary adenomas, adrenal disorders, or ectopic sources—and is sometimes used to assess mild autonomous cortisol secretion in adrenal incidentalomas.^[3]^[2] Preparation is crucial, including avoiding interfering medications like oral estrogens, anticonvulsants, or other glucocorticoids, as they can alter dexamethasone metabolism or cortisol binding.^[1] False positives may arise from stress, obesity, or pseudo-Cushing states (e.g., alcoholism, depression), necessitating confirmatory testing like ACTH levels or imaging.^[1]^[2] Risks are minimal, primarily limited to those of venipuncture, such as bruising or infection.^[1]

Overview and Background

Purpose and Clinical Indications

The dexamethasone suppression test (DST) serves as a diagnostic tool to evaluate the hypothalamic-pituitary-adrenal (HPA) axis by assessing the suppression of cortisol production following administration of the synthetic glucocorticoid dexamethasone.^[2] This test exploits the negative feedback mechanism of the HPA axis to identify dysregulation in cortisol secretion.^[4] Its primary application is in screening for Cushing's syndrome, a condition characterized by chronic endogenous hypercortisolism, including subtypes such as pituitary-dependent Cushing's disease (ACTH-secreting pituitary adenoma), adrenal adenoma causing autonomous cortisol production, and ectopic ACTH production from non-pituitary tumors.^[5] The test aids in confirming the presence of hypercortisolism and provides initial clues toward differentiating these etiologies based on the degree of cortisol suppressibility.^[2] Secondary indications include the evaluation of adrenal incidentalomas discovered on imaging, where the DST helps detect mild autonomous cortisol secretion that may not fully manifest as overt Cushing's syndrome.^[4] It is also employed to distinguish ACTH-independent causes, such as adrenal tumors, from ACTH-dependent ones, guiding further diagnostic steps like imaging or additional endocrine testing.^[5]

Historical Development

The dexamethasone suppression test (DST) was introduced in 1960 by Grant W. Liddle and colleagues as a method to differentiate causes of Cushing's syndrome by assessing the suppressibility of the hypothalamic-pituitary-adrenal (HPA) axis with exogenous glucocorticoids.^[6] Liddle's protocol involved low- and high-dose dexamethasone administration to distinguish pituitary-dependent Cushing's disease from ectopic ACTH production or adrenal tumors, providing a key advancement in endocrinology before reliable ACTH assays were available.^[7] In the 1970s, the DST was extended to psychiatric applications, with Bernard J. Carroll and colleagues proposing its use as a biological marker for endogenous or melancholic depression based on observed cortisol nonsuppression rates exceeding 40% in affected patients.^[8] This adaptation, formalized in the early 1980s through the National Institute of Mental Health consensus criteria, positioned the overnight 1-mg DST as a potential diagnostic aid for major depression subtypes, sparking widespread research and clinical trials during that decade. However, enthusiasm waned following a 1988 JAMA analysis by Thomas R. Insel and Frederick K. Goodwin, which critiqued the DST's high false-positive rate (up to 30% in non-depressed populations) and poor specificity, rendering it unreliable for routine depression diagnosis and prompting its near-abandonment in psychiatry.^[9] Post-2000 refinements have revitalized the DST in endocrinology, emphasizing its role in Cushing's screening when combined with late-night salivary cortisol and 24-hour urinary free cortisol measurements, as reevaluated in a 2003 Journal of Clinical Endocrinology & Metabolism study that adjusted suppression thresholds for improved sensitivity (up to 98%) and specificity (up to 92%).^[10] These updates, integrated with advanced imaging like pituitary MRI, have enhanced its utility in confirming hypercortisolism while minimizing standalone limitations.^[2] As of 2025, further advancements include endorsements in the 2023 European Society of Endocrinology guidelines for using the 1-mg overnight DST to screen for autonomous cortisol secretion in adrenal incidentalomas, and proposals to lower cortisol cut-off thresholds (e.g., below 1.8 µg/dL) to improve detection of mild hypercortisolism, based on recent reviews enhancing diagnostic precision.^[11]^[12]

Physiology

Hypothalamic-Pituitary-Adrenal Axis

The hypothalamic-pituitary-adrenal (HPA) axis is a central neuroendocrine system that orchestrates the body's response to stress and sustains physiological balance through coordinated hormonal signaling. The hypothalamus initiates the cascade by secreting corticotropin-releasing hormone (CRH) from neurons in the paraventricular nucleus, which travels to the anterior pituitary gland via the hypophyseal portal system. CRH then stimulates pituitary corticotroph cells to release adrenocorticotropic hormone (ACTH) into the systemic circulation. ACTH subsequently binds to receptors on the zona fasciculata of the adrenal cortex, promoting the synthesis and secretion of cortisol, the primary glucocorticoid hormone.^[13]^[14] A key regulatory feature of the HPA axis is its negative feedback loop, which prevents excessive cortisol production and maintains equilibrium. Circulating cortisol diffuses into cells of the hypothalamus and pituitary, where it binds to intracellular glucocorticoid receptors (GRs). This binding induces conformational changes that translocate the receptor-cortisol complex to the nucleus, repressing transcription of the CRH and pro-opiomelanocortin (POMC) genes, thereby inhibiting further release of CRH and ACTH. This feedback operates at multiple timescales, with rapid non-genomic effects occurring within minutes and slower genomic actions over hours, ensuring adaptive control.^[15]^[13] The HPA axis exhibits a robust diurnal rhythm, synchronized by the suprachiasmatic nucleus of the hypothalamus in response to light-dark cycles, resulting in cortisol levels that peak in the early morning (typically around 6-8 AM) and decline to a nadir at midnight. This pattern supports daily energy demands, with higher morning levels aiding arousal and metabolic readiness. External factors such as acute stress or illness can override this rhythm; psychological or physical stressors activate neural inputs to the hypothalamus, boosting CRH and thus elevating cortisol to mobilize glucose, suppress inflammation, and enhance cardiovascular function, while inflammatory cytokines from illness can similarly stimulate the axis.^[14]^[15] Through these mechanisms, the HPA axis plays an essential role in homeostasis by integrating environmental cues with internal metabolic and immune needs, facilitating adaptation to challenges. Dysregulation of this system, often manifesting as sustained hypercortisolism from impaired feedback or chronic activation, disrupts these processes and contributes to disorders involving excess cortisol exposure.^[13]^[14]

Mechanism of Dexamethasone Suppression

Dexamethasone is a potent synthetic glucocorticoid that effectively crosses the blood-brain barrier and binds with high affinity to glucocorticoid receptors in the hypothalamus and pituitary gland.^[2] This binding activates negative feedback mechanisms within the hypothalamic-pituitary-adrenal (HPA) axis, mimicking the regulatory effects of endogenous cortisol.^[16] In the suppression process, dexamethasone inhibits the secretion of corticotropin-releasing hormone (CRH) from the hypothalamus and adrenocorticotropic hormone (ACTH) from the anterior pituitary, which in turn reduces the production and release of endogenous cortisol from the adrenal cortex in individuals with a normally functioning HPA axis.^[2] This feedback inhibition leads to measurable decreases in circulating cortisol levels, typically within hours of administration, confirming the integrity of the axis's regulatory loop.^[16] Unlike endogenous cortisol, dexamethasone has a prolonged biological half-life of 36 to 54 hours and negligible mineralocorticoid activity, enabling isolated assessment of glucocorticoid-mediated effects without influencing electrolyte balance or interfering with standard cortisol assays in plasma, urine, or saliva.^[2] In pathological conditions, such as those involving autonomous ACTH secretion from pituitary adenomas or cortisol overproduction from adrenal tumors, the suppression fails because the HPA axis exhibits resistance to negative feedback, allowing persistent elevation of cortisol levels despite dexamethasone administration.^[2]^[16]

Test Procedures

Patient Preparation and Administration

Patients undergoing the dexamethasone suppression test (DST) should receive clear instructions to ensure accurate results. They are advised to avoid acute physical or emotional stress, as it can elevate cortisol levels and mimic pathological responses. Maintaining a normal sleep schedule is essential, with the dexamethasone dose typically administered at bedtime (around 11 PM to midnight) to align with the natural cortisol rhythm. Fasting is not always required but may be recommended for the morning blood draw to standardize conditions; patients should confirm with their healthcare provider. Additionally, individuals must disclose all medications and supplements, as certain drugs can interfere with the test by altering dexamethasone metabolism or cortisol binding. For instance, estrogens (such as oral contraceptives) should be discontinued at least 6 weeks prior due to their effect on increasing corticosteroid-binding globulin levels. Anticonvulsants like phenytoin or carbamazepine, which induce CYP3A4 enzymes, accelerate dexamethasone clearance and may lead to false results.^[17]^[4]^[2] An overview of contraindications includes active infections, which can cause stress-induced hypercortisolism. Psychiatric conditions like psychosis require caution due to potential exacerbation by dexamethasone, though the low doses used pose minimal risk and it is not an absolute contraindication. Conditions such as obesity or depression can also produce patterns resembling hypercortisolism, warranting cautious interpretation rather than absolute contraindication. Full details on these and special populations are addressed elsewhere. Patients with known hypersensitivity to glucocorticoids or those on medications strongly affecting the hypothalamic-pituitary-adrenal axis should consult their provider before proceeding.^[17]^[4] Administration of the DST involves oral dexamethasone tablets, typically given as a single low dose or multiple doses over 48 hours, depending on the protocol. The timing is critical: dexamethasone is administered in the evening or at specified intervals, with serum cortisol sampled the following morning between 8 and 9 AM to capture the nadir of the diurnal rhythm. Blood collection occurs after the patient has been supine for at least 30 minutes to minimize postural effects on cortisol. Laboratory measurement of serum cortisol is performed using immunoassays, such as radioimmunoassay or chemiluminescent immunoassay, which provide reliable quantification with a lower detection limit around 0.5 μg/dL. To validate administration, serum dexamethasone levels may be measured if non-suppression occurs, with levels above 200 ng/dL confirming compliance.^[2]^[4]^[17] Safety monitoring is straightforward, as the short-term use of dexamethasone at screening doses poses minimal risk. Rare side effects include transient insomnia, vivid dreams, or mild gastrointestinal upset, which usually resolve without intervention. Patients are instructed to report any unusual symptoms, such as severe headache or mood changes, though these are uncommon. The test does not typically require ongoing monitoring beyond the procedure itself, but follow-up is advised if results are equivocal.^[18]^[2]

Low-Dose Test Protocol

The low-dose dexamethasone suppression test (LDDST) serves as the standard initial screening procedure to detect hypercortisolism suggestive of Cushing's syndrome by administering a minimal dose of dexamethasone to evaluate the hypothalamic-pituitary-adrenal axis's responsiveness.^[2] This test is preferred for its simplicity and high sensitivity in outpatient settings, often confirming or ruling out the need for further diagnostic evaluation.^[19] In the overnight variant, a single 1 mg dose of dexamethasone is administered orally between 11 p.m. and midnight, followed by measurement of plasma or serum cortisol levels the next morning between 8 a.m. and 9 a.m.^[2]^[1] This approach mimics the natural diurnal cortisol rhythm and requires minimal patient compliance, making it suitable for initial screening.^[19] The two-day variant involves administering 0.5 mg of dexamethasone orally every 6 hours for 48 hours, totaling eight doses, with cortisol levels assessed via plasma or serum draw 6 hours after the final dose, or through 24-hour urinary free cortisol collection.^[2]^[1] This protocol provides a more prolonged suppression challenge and is particularly useful when the overnight test yields equivocal results or in settings requiring higher specificity.^[19] As an initial screen, the LDDST demonstrates high sensitivity for detecting Cushing's syndrome, reaching up to 98% in clinical studies, though it may require confirmatory testing to address potential false positives.^[19] For outpatient convenience, some protocols incorporate salivary cortisol measurement instead of plasma sampling post-dexamethasone administration, offering comparable accuracy with easier collection.^[20]^[21]

High-Dose Test Protocol

The high-dose dexamethasone suppression test (HDST) is a confirmatory procedure used after initial screening to subclassify causes of Cushing's syndrome, particularly distinguishing pituitary-dependent (Cushing's disease) from ectopic ACTH sources in ACTH-dependent cases.^[2] It is indicated following a positive low-dose test and plasma ACTH measurement, guiding differentiation by assessing cortisol suppressibility under higher glucocorticoid loads.^[2] The standard protocol, Liddle's two-day regimen, administers 2 mg of oral dexamethasone every 6 hours for 48 hours (total 16 mg), with serum cortisol measured 6 hours after the last dose to evaluate suppression.^[22]^[2] An alternative overnight high-dose test gives a single 8 mg oral dose at 11 p.m., followed by morning cortisol assessment at 8-9 a.m. the next day.^[2] In research settings, an intravenous variant infuses 1 mg of dexamethasone per hour for 4-7 hours after baseline sampling, providing precise suppression assessment through serial cortisol measurements.^[23]

Interpretation and Results

Normal Response Criteria

In healthy individuals, the low-dose overnight dexamethasone suppression test elicits a normal response characterized by suppression of serum cortisol to less than 1.8 μg/dL (50 nmol/L) measured at 8-9 a.m. the following day after administration of 1 mg dexamethasone at 11 p.m. to midnight.^[2] This threshold reflects intact feedback inhibition of the hypothalamic-pituitary-adrenal axis, with suppression rates exceeding 95% in non-stressed adults.^[2] For the two-day low-dose protocol (0.5 mg every 6 hours for 48 hours), normal suppression is similarly defined by morning cortisol below 1.8 μg/dL or a greater than 90% reduction from baseline urinary free cortisol.^[2] Several physiological factors can influence normal response thresholds. Age-related declines in HPA axis sensitivity lead to progressively higher post-dexamethasone cortisol levels and reduced suppression rates, with median cortisol rising from approximately 16 nmol/L in those under 50 years to 35 nmol/L in individuals over 70, potentially necessitating adjusted cutoffs in the elderly to avoid misinterpretation.^[24] Sex differences may also play a role, as females often exhibit higher post-suppression cortisol levels compared to males, possibly due to estrogen-mediated effects on cortisol-binding globulin, though standardized thresholds apply across sexes. Time of day is critical for standardization; the morning cortisol measurement aligns with the nadir of the diurnal rhythm to ensure reliable suppression assessment, as earlier or later sampling can artifactually elevate readings.^[4] Assay method variations impact measurement precision and threshold interpretation. Immunometric assays, such as chemiluminescent immunoassays (e.g., Elecsys or Access), can overestimate or underestimate cortisol by up to 32% at low concentrations compared to liquid chromatography-tandem mass spectrometry (LC-MS/MS), leading to discrepancies in suppression classification; LC-MS/MS is recommended over immunoassays in recent studies (as of 2025) for its superior specificity and accuracy in detecting subtle changes near the 50 nmol/L cutoff.^[25] Urine free cortisol or late-night salivary cortisol measurements serve as adjuncts to confirm normal suppression in equivocal cases, with 24-hour urine free cortisol below 20-50 mcg/day or salivary cortisol under 0.13 μg/dL (3.6 nmol/L) supporting a negative DST result in healthy subjects.^[2]^[26] Confirming adequate dexamethasone absorption, such as by measuring serum dexamethasone levels (typically >180 pg/mL post-dose), is increasingly recommended to validate suppression results and rule out false negatives due to poor compliance or altered metabolism.^[25]

Pathological Patterns and Differential Diagnosis

In the dexamethasone suppression test (DST), failure to suppress cortisol levels in the low-dose protocol (1 mg overnight or 2 mg over 2 days) indicates hypercortisolism consistent with Cushing's syndrome, necessitating further differentiation of its etiology.^[2] A key pathological pattern occurs when low-dose DST shows non-suppression, but high-dose DST (8 mg overnight or 2 mg every 6 hours for 2 days) achieves greater than 50% cortisol reduction; this suggests pituitary-dependent Cushing's disease, where the pituitary adenoma retains partial sensitivity to glucocorticoid feedback.^[17] This pattern accounts for approximately 60-70% of endogenous Cushing's syndrome cases, making it the most common form.^[27] Another distinct pattern is non-suppression of cortisol in both low- and high-dose DST protocols accompanied by elevated plasma adrenocorticotropic hormone (ACTH) levels (>20 pg/mL); this points to ectopic ACTH syndrome, often arising from non-pituitary tumors such as small cell lung carcinoma, which lack feedback inhibition.^[2] Ectopic sources represent about 10-15% of Cushing's syndrome etiologies and typically present with more severe, rapid-onset hypercortisolism.^[27] In contrast, non-suppression across both DST doses with low or undetectable ACTH levels (<5 pg/mL) indicates ACTH-independent Cushing's syndrome due to primary adrenal pathology, such as adenoma or carcinoma, where autonomous cortisol production bypasses pituitary regulation.^[17] Adrenal causes comprise roughly 15-20% of cases and are often unilateral, amenable to surgical intervention.^[27] Differential diagnosis integrates DST results with basal ACTH measurement to classify as ACTH-dependent (pituitary or ectopic) or independent (adrenal).^[2] For ACTH-dependent cases, especially when pituitary MRI is negative or equivocal, bilateral inferior petrosal sinus sampling (IPSS) provides central-to-peripheral ACTH gradients to confirm pituitary origin with high accuracy (>95% sensitivity and specificity).^[17] Adjunctive imaging, including pituitary MRI for microadenomas, chest/abdominal CT for ectopic sources, and adrenal CT/MRI, further localizes pathology and guides management.^[2]

Clinical Considerations

Limitations and Potential Errors

The dexamethasone suppression test (DST) is susceptible to false-positive results, where cortisol levels fail to suppress appropriately despite the absence of Cushing's syndrome. Common causes include medications that accelerate dexamethasone metabolism via induction of CYP3A4 enzymes, such as phenytoin, phenobarbital, carbamazepine, and rifampicin, leading to subtherapeutic dexamethasone levels and inadequate HPA axis suppression.^[28]^[4] Alcohol consumption or withdrawal can similarly enhance dexamethasone clearance or disrupt cortisol regulation, contributing to non-suppression.^[28] Psychiatric conditions, particularly major depression, are associated with HPA axis hyperactivity and elevated baseline cortisol, resulting in false positives; however, this unreliability in depression has been noted historically but is less emphasized in modern Cushing's screening. Stress, weight loss, poor dexamethasone absorption (e.g., due to vomiting or gastrointestinal issues), and factors like exercise or sleep disruption post-administration can also yield false positives by mimicking autonomous cortisol production.^[28]^[2]^[29] False-negative results occur when the test shows appropriate suppression in patients with true endogenous hypercortisolism, particularly in cases of mild or cyclic Cushing's syndrome. In mild forms, cortisol excess may be subtle and intermittent, allowing partial suppression during testing and evading detection.^[4] Cyclic hypercortisolism, characterized by fluctuating periods of hyper- and normocortisolism, can lead to false negatives if the test coincides with a low-cortisol phase, as spontaneous remission may permit dexamethasone-induced suppression.^[30]00150-X/abstract) Overall, the DST exhibits a false-result rate of approximately 10-20%, primarily driven by its specificity of 80-90% in screening for Cushing's syndrome.^[31] Recent 2025 research proposes lowering the cortisol cutoff to around 1.2 µg/dL in specific contexts like adrenal incidentalomas to better detect mild autonomous cortisol secretion, potentially improving sensitivity without excessively reducing specificity.^[12] As of 2025, clinical guidelines increasingly advocate for combined testing strategies to address these limitations, integrating the DST with late-night salivary cortisol or 24-hour urinary free cortisol measurements to improve diagnostic accuracy and reduce false results.^[32]^[33] To mitigate errors, clinicians recommend measuring post-test serum dexamethasone levels (target >5.6 nmol/L) to confirm adequate drug exposure and rule out metabolism-related false positives.^[2] Implementing drug washout periods (e.g., 4-6 weeks for CYP3A4 inducers), repeating the test under controlled conditions, and correlating results with clinical symptoms and additional biochemical assays are essential for reliable interpretation.^[34]^[35]

Contraindications and Special Populations

The dexamethasone suppression test (DST), which involves a low single dose of dexamethasone, has few absolute contraindications, primarily known hypersensitivity to dexamethasone. Conditions such as systemic fungal infections, cerebral malaria, active systemic infections, uncontrolled diabetes mellitus, and glaucoma are relative contraindications or require caution, as even short-term glucocorticoid exposure may exacerbate them; close monitoring (e.g., for hyperglycemia or intraocular pressure) or alternative diagnostic approaches are recommended in these cases.^[36]^[37]^[38] Relative contraindications include conditions where the test may be performed with caution and modifications, such as iatrogenic hypercortisolism from exogenous corticosteroids, necessitating tapering prior to testing to avoid misleading results.^[2] In patients with hepatic or renal impairment, dexamethasone metabolism and clearance are altered, potentially leading to reduced efficacy or prolonged effects; serum dexamethasone levels should be measured to confirm adequate drug exposure (>200 ng/dL), enabling proper assessment of cortisol suppression, and alternative tests like 24-hour urinary free cortisol may be preferred.^[37]^[39] Pregnancy is a relative contraindication for the DST, as elevated corticosteroid-binding globulin levels can cause false positives, and the test is not recommended; alternatives such as late-night salivary cortisol or 24-hour urinary free cortisol are advised for initial screening in pregnant individuals.^[2]^[40] In the elderly, dose adjustments may be necessary due to age-related changes in dexamethasone clearance and higher false-positive rates; higher doses or confirmatory testing with serum dexamethasone measurement can improve accuracy, alongside caution for osteoporosis risk.^[24]^[41] For pediatric patients, the DST is used to screen for Cushing's syndrome and requires weight- or body surface area-based dosing, such as 0.3 mg/m² for the overnight test, with interpretation adjusted for age-related cortisol norms and growth impacts; meticulous blood glucose monitoring is essential in children with diabetes.^[38]^[42]^[37] In outpatient settings, cultural or socioeconomic factors affecting compliance, such as access to medication timing or follow-up blood draws, should be considered to ensure test validity, though these do not alter the protocol itself.^[2]

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