Dexamethasone Suppression Tests in Clinical Practice: A Practical Primer for the Internist

Dr Neeraj Manikath , claude.ai

Abstract

Dexamethasone suppression tests (DST) remain cornerstone investigations in the evaluation of hypercortisolism, yet their selection and interpretation often perplex clinicians. This comprehensive review elucidates the physiological principles, diagnostic algorithms, and practical nuances of various DST protocols. We provide evidence-based guidance on test selection, common pitfalls, and interpretative strategies to optimize diagnostic accuracy in real-world clinical scenarios.

Introduction

Cushing's syndrome, though rare with an incidence of 2-3 per million population annually, presents a diagnostic challenge that demands methodical investigation.¹ The dexamethasone suppression test exploits the principle of negative feedback in the hypothalamic-pituitary-adrenal (HPA) axis. Exogenous glucocorticoid administration should suppress ACTH and consequently cortisol secretion in healthy individuals, while pathological hypercortisolism demonstrates resistance to this suppression.² Understanding which DST to deploy and when represents a critical skill for the practicing internist.

Physiological Foundation

The HPA axis operates through tightly regulated negative feedback. Corticotropin-releasing hormone (CRH) from the hypothalamus stimulates pituitary ACTH secretion, which drives adrenal cortisol production. Cortisol, in turn, suppresses both CRH and ACTH through glucocorticoid receptors.³ Dexamethasone, a synthetic glucocorticoid with 25-30 times the potency of cortisol, binds these receptors without cross-reacting in cortisol immunoassays—an elegant pharmacological property that makes it ideal for suppression testing.⁴

Pearl #1: Dexamethasone's lack of mineralocorticoid activity and minimal cross-reactivity with cortisol assays distinguishes it from other glucocorticoids, making it the superior agent for suppression testing.

The Overnight 1-mg Dexamethasone Suppression Test

Protocol

The patient takes 1 mg of dexamethasone orally between 11 PM and midnight. Plasma cortisol is measured at 8-9 AM the following morning.⁵

Interpretation

Normal response: Cortisol <1.8 μg/dL (50 nmol/L)
Equivocal: 1.8-5 μg/dL (50-138 nmol/L)
Abnormal: >5 μg/dL (138 nmol/L)

Clinical Utility

This test serves as the first-line screening tool for Cushing's syndrome with sensitivity of 95-98% and specificity of 80-85%.⁶ Its simplicity, low cost, and minimal inconvenience make it ideal for outpatient evaluation of suspected hypercortisolism.

Pitfalls and Hacks

Oyster #1: False-positive results occur with increased cortisol-binding globulin (CBG) states including estrogen therapy, pregnancy, and mitotane use. Consider measuring free cortisol or using higher cutoffs (>7 μg/dL) in these situations.⁷

Oyster #2: Drugs inducing hepatic CYP3A4 enzymes (phenytoin, carbamazepine, rifampin, phenobarbital) accelerate dexamethasone metabolism, leading to false-positive results. Conversely, CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir) may cause false negatives.⁸

Hack #1: If false-positive suspected due to rapid dexamethasone metabolism, measure dexamethasone levels simultaneously with morning cortisol. Levels <220 ng/dL suggest inadequate exposure.⁹

Hack #2: In patients with depression or severe stress, which can activate the HPA axis, use the 2-mg overnight DST (see below) for improved specificity while maintaining sensitivity.

The Low-Dose (2-day) Dexamethasone Suppression Test

Protocol

Dexamethasone 0.5 mg is administered orally every 6 hours for 48 hours (8 doses total). Plasma cortisol is measured 6 hours after the final dose.¹⁰

Interpretation

Normal: Cortisol <1.8 μg/dL (50 nmol/L)
Abnormal: >1.8 μg/dL

Clinical Utility

The 2-day low-dose DST offers superior specificity (95-98%) compared to the overnight test while maintaining high sensitivity (95-97%).¹¹ It represents the definitive screening test when overnight DST results are equivocal or in populations with high false-positive risk.

Pearl #2: The 2-day test provides more sustained dexamethasone exposure, overcoming issues with rapid metabolism and reducing false-positives from stress or psychiatric conditions.

Practical Considerations

Oyster #3: Patient compliance becomes challenging with eight doses over 48 hours. Provide written instructions and consider supervised administration in hospitalized patients when diagnostic certainty is paramount.

Hack #3: Collect 24-hour urine for cortisol measurement during the second day of testing. This adds minimal burden and provides confirmatory data, particularly valuable when plasma results fall in the equivocal range.¹²

The High-Dose (8-mg) Dexamethasone Suppression Test

Protocol

After confirming ACTH-dependent Cushing's syndrome, administer 8 mg dexamethasone at 11 PM, with plasma cortisol measured at 8 AM the next morning (overnight protocol). Alternatively, give 2 mg every 6 hours for 48 hours (2-day protocol).¹³

Interpretation

>50% suppression from baseline: Suggests pituitary Cushing's disease
<50% suppression: Suggests ectopic ACTH secretion

Clinical Utility

This test differentiates pituitary from ectopic ACTH-dependent Cushing's syndrome with approximately 80-90% accuracy for pituitary disease.¹⁴ However, its role has diminished with the advent of inferior petrosal sinus sampling (IPSS).

Pearl #3: The high-dose DST should never be the sole determinant for surgical decision-making. Modern practice mandates pituitary MRI and, when necessary, IPSS for definitive localization.¹⁵

Modern Context

Oyster #4: Approximately 10-20% of ectopic ACTH-secreting tumors suppress with high-dose dexamethasone, and 5-10% of pituitary adenomas do not suppress—limiting diagnostic certainty.¹⁶ Bronchial carcinoids, in particular, may show paradoxical suppression.

Hack #4: When high-dose DST results conflict with imaging, proceed directly to IPSS rather than repeating tests. Time to diagnosis impacts morbidity in Cushing's syndrome.

The Overnight 2-mg Dexamethasone Suppression Test

Protocol

Administer 2 mg dexamethasone at midnight, with cortisol measured at 8 AM.

Clinical Utility

This modified screening test improves specificity over the 1-mg test while preserving convenience. It's particularly valuable in patients with psychiatric comorbidities or obesity, where HPA axis activation may cause false-positives with standard testing.¹⁷

Hack #5: Use the 2-mg overnight test as first-line screening in patients with depression, chronic pain, or significant psychosocial stress—populations where standard 1-mg testing yields unacceptably high false-positive rates.

Selecting the Appropriate Test: A Clinical Algorithm

Initial Screening Decision Tree

For typical outpatients with clinical suspicion of Cushing's syndrome: → Overnight 1-mg DST

For patients with depression, chronic stress, or psychiatric illness: → Overnight 2-mg DST or proceed directly to 2-day low-dose DST

For patients on CYP3A4-inducing medications: → 2-day low-dose DST with dexamethasone level measurement, or consider alternative screening (late-night salivary cortisol, 24-hour urinary free cortisol)

When overnight 1-mg DST shows cortisol 1.8-5 μg/dL: → Proceed to 2-day low-dose DST for confirmation

When overnight 1-mg DST shows cortisol >5 μg/dL: → Consider 2-day low-dose DST if clinical suspicion moderate → Proceed to establishing ACTH-dependency if clinical suspicion high

Differential Diagnosis Decision Tree

After confirming Cushing's syndrome with ACTH >20 pg/mL: → Pituitary MRI as initial localization → If MRI negative or shows lesion <6 mm, consider high-dose DST as preliminary differentiation tool → Proceed to IPSS for definitive diagnosis before surgery

Pearl #4: The modern diagnostic pathway prioritizes IPSS over high-dose DST for ACTH-dependent disease differentiation, but high-dose DST remains useful when IPSS is unavailable or contraindicated.

Special Populations and Circumstances

Pregnancy

Pregnancy poses unique challenges as cortisol and CBG levels increase physiologically by 2-3 fold.¹⁸ DST has reduced specificity, and urinary free cortisol cutoffs must be adjusted upward. When Cushing's syndrome is suspected in pregnancy, consultation with endocrinology and maternal-fetal medicine is essential, as untreated disease carries significant maternal and fetal morbidity.

Hack #6: In pregnant patients, use gestational age-specific reference ranges and consider late-night salivary cortisol as the primary screening tool over DST.

Renal and Hepatic Impairment

Severe renal impairment (CrCl <30 mL/min) may alter dexamethasone metabolism unpredictably. Significant hepatic dysfunction affects both dexamethasone metabolism and cortisol production. In these populations, measurement of dexamethasone levels alongside cortisol becomes particularly valuable for result interpretation.¹⁹

Pseudo-Cushing's States

Alcohol dependence, severe depression, and morbid obesity can cause physiological hypercortisolism mimicking Cushing's syndrome—termed pseudo-Cushing's states. These conditions may show abnormal low-dose DST results.²⁰

Hack #7: The combined dexamethasone-CRH test (1 mg dexamethasone at 11 PM, followed by 100 μg CRH at 8 AM the next morning, with cortisol measured 15 minutes post-CRH) achieves near-perfect differentiation between Cushing's syndrome and pseudo-Cushing's states.²¹ Consider this advanced protocol when clinical context suggests pseudo-Cushing's.

Technical and Laboratory Considerations

Assay Methodology

Modern cortisol assays use either immunoassay or liquid chromatography-tandem mass spectrometry (LC-MS/MS). While immunoassays offer convenience, they may show cross-reactivity with synthetic glucocorticoids and demonstrate variable specificity.²² LC-MS/MS provides superior accuracy and is preferred when available.²³

Oyster #5: Verify your laboratory's cortisol assay methodology and reference ranges. Cutoffs established for one assay may not apply to another. When results seem discordant with clinical presentation, request LC-MS/MS confirmation.

Sample Timing and Handling

Cortisol exhibits significant circadian variation with peak levels at 8 AM and nadir at midnight. Strict adherence to specified collection times is essential for valid interpretation. Samples should be collected in tubes appropriate for the assay method and processed promptly, as cortisol degrades in improperly stored samples.²⁴

Hack #8: Instruct patients to fast after midnight before morning blood draw, as glucose ingestion may transiently suppress cortisol, potentially causing false-negative results.

Interpreting Equivocal Results

When DST results fall in borderline ranges despite careful test selection and execution, consider this systematic approach:

Review medication list exhaustively for drugs affecting dexamethasone or cortisol metabolism
Assess for physiological stressors including recent surgery, acute illness, severe pain, or psychological trauma
Measure dexamethasone levels to confirm adequate drug exposure
Utilize complementary tests: late-night salivary cortisol (2 measurements) and 24-hour urinary free cortisol
Consider repeat testing after 6-8 weeks if acute stressors resolve
Proceed to CRH stimulation or dexamethasone-CRH testing for definitive differentiation

Pearl #5: The diagnosis of Cushing's syndrome should never rest on a single abnormal test. Biochemical confirmation requires at least two concordant abnormal results from different test modalities.²⁵

Cost-Effectiveness and Practical Implementation

From a healthcare systems perspective, the overnight 1-mg DST represents the most cost-effective initial screening strategy with an estimated cost of $50-100 including medication and laboratory fees.²⁶ The 2-day low-dose DST costs approximately $200-300 but saves money by reducing false-positives and subsequent unnecessary investigations.

Hack #9: When ordering DST in outpatient settings, provide patients with a printed protocol sheet including medication timing, food/drink restrictions, and explicit instructions for blood draw timing. This simple intervention dramatically improves test validity.

Emerging Perspectives

Recent research explores late-night dexamethasone suppression testing, where dexamethasone is administered in the late afternoon with late-night cortisol measurement, potentially improving diagnostic accuracy by respecting circadian rhythms.²⁷ Additionally, hair cortisol analysis may provide integrated measures of cortisol exposure over months, though standardization remains incomplete.²⁸

Conclusion

Dexamethasone suppression testing remains an indispensable tool in the evaluation of hypercortisolism, but test selection must be tailored to individual clinical contexts. The overnight 1-mg DST serves as the optimal initial screening test for most patients, while the 2-day low-dose DST provides definitive screening when higher specificity is required. The high-dose DST maintains limited utility in the modern era of advanced imaging and IPSS. Familiarity with the physiological principles, awareness of common pitfalls, and systematic interpretation of results enable internists to navigate the diagnostic pathway efficiently, minimizing patient burden while maximizing diagnostic accuracy.

Final Pearl: When in doubt about DST interpretation, remember that Cushing's syndrome is a serious, treatable condition where delayed diagnosis increases morbidity. A low threshold for endocrinology consultation and use of complementary testing modalities serves patients better than extended observation of borderline results.

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