What laboratory tests should be ordered for an adult with a history of steroid self-medication suspected of having Cushing syndrome?

Laboratory Evaluation for Cushing Syndrome Due to Steroid Self-Medication

Initial Critical Step: Exclude Exogenous Steroid Use

Before ordering any laboratory tests, you must first document and discontinue all exogenous steroid use, as this is the most common cause of Cushing syndrome and will confound all diagnostic testing. 1, 2 Once exogenous steroids are stopped and sufficient washout time has elapsed (typically 4-6 weeks depending on the steroid formulation), proceed with the diagnostic workup if clinical suspicion for endogenous Cushing syndrome remains.

First-Line Screening Tests

Order at least 2-3 measurements of any combination of the following three first-line screening tests to confirm hypercortisolism: 3, 4

Late-Night Salivary Cortisol (LNSC)

• Collect 2-3 samples on different nights to account for variability and detect cyclic hypercortisolism 3, 5, 4
• Sensitivity: 95%, Specificity: 100% 5
• Avoid in shift workers or patients with irregular sleep schedules, as the test becomes unreliable 4
• Ensure no topical hydrocortisone contamination, which falsely elevates results 4

24-Hour Urinary Free Cortisol (UFC)

• Collect 2-3 separate 24-hour urine collections due to up to 50% intra-patient variability 3, 4
• Diagnostic cut-off: >193 nmol/24h (>70 μg/m²) 5
• Sensitivity: 89-97%, Specificity: 91-100% 4, 6
• Preferred over LNSC in patients with renal impairment or significant polyuria 4
• Ensure complete urine collections with appropriate total volumes to avoid false results 6

Overnight 1-mg Dexamethasone Suppression Test (DST)

• Administer 1 mg dexamethasone at 11 PM, measure serum cortisol at 8 AM 4
• Diagnostic cut-off: ≥50 nmol/L (≥1.8 μg/dL) indicates Cushing syndrome 3, 5
• Sensitivity: 95%, Specificity: 80% 5
• Measure dexamethasone levels simultaneously if false-positive suspected (e.g., medications affecting CYP3A4 metabolism, malabsorption) 3, 4
• May be preferred for shift workers with disrupted circadian rhythm 3

Determining Etiology After Confirmed Hypercortisolism

Morning Plasma ACTH (08:00-09:00h)

Once hypercortisolism is confirmed with abnormal screening tests, measure morning plasma ACTH to differentiate ACTH-dependent from ACTH-independent causes: 3, 7, 4

• ACTH <5 ng/L (<1.1 pmol/L): ACTH-independent (adrenal) Cushing syndrome 7, 4
• ACTH ≥5 ng/L (≥1.1 pmol/L): ACTH-dependent Cushing syndrome (pituitary or ectopic source) 5, 7
• ACTH >29 ng/L: 70% sensitivity and 100% specificity for Cushing's disease 5, 7
• Critical timing: Must be collected in the morning (08:00-09:00h) when ACTH levels are highest; afternoon measurements are unreliable 7
• No fasting required for ACTH measurement 7

Additional Laboratory Tests Based on ACTH Results

If ACTH-Independent (Low/Undetectable ACTH)

Proceed directly to adrenal imaging (CT or MRI) to identify adrenal lesion(s) 7, 4

If ACTH-Dependent (Normal/Elevated ACTH)

Order pituitary MRI as the next step to identify potential pituitary adenoma 7, 4

If pituitary MRI shows:

• Adenoma ≥10 mm: Presume Cushing's disease, proceed to surgery 4
• Adenoma 6-9 mm: Consider CRH stimulation test or bilateral inferior petrosal sinus sampling (BIPSS) 3, 4
• No adenoma or <6 mm lesion: Perform BIPSS to distinguish pituitary from ectopic ACTH source 3, 4

CRH Stimulation Test (if indicated)

• Cortisol rise >38 nmol/L at 15 minutes after CRH indicates Cushing's disease 7
• Sensitivity: 74-100% for ACTH-dependent Cushing syndrome 5
• Can be combined with dexamethasone suppression (Dex-CRH test) for improved accuracy: sensitivity 90%, specificity 95% 7

Bilateral Inferior Petrosal Sinus Sampling (BIPSS)

• Gold standard for differentiating pituitary from ectopic ACTH sources 3, 4
• Diagnostic criteria: Central-to-peripheral ACTH ratio ≥2:1 baseline or ≥3:1 after CRH/desmopressin stimulation 3, 4
• Sensitivity: 96-100%, Specificity: approaching 100% 7
• Must be performed at specialized center by experienced interventional radiologist 7, 4
• Never use BIPSS to diagnose hypercortisolism itself—only to localize ACTH source after diagnosis is confirmed 3, 4

Critical Pitfalls to Avoid

• Never proceed with etiologic workup before confirming hypercortisolism with abnormal screening tests 4
• Do not rely on single screening test results—always repeat 1-2 tests to confirm 4
• Beware of pseudo-Cushing's states (depression, alcoholism, severe obesity) that can cause mildly elevated results, typically UFC <3-fold above normal 4
• Ensure adequate washout of exogenous steroids before testing, as residual suppression can persist for weeks 1, 2
• Measure dexamethasone levels with DST when medications affecting CYP3A4 metabolism are present (anticonvulsants, rifampin) 3, 4
• In cyclic Cushing syndrome, repeat testing during active hypercortisolism phase confirmed by LNSC, DST, or UFC 3, 7

Supplementary Laboratory Monitoring

While not diagnostic, consider checking for complications of hypercortisolism:

• Serum potassium: Hypokalemia common, especially with ectopic ACTH syndrome 3
• Fasting glucose/HbA1c: Screen for hyperglycemia/diabetes 3
• Lipid panel: Assess for dyslipidemia 3