What is the recommended diagnostic approach for suspected Cushing syndrome?

Diagnostic Approach for Cushing Syndrome

Exclude Exogenous Glucocorticoids First

Before ordering any biochemical test, obtain a comprehensive medication history to rule out all sources of exogenous glucocorticoids—oral tablets, inhaled agents (fluticasone, budesonide), topical creams, intra-articular or epidural injections, and over-the-counter supplements. Failure to exclude iatrogenic exposure is the most common pitfall and leads to unnecessary testing without clinical benefit. 1, 2

First-Line Screening Tests (Perform 2–3 of the Following)

The Endocrine Society recommends performing at least two to three first-line screening tests, with each test repeated 2–3 times on separate days to account for biological variability and detect cyclic disease. 1, 2

Late-Night Salivary Cortisol (LNSC)

• Collect saliva samples at the patient's usual bedtime (11 PM–midnight) on 2–3 consecutive nights. 1
• Sensitivity: 95%; Specificity: 93–100%—the highest specificity among all screening tests. 1, 2
• Abnormal threshold: >3.6 nmol/L (>0.1 μg/dL). 2, 3
• Absolute contraindication: Do NOT use in night-shift workers or anyone with disrupted sleep-wake cycles, because the test relies on the normal nocturnal cortisol nadir. 1
• Pitfall: Topical hydrocortisone can contaminate samples, especially with mass spectrometry assays. 1

24-Hour Urinary Free Cortisol (UFC)

• Collect 2–3 separate 24-hour urine specimens on different days. 1, 2
• Sensitivity: 89%; Specificity: 100%. 1
• Abnormal threshold: >193 nmol/24h (>70 μg/m²/24h) or >100 μg/24h (1.6 μmol/24h). 1, 2
• Measure total volume and creatinine excretion to confirm completeness of collection—incomplete collections are the most common cause of false results. 1
• Pitfall: Avoid in patients with renal impairment (CrCl <60 mL/min) or significant polyuria; use LNSC instead. 2

Overnight 1-mg Dexamethasone Suppression Test (DST)

• Administer 1 mg dexamethasone orally at 11 PM–midnight; measure serum cortisol at 8 AM the next morning. 1, 2
• Normal suppression: cortisol <1.8 μg/dL (<50 nmol/L); cortisol >5 μg/dL (>138 nmol/L) indicates overt Cushing's syndrome. 1, 3
• Simultaneously measure dexamethasone levels with cortisol to confirm adequate drug absorption and exclude false-positive results from rapid metabolism (CYP3A4 inducers) or false-negative results from CYP3A4 inhibitors. 1, 2
• Pitfall: Oral estrogen-containing contraceptives increase cortisol-binding globulin and can falsely elevate total cortisol, causing false-positive results. 1, 3
• Preferred test for patients with irregular sleep schedules or shift work because it does not depend on a fixed bedtime collection. 1

Interpretation of Screening Results

• If any screening test is abnormal, repeat 1–2 additional screening tests to confirm hypercortisolism before proceeding further. 1, 2
• If all tests are normal and clinical suspicion is low to moderate, Cushing's syndrome is unlikely. 1
• If clinical suspicion remains high despite normal tests, consider cyclic Cushing's syndrome and perform extended monitoring with multiple sequential LNSC measurements over weeks to months. 1

Exclude Pseudo-Cushing's States

Pseudo-Cushing's states—severe obesity, major depression, chronic alcoholism, uncontrolled diabetes, polycystic ovary syndrome—can produce modest cortisol elevations that mimic true Cushing's syndrome. 1, 2, 3

• If mild hypercortisolism is detected in the context of a possible pseudo-Cushing's state, treat the underlying condition and repeat biochemical testing after 3–6 months. 1
• For equivocal cases, consider the desmopressin test or Dex-CRH test to distinguish true Cushing's syndrome from pseudo-Cushing's. 1, 2

Determine Etiology: Measure Morning Plasma ACTH

Once hypercortisolism is confirmed by abnormal screening tests, measure a single 9 AM plasma ACTH to differentiate ACTH-dependent from ACTH-independent disease. 1, 2

ACTH-Independent (Adrenal) Cushing's Syndrome

• Low or undetectable ACTH (<5 ng/L or <1.1 pmol/L) indicates an adrenal source. 1, 2
• Proceed to adrenal CT or MRI. 1
• Imaging features suspicious for adrenocortical carcinoma: mass >4 cm, irregular margins, heterogeneous enhancement, unenhanced Hounsfield units >10. 1
• Perform comprehensive steroid profiling (DHEA-S, 17-OH-progesterone, androstenedione, testosterone, 17-β-estradiol) and 24-hour urine steroid metabolites to distinguish benign adenomas from carcinoma. 1

ACTH-Dependent (Pituitary or Ectopic) Cushing's Syndrome

• Normal or elevated ACTH (>5 ng/L or >1.1 pmol/L) suggests a pituitary or ectopic source. 1, 2
• Proceed to pituitary MRI (sensitivity 63%, specificity 92%). 1
• If pituitary MRI is negative or equivocal, perform bilateral inferior petrosal sinus sampling (BIPSS) with CRH or desmopressin stimulation. 1, 2
  • Diagnostic criteria: central-to-peripheral ACTH ratio ≥2:1 before stimulation and ≥3:1 after CRH stimulation confirms pituitary source. 1
• CRH stimulation test: ≥20% increase in cortisol from baseline supports pituitary origin. 1

Special Considerations for Ectopic ACTH Syndrome

In patients with known or suspected lung cancer (especially small-cell carcinoma or bronchial carcinoid), suspect paraneoplastic Cushing's syndrome if moon facies, acne, and purple striae are present. 1

• Characteristic features: prominent skin hyperpigmentation (more common with ectopic ACTH), hypokalemia with metabolic alkalosis, and weight loss (occurs in ~10% of small-cell lung cancer cases). 1

Pediatric Considerations

In children and adolescents, screen only if weight gain is inexplicable and combined with either decreased height standard deviation score or decreased height velocity. 1

• Late-night salivary cortisol has 95% sensitivity and 100% specificity in pediatric Cushing's disease. 1
• Sleeping midnight serum cortisol ≥50 nmol/L provides 100% sensitivity and 60% specificity in children. 1

Critical Pitfalls to Avoid

• Never use bilateral inferior petrosal sinus sampling (BIPSS) to diagnose hypercortisolism—it is reserved solely for localizing the source of ACTH after hypercortisolism has been biochemically confirmed. 1
• In cyclic Cushing's syndrome, verify active hypercortisolism with a screening test immediately before performing BIPSS or other dynamic localization studies. 1
• A single abnormal screening test is insufficient for diagnosis—repeat testing is essential to minimize false-positive diagnoses. 1
• Measure dexamethasone levels during DST to exclude false results from abnormal drug metabolism. 1, 2