What is the initial work‑up for a patient suspected of Cushing syndrome?

Initial Workup for Suspected Cushing Syndrome

Before any testing, you must first exclude exogenous glucocorticoid use—including oral, inhaled, injectable, and topical corticosteroids—as this is the most common cause of Cushing syndrome and further biochemical testing is unnecessary if iatrogenic exposure is confirmed. 1, 2

Step 1: Exclude Exogenous Steroid Exposure

• Obtain a detailed medication history covering all routes of glucocorticoid administration: oral prednisone, inhaled budesonide or fluticasone, intra-articular injections, topical creams, and nasal sprays 1, 3
• Document any supplements or over-the-counter products, as some contain undeclared corticosteroids 4
• Only proceed with biochemical testing once iatrogenic Cushing syndrome is definitively ruled out 5, 1

Step 2: Initial Biochemical Screening Tests

Perform 2-3 of the following first-line screening tests to confirm hypercortisolism: 1, 3

Recommended First-Line Tests:

• 24-hour urinary free cortisol (UFC): Sensitivity 89%, specificity 100% 3

  • Collect at least two separate 24-hour urine samples to account for day-to-day variability 1, 4
  • Ensure adequate collection volume (>1 liter) to validate the specimen 4

• Late-night salivary cortisol (LNSC): Sensitivity 95%, specificity 93-100% 3

  • Collect 2-3 samples at 11 PM-midnight when cortisol should be at its nadir 1, 3
  • This test detects loss of normal circadian rhythm, which is characteristic of Cushing syndrome 1

• Overnight 1 mg dexamethasone suppression test (DST): 1, 3

  • Administer 1 mg dexamethasone at 11 PM, measure plasma cortisol at 8 AM the next morning 1
  • Normal suppression is cortisol <1.8 μg/dL (50 nmol/L); values above this indicate abnormal feedback inhibition 1, 3
  • Measure plasma dexamethasone level simultaneously to exclude false positives from malabsorption or drug interactions 1, 3

Important Caveats for Screening Tests:

• False positives can occur with severe obesity, uncontrolled diabetes, alcoholism, depression, or polycystic ovary syndrome (pseudo-Cushing states) 1, 4
• Drug interactions affecting dexamethasone metabolism (CYP3A4 inducers like phenytoin, rifampin) can cause false positive DST results 1
• Oral estrogens or pregnancy increase corticosteroid-binding globulin and can elevate total cortisol measurements 1
• If screening tests are mildly abnormal and clinical suspicion is moderate, consider cyclic Cushing syndrome and repeat testing during symptomatic periods 1, 4

Step 3: Confirm Hypercortisolism Before Proceeding

• If one screening test is abnormal but clinical suspicion is low-to-moderate, repeat testing or perform an alternative screening test 5, 1
• If screening tests are normal but clinical suspicion remains high (progressive symptoms, multiple classic features), repeat testing in 3-6 months or refer to endocrinology 5, 1
• Do not proceed to localization studies (imaging, ACTH measurement) until hypercortisolism is biochemically confirmed 5, 6

Step 4: Determine ACTH-Dependent vs. ACTH-Independent Disease

Once hypercortisolism is confirmed, measure morning (8-9 AM) plasma ACTH to differentiate the etiology: 1, 3

ACTH Interpretation:

• ACTH >5 ng/L (>1.1 pmol/L): Indicates ACTH-dependent Cushing syndrome (pituitary adenoma or ectopic ACTH source) 1, 3
• ACTH >29 ng/L (>6.4 pmol/L): 70% sensitivity and 100% specificity for Cushing disease (pituitary source) 1
• ACTH low or undetectable (<5 ng/L): Indicates ACTH-independent Cushing syndrome (adrenal adenoma, carcinoma, or bilateral hyperplasia) 1, 3

Critical Technical Points:

• Collect ACTH in the morning (8-9 AM) when levels are highest and correspond to established diagnostic thresholds 1
• Fasting is not required for ACTH measurement 1
• Ensure the patient is not on exogenous steroids, as these suppress ACTH and confound interpretation 1
• Use an ACTH assay capable of reliably differentiating ACTH-dependent from ACTH-independent disease 7

Step 5: Initial Imaging Based on ACTH Results

For ACTH-Independent Disease (Low ACTH):

• Order adrenal CT or MRI to identify adrenal lesions (adenoma, carcinoma, or bilateral hyperplasia) 1, 3
• CT is preferred for adrenal imaging due to superior spatial resolution 3

For ACTH-Dependent Disease (Elevated ACTH):

• Order high-quality pituitary MRI with thin slices (3T preferred over 1.5T) to detect pituitary adenoma 1, 3
• MRI has only 63% sensitivity for ACTH-secreting microadenomas, so a negative MRI does not exclude Cushing disease 1
• If pituitary adenoma ≥10 mm is found, this strongly suggests Cushing disease and the patient can proceed to neurosurgical evaluation 1
• If MRI shows no adenoma or a lesion <6 mm, bilateral inferior petrosal sinus sampling (BIPSS) is required to differentiate pituitary from ectopic ACTH sources 1, 3

Common Pitfalls to Avoid

• Do not order ACTH or imaging studies before confirming hypercortisolism biochemically—this leads to unnecessary testing and false localization 5, 6
• Do not rely on a single abnormal screening test in patients with low clinical suspicion—repeat or use alternative tests 1
• Do not assume all Cushing syndrome is exogenous—always confirm with biochemical testing if clinical features are progressive despite stopping steroids 3
• Do not skip measuring dexamethasone levels during DST in patients on medications affecting drug metabolism—this prevents false positive results 1, 3
• Recognize that factitious Cushing syndrome can mimic ACTH-dependent disease—if results are discordant (e.g., very high UFC but normal morning cortisol), consider measuring synthetic corticosteroids by mass spectrometry 7

When to Refer to Endocrinology

• Any patient with confirmed hypercortisolism requires endocrinology referral for further evaluation and management 5, 1
• Patients with equivocal screening results but high clinical suspicion should be referred for specialized testing 5, 1
• BIPSS should only be performed at specialized centers with experienced interventional radiologists 1, 3