In a female patient with classic Cushing’s syndrome signs and negative contrast CT scans of the chest, abdomen, and pelvis, could the cause be an upstream source, and should the initial evaluation begin with laboratory testing (cortisol, ACTH, dexamethasone suppression) rather than a physical exam?

Initial Evaluation Should Begin with Laboratory Testing, Not Physical Exam

Yes, laboratory testing is the correct first step to screen for Cushing's syndrome, and a negative CT scan does not rule out the diagnosis. The physical exam findings you describe are important clinical clues, but biochemical confirmation must come first before pursuing imaging for tumor localization 1.

Why Laboratory Testing Comes First

The Endocrine Society guidelines explicitly recommend starting with biochemical screening tests rather than imaging 1. You should obtain at least two of the following first-line tests:

• 24-hour urinary free cortisol (UFC)
• 1-mg overnight dexamethasone suppression test (DST)
• Late-night salivary cortisol (measured twice)

These tests confirm whether hypercortisolism exists before you invest time and resources searching for its source 1, 2. The physical exam findings suggest Cushing's syndrome, but many features overlap with common conditions in the general population, so biochemical proof is essential 3.

Why the CT Was Negative

A negative CT of chest, abdomen, and pelvis does not exclude Cushing's syndrome for several critical reasons:

Pituitary Source (Most Common)

• 80-85% of ACTH-dependent Cushing's syndrome originates from pituitary microadenomas (Cushing's disease), which would not appear on chest/abdominal/pelvic CT 2
• These require dedicated pituitary MRI (preferably 3T over 1.5T) for detection 4
• Even with optimal MRI, 40% of pituitary corticotroph adenomas may not be visible initially

Small Ectopic Tumors

• Ectopic ACTH-secreting tumors can be very small (often <1 cm) and easily missed on standard CT 4
• Well-differentiated neuroendocrine tumors may require specialized imaging like 68Ga-DOTATATE PET/CT for detection 4

Adrenal Micronodules

• Early adrenal adenomas or bilateral micronodular hyperplasia may be too small to detect on routine CT 5, 6

The Diagnostic Algorithm

Step 1: Confirm Hypercortisolism (Do This Now)

Order two different first-line screening tests 1:

• If both abnormal → proceed to Step 2
• If both normal → Cushing's syndrome unlikely (unless cyclic disease suspected)
• If discordant → refer to endocrinology for additional testing

Critical pitfall: Ensure the patient is not taking exogenous glucocorticoids (pills, creams, injections, inhalers) before testing 1, 2. This is the most common cause of Cushingoid appearance.

Step 2: Measure Plasma ACTH

Once hypercortisolism is confirmed, a single ACTH level differentiates:

• ACTH elevated or normal → ACTH-dependent (pituitary or ectopic source)
• ACTH suppressed (<5 pg/mL) → ACTH-independent (adrenal source)

Step 3: Localize the Source

For ACTH-dependent disease:

• Order pituitary MRI with contrast (3T preferred) 4
• If MRI negative or equivocal, consider CRH stimulation test and/or desmopressin stimulation test 4
• Bilateral inferior petrosal sinus sampling (IPSS) remains the gold standard when non-invasive tests are discordant 4

For ACTH-independent disease:

• Dedicated adrenal CT protocol or MRI to characterize the adrenal mass

Step 4: Whole-Body Imaging if Ectopic Source Suspected

If tests suggest ectopic ACTH secretion:

• 68Ga-DOTATATE PET/CT is superior to conventional CT for detecting small neuroendocrine tumors 4
• Standard chest/abdomen/pelvis CT with thin cuts if PET unavailable

Important Caveats

Drug interactions: Phenytoin, phenobarbital, rifampin, and indomethacin can cause false results on dexamethasone suppression testing 7. Ask specifically about these medications.

Cyclic Cushing's: Some patients have intermittent hypercortisolism. If initial tests are normal but clinical suspicion remains high, repeat testing during symptomatic periods or use multiple late-night salivary cortisol measurements over weeks 1, 8.

Pseudo-Cushing's states: Depression, alcoholism, obesity, and poorly controlled diabetes can cause mild cortisol elevation. These typically show some suppression on testing, unlike true Cushing's syndrome 2, 9.

Bottom Line

Start with blood and urine tests to prove hypercortisolism exists—do not start with imaging. The negative CT scan is expected and does not exclude any cause of Cushing's syndrome. Once biochemical confirmation is obtained, ACTH levels will guide you toward the correct imaging modality (pituitary MRI for ACTH-dependent disease, adrenal imaging for ACTH-independent disease). Refer to endocrinology early if screening tests are abnormal or discordant 1.