Diagnostic protocol for Cushing syndrome

Step 0: Confirm this is a patient who should be tested

• Easy bruising, wide reddish-purple striae, facial plethora
• Proximal muscle weakness
• Unexplained osteoporosis or fractures at a young age
• New or difficult-to-control hypertension or diabetes
• Menstrual irregularity, hirsutism, reduced libido
• Growth failure with weight gain in children
• Adrenal incidentaloma with suggestive features

Step 1: Screen for endogenous cortisol excess

A. 1-mg overnight dexamethasone suppression test

• Give dexamethasone 1 mg orally at 11 PM to midnight.
• Measure serum cortisol at 8 to 9 AM the next day.

• Morning cortisol < 1.8 micrograms/dL (< 50 nmol/L)
• This indicates normal suppression and makes Cushing syndrome unlikely.

• Morning cortisol ≥ 1.8 micrograms/dL
• This is failure of suppression and suggests possible Cushing syndrome, but it must be confirmed with another test.

• Poor dexamethasone absorption or non-adherence
• Drugs that increase dexamethasone metabolism: phenytoin, phenobarbital, carbamazepine, rifampicin
• Estrogen therapy or oral contraceptive use can raise cortisol-binding globulin and total serum cortisol
• Depression, alcohol excess, obesity, acute illness

B. Late-night salivary cortisol, preferably two samples

• Collect saliva at 11 PM to midnight, on two separate nights.
• The patient should avoid smoking, eating, and brushing teeth shortly before collection.

• Low late-night salivary cortisol, within the assay-specific reference range.

• Elevated late-night salivary cortisol on two occasions supports Cushing syndrome.

• Convenient outpatient test.
• Particularly useful if cyclic Cushing syndrome is suspected, because it can be repeated during symptomatic periods.

• Shift work, irregular sleep, acute stress, smoking, oral bleeding, and incorrect collection can give misleading results.

C. 24-hour urinary free cortisol, preferably two collections

• Collect all urine for 24 hours, ideally on two separate days.
• Urinary creatinine should be checked to assess completeness of collection.

• Urinary free cortisol within the laboratory reference range.

• Urinary free cortisol above the assay's upper limit of normal suggests hypercortisolism.
• A value more than four times the upper limit of normal is strongly supportive of Cushing syndrome.

• Incomplete collection causes falsely low values.
• Renal impairment, especially GFR below 30 mL/minute, may cause falsely low urinary cortisol.
• Mild elevation may occur in obesity, alcohol excess, depression, pregnancy, and poorly controlled diabetes.

Tests that should not be used alone for screening

• Random serum cortisol
• Single morning cortisol
• Random ACTH
• Urinary 17-ketosteroids
• CT or MRI before biochemical confirmation

Step 2: Confirm endogenous Cushing syndrome

Step 3: Measure plasma ACTH to determine whether cortisol excess is ACTH-dependent

A. ACTH-independent Cushing syndrome

• High cortisol
• Low ACTH because cortisol suppresses pituitary ACTH secretion

• Thin-section CT or MRI of the adrenal glands.

• Unilateral adrenal mass: adrenal adenoma or carcinoma, assessed using imaging appearance and hormone profile.
• Bilateral nodularity or enlargement: bilateral adrenal hyperplasia/nodular disease.
• Normal-appearing glands: seek specialist assessment, since imaging alone does not prove the source.

B. ACTH-dependent Cushing syndrome

• High cortisol
• ACTH normal or high

1. Pituitary ACTH-secreting adenoma, called Cushing disease, approximately the commonest endogenous cause.
2. Ectopic ACTH secretion, often from a neuroendocrine tumor, small-cell lung cancer, bronchial carcinoid, thymic carcinoid, or pancreatic neuroendocrine tumor.

Step 4: Pituitary MRI

Step 5: Inferior petrosal sinus sampling, IPSS

• Catheters are inserted through the femoral veins into both inferior petrosal sinuses, which drain the pituitary.
• ACTH is measured simultaneously in right and left petrosal sinus samples and peripheral blood.
• CRH or desmopressin stimulation may be used.

Older dynamic tests: how to interpret them

High-dose dexamethasone suppression test

CRH or desmopressin stimulation

Simplified examination algorithm

Clinical suspicion of Cushing syndrome
        ↓
Exclude exogenous glucocorticoid exposure and pseudo-Cushing conditions
        ↓
Screen with one of:
  - 1 mg overnight dexamethasone suppression test
  - Late-night salivary cortisol x 2
  - 24-hour urinary free cortisol x 2
        ↓
Two abnormal results?
   ├─ No: repeat / investigate cyclic Cushing or pseudo-Cushing
   └─ Yes: measure plasma ACTH
              ↓
      ACTH low                     ACTH normal/high
        ↓                                  ↓
 Adrenal CT/MRI                    Pituitary MRI
        ↓                                  ↓
 Adrenal cause?                 MRI equivocal or <6 mm?
                                           ↓
                                         IPSS
                                  ┌────────┴────────┐
                             Central gradient    No gradient
                                  ↓                 ↓
                          Pituitary Cushing      Ectopic ACTH search
                           disease likely