Workup for Raised Cortisol Level
The 1 mg dexamethasone suppression test (DST) is the preferred initial screening test for hypercortisolism, with cortisol measured at 8 AM after taking 1 mg dexamethasone at 11 PM the night before. 1
Initial Screening Tests
The diagnostic workup should begin with biochemical confirmation using one or more of the following tests:
1 mg overnight DST: Take 1 mg dexamethasone at 11 PM, measure serum cortisol at 8 AM. Cortisol <50 nmol/L excludes hypercortisolism, 51-138 nmol/L suggests possible autonomous cortisol secretion, and >138 nmol/L indicates cortisol hypersecretion. 1
24-hour urinary free cortisol (UFC): Collect at least 2-3 samples to account for variability. UFC is always markedly elevated in clinically apparent Cushing's syndrome and is independent of cortisol-binding globulin changes. 1, 2
Late-night salivary cortisol (LNSC): Multiple measurements can be easier for patients to complete and are useful when circadian rhythm disruption is present. 1, 3
Important caveat: No single test is perfect, so multiple tests are recommended to confirm the diagnosis. 3
Determining the Etiology
Once hypercortisolism is confirmed, measure plasma ACTH levels to differentiate between ACTH-dependent and ACTH-independent causes:
ACTH-Independent (Low/Undetectable ACTH <5 ng/L)
- Indicates primary adrenal source 3, 4
- Order adrenal CT or MRI to identify adenoma, carcinoma, or bilateral hyperplasia 3, 4
- MRI correctly diagnoses adenomas in 5 of 6 cases and can distinguish carcinoma 4
- Check DHEAS and testosterone if adrenocortical carcinoma or virilization is suspected 1
ACTH-Dependent (ACTH >5 ng/L)
- Suggests pituitary (Cushing's disease) or ectopic ACTH source 3, 5
- Order pituitary MRI with contrast to look for adenoma 3
- If MRI is negative or equivocal, bilateral inferior petrosal sinus sampling (BIPSS) is the gold standard to confirm pituitary source. A central-to-peripheral ACTH gradient ≥2 before or ≥3 after CRH stimulation confirms Cushing's disease. 3
- CRH stimulation test may help differentiate pituitary from ectopic sources 3
Clinical Assessment
Look for specific clinical features that support the diagnosis:
History findings: Weight gain, central obesity, easy bruising, severe hypertension, diabetes, proximal muscle weakness, fatigue, depression, sleep disturbances, menstrual irregularities, virilization (females), or fragility fractures 1
Physical examination findings: Hypertension, central obesity, supraclavicular fat accumulation, dorsocervical fat pad, facial plethora, thinned skin, purple striae >1 cm wide, acne, ecchymoses, hirsutism, proximal muscle weakness or wasting 1
Critical Pitfalls to Avoid
Always exclude exogenous glucocorticoid use first before proceeding with biochemical testing 3
Measure dexamethasone levels during DST to rule out abnormal metabolism from CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) or inhibitors (fluoxetine, cimetidine, diltiazem) 1
Consider pseudo-Cushing's states: Psychiatric disorders, alcohol use disorder, obesity, and polycystic ovary syndrome can cause mild hypercortisolism with abnormal screening tests but UFC is typically <3-fold normal 1
Account for increased cortisol-binding globulin: Oral estrogens, pregnancy, and chronic active hepatitis increase total cortisol levels, causing false-positive DST results 1
Avoid UFC in renal impairment: Use LNSC instead when creatinine clearance <60 mL/min or polyuria >5 L/24h 1
Don't rely on imaging alone: Biochemical confirmation and ACTH determination must precede imaging to avoid misdiagnosing incidental findings 3
Consider cyclic Cushing's syndrome: Inconsistent results may require periodic re-evaluation 3