What labs should be checked for hypercortisolism (high cortisol levels)?

Laboratory Tests for Diagnosing Hypercortisolism

For patients with suspected hypercortisolism, a comprehensive diagnostic workup should include three first-line screening tests: 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), and the 1 mg overnight dexamethasone suppression test (DST), followed by ACTH measurement to determine the source of excess cortisol. 1, 2

Initial Screening Tests

First-Line Tests

• 24-hour urinary free cortisol (UFC): Collect 2-3 samples to account for variability; values >193 nmol/24h (>70 μg/m²) have 89% sensitivity and 100% specificity for Cushing syndrome 1, 2
• Late-night salivary cortisol (LNSC): Obtain at least 2-3 samples on separate days; offers 95% sensitivity and 100% specificity 2, 3
• 1 mg overnight dexamethasone suppression test (DST): Dexamethasone taken at 11 PM with serum cortisol measured at 8 AM; failure to suppress cortisol to <50 nmol/L (<1.8 μg/dL) suggests hypercortisolism 1, 2

Interpretation of First-Line Tests

• A positive result on any one test warrants further investigation 2
• UFC values within 3-fold of the upper limit of normal may indicate pseudo-Cushing states rather than true Cushing syndrome 1, 4
• In children and adolescents, unexplained weight gain combined with decreased growth velocity strongly suggests true Cushing syndrome rather than simple obesity 1, 4

Determining the Source of Hypercortisolism

After confirming hypercortisolism, the next step is to determine whether it is ACTH-dependent or ACTH-independent:

• Morning plasma ACTH level: Essential to differentiate causes 1, 2

  • Normal/elevated ACTH (>1.1 pmol/L or >5 ng/L): Suggests ACTH-dependent Cushing syndrome (pituitary or ectopic source) 1, 2
  • Low/undetectable ACTH: Indicates ACTH-independent Cushing syndrome (adrenal source) 2

• CRH stimulation test: For ACTH-dependent cases 1, 2

  • ≥20% increase in cortisol from baseline supports pituitary origin 1, 2

Additional Tests for Equivocal Cases

• Bilateral inferior petrosal sinus sampling (BIPSS): For ACTH-dependent cases with no identified adenoma on pituitary MRI 1

  • Central-to-peripheral ACTH ratio ≥2:1 before CRH/desmopressin and ≥3:1 after stimulation confirms pituitary source 1
  • Should only be performed in specialized centers by experienced interventional radiologists 1

• Desmopressin test: Can differentiate Cushing disease from pseudo-Cushing states 1, 4

  • Has high specificity for Cushing disease and is less complex than the Dex-CRH test 1

Special Considerations

• Pseudo-Cushing states: Psychiatric disorders, alcoholism, obesity, and polycystic ovary syndrome can activate the HPA axis and cause mildly elevated cortisol levels 1, 4

• Laboratory methods: LC-MS/MS offers improved specificity and sensitivity compared to immunoassays for measuring cortisol in serum, urine, and saliva 5, 6

• Renal impairment: For patients with renal impairment (CrCl <60mL/min) or significant polyuria, LNSC may be preferred over UFC 1

• Medications: Oral estrogen therapy may affect DST results; measuring dexamethasone levels may help identify false positives 1

Common Pitfalls

• Do not use BIPSS to diagnose hypercortisolism; it should only be used to determine the source after confirming the diagnosis 1

• A single normal test does not exclude Cushing syndrome, especially in mild or cyclic cases; multiple tests are often needed 2, 3

• UFC measurement alone has limitations due to collection errors, assay variability, and the influence of renal function 6, 7

• Ensure patients are not taking exogenous glucocorticoids before testing, as these can cause false results 2