What is the proper way to check cortisol levels?

Proper Methods for Checking Cortisol Levels

The proper way to check cortisol levels depends on the clinical context, with the overnight 1-mg dexamethasone suppression test (DST) being the preferred screening test for identifying autonomous cortisol secretion when evaluating for hypercortisolism. 1

First-Line Testing Options

• Overnight 1-mg Dexamethasone Suppression Test (DST):

  • Administer 1 mg dexamethasone at 11 PM, measure serum cortisol at 8 AM the following morning 1
  • Interpretation:
    • <50 nmol/L (<1.8 μg/dL) excludes cortisol hypersecretion 1
    • 51-138 nmol/L suggests possible autonomous cortisol secretion 1

    • 138 nmol/L indicates evidence of cortisol hypersecretion 1

  • Highest sensitivity among cortisol tests (>90%) 1

• Late Night Salivary Cortisol (LNSC):

  • Based on the principle that patients with Cushing's syndrome lose normal circadian nadir of cortisol secretion 1
  • Collect at least 2-3 samples at usual bedtime 1
  • Highly specific test for diagnosing hypercortisolism 1
  • Should NOT be performed in patients with disrupted day/night cycles (e.g., night-shift workers) 1, 2

• 24-hour Urinary Free Cortisol (UFC):

  • Measures increased bioavailable cortisol 1
  • Lower sensitivity compared to DST and LNSC 1
  • Useful as an ancillary test when DST or LNSC results are equivocal 1

Testing Algorithm

1. For suspected hypercortisolism/Cushing's syndrome:

   • Start with overnight 1-mg DST as the preferred screening test 1
   • Consider adding LNSC and/or 24-hour UFC as complementary tests 1
   • For cyclic Cushing's syndrome, multiple sequential LNSC tests are particularly useful 1

2. For suspected adrenal insufficiency:

   • Morning serum cortisol (8 AM-12 PM) can be used as initial screening 3, 4
   • Thresholds vary by assay: values ≥336-506 nmol/L (depending on assay) may predict adequate adrenal reserve 5
   • Values <275 nmol/L warrant further dynamic testing 3

Special Considerations

• Potential false positives with DST:

  • Medications affecting dexamethasone metabolism (CYP3A4 inducers like phenobarbital, carbamazepine) 1, 6
  • Increased corticosteroid binding globulin from oral estrogens or pregnancy 1
  • Consider measuring dexamethasone levels concurrently with cortisol to reduce false positives 1

• Shift workers and disrupted circadian rhythms:

  • Avoid LNSC testing in these populations 1, 2
  • DST is the preferred diagnostic test for evaluating cortisol status in shift workers 2

• Laboratory methods:

  • Mass spectrometry methods (LC-MS/MS) offer higher specificity than immunoassays 7
  • Cortisol assays vary significantly between platforms; reference ranges are assay-specific 5

Confirmatory Testing

• For equivocal results, consider the 2-day low-dose dexamethasone suppression test (LDDST) 6:
  • Administer 0.5 mg dexamethasone every 6 hours for 48 hours 6
  • Measure cortisol at 0,24, and 48 hours 6
  • Cortisol suppression below 50 nmol/L (1.8 μg/dL) is considered normal 6

By following this structured approach to cortisol testing, clinicians can effectively screen for and diagnose disorders of the hypothalamic-pituitary-adrenal axis while minimizing unnecessary testing.