How do you evaluate cortisol levels in a patient suspected of having an endocrine disorder?

Evaluating Cortisol Levels for Suspected Endocrine Disorders

The 1 mg dexamethasone suppression test (DST) taken at 11 PM with serum cortisol measured at 8 AM is the preferred screening test for autonomous cortisol secretion in patients with adrenal incidentalomas, while early morning (8 AM) serum cortisol and ACTH measurements are the first-line tests for suspected adrenal insufficiency. 1

Initial Screening Approach Based on Clinical Scenario

For Suspected Hypercortisolism (Cushing's Syndrome)

Screen all patients with adrenal incidentalomas for autonomous cortisol secretion using the 1 mg DST. 1 The test protocol involves:

• 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 (1.8-5.0 μg/dL): Possible autonomous cortisol secretion—proceed to ancillary testing 1
• >138 nmol/L (>5.0 μg/dL): Evidence of cortisol hypersecretion 1

Ancillary testing for indeterminate or positive results includes:

• 24-hour urinary free cortisol (UFC) for 3 consecutive days (diagnostic cut-off >193 nmol/24h or >70 μg/m²) 1
• Midnight salivary cortisol 1
• Serum cortisol circadian rhythm study at 9 AM, 6 PM, and midnight while sleeping (midnight cortisol ≥50 nmol/L suggests hypercortisolism) 1
• Plasma ACTH to confirm ACTH independency in patients considering intervention 1

For Suspected Adrenal Insufficiency

Obtain early morning (8 AM) measurements of serum cortisol, ACTH, and DHEAS as the first-line diagnostic tests. 2, 3

Critical interpretation thresholds:

Morning cortisol <275 nmol/L (<10 μg/dL):

• With high ACTH (>300 pg/mL): Strongly suggests primary adrenal insufficiency 2, 3, 4
• With low or inappropriately normal ACTH: Suggests secondary adrenal insufficiency 1, 2, 3

Morning cortisol <100 nmol/L (<3.6 μg/dL):

• Proceed directly to ACTH stimulation testing—93.2% positive predictive value for adrenal insufficiency 5

Morning cortisol 100-275 nmol/L (3.6-10 μg/dL):

• Indeterminate zone—requires ACTH stimulation testing for definitive diagnosis 2, 6, 5

Morning cortisol >450 nmol/L (>16.3 μg/dL):

• Rules out adrenal insufficiency with 98.7% negative predictive value—no further testing needed 5

ACTH Stimulation Test (Cosyntropin/Synacthen Test)

When morning cortisol is indeterminate (100-450 nmol/L), perform the standard high-dose ACTH stimulation test. 2, 5

Test protocol:

• Administer 0.25 mg (250 μg) cosyntropin IV or IM 1, 2
• Measure serum cortisol at baseline, 30 minutes, and 60 minutes post-administration 1, 2

Interpretation:

• Peak cortisol <500 nmol/L (<18 μg/dL): Diagnostic of adrenal insufficiency 1, 2, 3
• Peak cortisol >550 nmol/L (>18-20 μg/dL): Normal response, excludes adrenal insufficiency 1, 2, 5

After confirming adrenal insufficiency, measure plasma ACTH to distinguish primary from secondary:

• ACTH >1.1 pmol/L (>5 ng/L) with low cortisol: Primary adrenal insufficiency 1
• ACTH low or inappropriately normal with low cortisol: Secondary adrenal insufficiency 1, 2, 3

Additional Testing for Primary Adrenal Insufficiency

Once primary adrenal insufficiency is confirmed, determine the underlying cause:

• Measure 21-hydroxylase (anti-adrenal) autoantibodies—accounts for ~85% of cases in Western populations 2
• If autoantibodies are negative, obtain adrenal CT imaging to evaluate for hemorrhage, tumor, tuberculosis, or other structural causes 2
• In males with negative autoantibodies, measure very long-chain fatty acids (VLCFA) to screen for adrenoleukodystrophy 2

Critical Pitfalls to Avoid

Never delay treatment of suspected acute adrenal crisis for diagnostic procedures. 1, 2 If a patient presents with unexplained hypotension, collapse, or severe gastrointestinal symptoms:

• Immediately administer IV hydrocortisone 100 mg and 0.9% saline infusion at 1 L/hour 1, 2
• Draw blood for cortisol and ACTH before treatment if possible, but do not delay therapy 1, 2

Do not attempt cortisol testing in patients actively taking exogenous corticosteroids. 2 Therapeutic glucocorticoids suppress the HPA axis and cause iatrogenic secondary adrenal insufficiency, making morning cortisol levels unreliable. 2 Wait until the patient has been weaned off corticosteroids before performing definitive testing. 2

Do not rely solely on electrolyte abnormalities for diagnosis. Hyponatremia is present in 90% of newly diagnosed adrenal insufficiency cases, but hyperkalemia occurs in only ~50% of cases—its absence does not rule out the diagnosis. 2

Recognize that approximately 10% of patients with primary adrenal insufficiency present with normal basal cortisol concentrations. 4 When the clinical history is suggestive of Addison's disease and ACTH is clearly elevated (>300 pg/mL), normal cortisol levels should still raise suspicion for early primary adrenal insufficiency. 4

For patients with pituitary lesions or suspected secondary adrenal insufficiency, remember that TSH is not accurate in central hypothyroidism—target free T4 in the upper half of the reference range instead. 1