How should cortisol be evaluated in a patient with suspected Addison disease?

How to Check Cortisol in Addison Disease

For suspected Addison disease (primary adrenal insufficiency), perform a morning (8 AM) cortisol paired with plasma ACTH, followed by a high-dose (250 μg) cosyntropin stimulation test if initial results are equivocal. 1

Diagnostic Algorithm

Step 1: Morning Cortisol and ACTH (First-Line)

• Draw at 8 AM: Obtain simultaneous morning cortisol and plasma ACTH levels 1
• Diagnostic for Addison disease:
  • Cortisol <140 nmol/L (<5 μg/dL) **AND** ACTH >2× upper limit of normal (typically >300 ng/L or 66 pmol/L) 1
  • This combination is highly predictive and confirms the diagnosis without further testing 1

Interpretation thresholds for morning cortisol alone:

• >332 nmol/L (>11.1 μg/dL): Rules out adrenal insufficiency with high specificity (93-100%), no further testing needed 2, 3
• <98-140 nmol/L (<3.5-5 μg/dL): Highly suggestive of adrenal insufficiency 1, 4
• Between these values: Proceed to cosyntropin stimulation test 1

Step 2: Cosyntropin Stimulation Test (When Needed)

The high-dose (250 μg) cosyntropin test is the gold standard for confirming adrenal insufficiency 1, 5

Protocol:

• Administer 250 μg cosyntropin (Synacthen) IV or IM
• Measure cortisol at baseline and 30 minutes (or 60 minutes)
• Normal response: Peak cortisol ≥500-550 nmol/L (≥18 μg/dL) using traditional cutoffs, or ≥14.6 μg/dL with newer generation immunoassays 1, 2
• Abnormal response: Peak cortisol below these thresholds confirms adrenal insufficiency

Important nuances:

• The 30-minute time point has been validated against insulin tolerance test 1
• Newer cortisol immunoassays with improved specificity may allow lower cutoffs (≥14.6 μg/dL), reducing false positives by 17% 2
• Low-dose (1 μg) cosyntropin test offers no additional sensitivity or specificity for primary adrenal insufficiency over high-dose test 1

Step 3: Confirm Primary vs Secondary Etiology

Measure plasma ACTH to distinguish primary from secondary adrenal insufficiency 1

• Primary AI (Addison disease): ACTH >2× upper limit of normal with low cortisol 1
• Secondary AI: ACTH normal-to-low with low cortisol 6, 7
• Early Addison disease: Elevated ACTH with cortisol still in normal range can be the first sign 1

Step 4: Assess Mineralocorticoid Status

Simultaneously measure plasma renin and aldosterone in all suspected primary adrenal insufficiency cases 1

• Diagnostic pattern: Elevated plasma renin activity/concentration with inappropriately normal or low aldosterone 1, 6
• Clinical significance: Mineralocorticoid deficiency may predominate in early PAI and can be the only initial sign 1
• This distinguishes primary from secondary AI (mineralocorticoid function preserved in secondary) 1

Critical Pitfalls to Avoid

1. ACTH assay variability: Significant analytical bias exists between ACTH assays; interpret based on your laboratory's specific reference ranges 1

2. Timing matters: Always draw morning (8 AM) samples when possible, as cortisol has diurnal variation (though this rhythm is absent in infants <4 months) 7

3. Don't delay treatment: If clinical suspicion is high (hypotension, hyponatremia, hyperkalemia, hyperpigmentation), start glucocorticoid therapy immediately and obtain baseline cortisol/ACTH before first dose—do not wait for stimulation test results 7, 8

4. Beware of "normal" cortisol in early disease: An elevated ACTH with cortisol in the normal range may represent evolving Addison disease 1

5. Assay-specific cutoffs: Be aware of which cortisol immunoassay your laboratory uses, as newer generation assays have different diagnostic thresholds 2