What laboratory tests are recommended to diagnose adrenal insufficiency?

Laboratory Tests for Diagnosing Adrenal Insufficiency

Obtain early morning (8 AM) serum cortisol and plasma ACTH simultaneously as your first-line diagnostic tests, and if these results are indeterminate (cortisol 5-18 μg/dL or 140-500 nmol/L), proceed immediately to a cosyntropin stimulation test to definitively confirm or exclude the diagnosis. 1, 2

Initial Diagnostic Laboratory Panel

Draw these tests before 8 AM whenever possible:

• Serum cortisol – A morning cortisol <250 nmol/L (<9 μg/dL or <5 μg/dL depending on assay) with elevated ACTH is diagnostic of primary adrenal insufficiency without need for further testing 3, 1, 4, 2
• Plasma ACTH – Markedly elevated ACTH (>300 pg/mL) with low cortisol confirms primary adrenal insufficiency; low or inappropriately normal ACTH with low cortisol indicates secondary adrenal insufficiency 3, 1, 5, 2
• Basic metabolic panel (sodium, potassium, CO2, glucose) – Hyponatremia occurs in 90% of cases; hyperkalemia is present in only ~50% of primary adrenal insufficiency cases, so its absence does not exclude the diagnosis 1, 4, 5
• Serum creatinine and BUN – Elevated due to prerenal renal failure from volume depletion 4, 5
• Serum glucose – Hypoglycemia may occur, particularly in children 4, 5

Critical interpretation thresholds:

• Morning cortisol <250 nmol/L (<9 μg/dL) with high ACTH = primary adrenal insufficiency confirmed 1, 4
• Morning cortisol <400 nmol/L (<14 μg/dL) with high ACTH in acute illness = strong suspicion of primary adrenal insufficiency 3, 1
• Morning cortisol 140-275 nmol/L (5-10 μg/dL) with low/normal ACTH = secondary adrenal insufficiency likely 1, 2
• Morning cortisol >550 nmol/L (>18-20 μg/dL) = adrenal insufficiency excluded 3, 1

Cosyntropin (Synacthen) Stimulation Test

When to perform: Use this test when morning cortisol is indeterminate (between 5-18 μg/dL or 140-500 nmol/L) or when clinical suspicion remains high despite borderline results 3, 1, 2, 6

Standard protocol:

• Administer 0.25 mg (250 mcg) cosyntropin intramuscularly or intravenously 3, 1
• Measure serum cortisol at baseline (before injection), then at exactly 30 minutes and/or 60 minutes post-administration 3, 1
• Peak cortisol <500 nmol/L (<18 μg/dL) at either 30 or 60 minutes = adrenal insufficiency confirmed 3, 1
• Peak cortisol >550 nmol/L (>18-20 μg/dL) = adrenal insufficiency excluded 3, 1

Important caveats about the cosyntropin test:

• The high-dose (250 mcg) test is preferred over low-dose (1 mcg) due to easier administration, comparable accuracy, and FDA approval 1
• This test can miss early secondary adrenal insufficiency because the supraphysiologic ACTH dose may still stimulate partially atrophied adrenals 7, 8
• Exogenous steroids (prednisone, prednisolone, inhaled fluticasone) suppress the HPA axis and cause false results—hold hydrocortisone for 24 hours before testing, but other steroids require longer washout 1, 7

Etiologic Workup (After Confirming Diagnosis)

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

For Primary Adrenal Insufficiency:

• 21-hydroxylase autoantibodies (21OH-Ab) – Positive in ~85% of autoimmune Addison's disease in Western populations; if positive, no further etiologic testing is needed 3, 1, 4
• CT scan of the adrenals – Obtain if 21OH-Ab is negative to evaluate for hemorrhage, tumor, tuberculosis, or infiltrative disease 3, 1, 4
• Very long-chain fatty acids (VLCFA) – Test in males with negative antibodies to screen for adrenoleukodystrophy 3, 1, 4
• Plasma renin activity – Elevated in primary adrenal insufficiency due to mineralocorticoid deficiency 4
• Serum aldosterone – Low in primary adrenal insufficiency 4

For Secondary Adrenal Insufficiency:

• Pituitary MRI – Evaluate for pituitary tumors, hemorrhage, infiltrative disease, or hypophysitis 1, 2
• Other pituitary hormone testing – TSH, free T4, LH, FSH, prolactin, IGF-1 to assess for additional pituitary deficiencies 1

Critical Pitfalls to Avoid

Never delay treatment in suspected acute adrenal crisis to perform diagnostic testing – If a patient presents with unexplained hypotension, collapse, severe vomiting, or altered mental status, give IV hydrocortisone 100 mg immediately and start 0.9% saline infusion at 1 L/hour, but draw blood for cortisol and ACTH before treatment if possible 3, 1, 4, 5

Do not rely on electrolyte abnormalities alone – Hyperkalemia is absent in 50% of primary adrenal insufficiency cases and is never present in secondary adrenal insufficiency because aldosterone secretion is preserved 1, 4, 5

Recognize that approximately 10% of patients with primary adrenal insufficiency present with normal basal cortisol concentrations – If ACTH is clearly elevated (>300 pg/mL) and clinical history is suggestive, proceed with cosyntropin testing even if morning cortisol appears "normal" 9

Do not attempt diagnostic testing in patients actively taking corticosteroids – Morning cortisol measurements will be falsely low due to iatrogenic secondary adrenal insufficiency; wait until corticosteroids are discontinued with adequate washout time before testing 1, 7, 8

Use dexamethasone 4 mg IV instead of hydrocortisone if you need to treat suspected adrenal crisis but still want to perform diagnostic testing later – Dexamethasone does not interfere with cortisol assays, allowing subsequent testing 1

Special Considerations for Hyponatremia

Adrenal insufficiency must be excluded before diagnosing SIADH – Both conditions present with euvolemic hypo-osmolar hyponatremia, inappropriately high urine osmolality, and elevated urinary sodium; the cosyntropin stimulation test is medically necessary to distinguish them 1