Diagnostic Approach to Adrenal Insufficiency Using ACTH Stimulation Tests

Limitations of the 250 µg Cosyntropin Test

• The 250 µg cosyntropin stimulation test is validated only for diagnosing primary (peripheral) adrenal insufficiency and cannot reliably differentiate primary from secondary (central) adrenal insufficiency because supraphysiologic ACTH can elicit a normal cortisol response even when the hypothalamic‑pituitary axis is deficient. [1][2]
• In secondary adrenal insufficiency the adrenal cortex remains structurally intact; a large 250 µg bolus can temporarily overcome chronic ACTH deficiency and produce a falsely reassuring cortisol peak, especially early in the disease before adrenal atrophy develops. [1][2]
• Relying on the 250 µg test alone to distinguish primary from secondary disease is discouraged; simultaneous measurement of basal plasma ACTH is required. [1][2]

Preferred Initial Evaluation (Basal Hormones)

• The most reliable method to separate primary from secondary adrenal insufficiency is simultaneous measurement of early‑morning cortisol and plasma ACTH. [1][2]
• Primary adrenal insufficiency is highly likely when morning cortisol is <140 nmol/L (<5 µg/dL) and ACTH is >2 × the upper limit of normal (approximately >66 pmol/L). [1][2]
• Secondary adrenal insufficiency is suggested when morning cortisol is <140 nmol/L but ACTH is low or inappropriately normal (not elevated). [1][2]
• DHEAS concentrations are markedly reduced in primary adrenal insufficiency but are not useful for diagnosing secondary forms and should not be used in isolation. [1][2]

Role of the Low‑Dose (1 µg) Cosyntropin Test

• Current clinical guidelines do not recommend the 1 µg cosyntropin test because it does not improve overall diagnostic accuracy, lacks a standardized commercial preparation, and has no consensus on cutoff values. [1][2] (Guideline strength: moderate recommendation)
• A 2016 meta‑analysis found no statistically significant difference in diagnostic performance between high‑dose (250 µg) and low‑dose (1 µg) ACTH stimulation tests for detecting secondary adrenal insufficiency. 2 (Evidence level: meta‑analysis, moderate quality)

Confirmatory Testing with 250 µg Cosyntropin

• The 250 µg cosyntropin test should be performed only to confirm primary adrenal insufficiency when basal cortisol/ACTH results are equivocal. [1][2] (Guideline strength: strong recommendation)
• Procedure: administer 250 µg cosyntropin intravenously or intramuscularly and obtain serum cortisol at baseline and at 30 minutes (or 60 minutes). 1
• The test can be carried out at any time of day because results are not affected by the normal diurnal cortisol rhythm. 1
• Interpretation: an abnormal (sub‑threshold) cortisol response plus an elevated basal ACTH confirms primary adrenal insufficiency; an abnormal response plus a low/normal basal ACTH suggests secondary adrenal insufficiency, recognizing that early secondary disease may still yield a normal response. [1][2]

Special Populations: Patients Tapering Exogenous Glucocorticoids

• Routine ACTH stimulation testing is not recommended for patients who are tapering glucocorticoids; morning cortisol alone is sufficient for most management decisions once physiologic replacement doses are reached. 3 (Guideline strength: strong recommendation)
• A morning cortisol > 300 nmol/L (>10 µg/dL) indicates recovery of the hypothalamic‑pituitary‑adrenal axis, allowing safe discontinuation of glucocorticoid therapy. 3
• A morning cortisol < 150 nmol/L (<5 µg/dL) signals persistent adrenal insufficiency; physiologic glucocorticoid replacement should be continued and the assessment repeated after several months. 3
• In this context, the 250 µg ACTH test has only moderate diagnostic accuracy and does not correlate with clinically important outcomes such as adrenal crisis. 3

Clinical Pitfalls to Avoid

• Do not rely on the 250 µg cosyntropin test alone for differentiating primary versus secondary adrenal insufficiency; always obtain a basal ACTH measurement concurrently. [1][2]
• In patients with suspected adrenal crisis, do not delay treatment for ACTH stimulation testing; obtain cortisol and ACTH if feasible, but administer 100 mg hydrocortisone intravenously immediately. 1
• Avoid performing ACTH stimulation tests in individuals who are still receiving supraphysiologic glucocorticoid doses, as the results are uninterpretable. 3

Diagnostic Workup for Suspected Adrenal Insufficiency

Initial Laboratory Evaluation

• In patients with suspected adrenal insufficiency, an early‑morning serum cortisol < 140 nmol/L (≈5 µg/dL) strongly suggests the disorder, especially when combined with a markedly elevated plasma ACTH (>2‑fold above the upper limit of normal, e.g., > 66 pmol/L) indicating a primary etiology, whereas low or low‑normal ACTH points toward secondary or glucocorticoid‑induced causes【4】.
• A markedly reduced DHEAS level—well below age‑ and sex‑adjusted lower limits—provides an additional early clue for primary adrenal insufficiency, although it cannot be used in isolation【4】.

Interpretation of Basal Results

• The combination of morning cortisol < 140 nmol/L and ACTH > 2‑fold above normal defines a “primary adrenal insufficiency” pattern, prompting confirmatory dynamic testing【4】.

Confirmatory Dynamic Testing

• The standard‑dose (250 µg) cosyntropin (synthetic ACTH) stimulation test is the diagnostic gold standard for confirming primary adrenal insufficiency【4】.
• Protocol: administer 250 µg cosyntropin intravenously or intramuscularly, then measure serum cortisol at baseline and at 30 minutes (or 60 minutes) post‑injection【4】.
• The test may be performed at any time of day because results are not significantly affected by diurnal cortisol variation【4】.
• A peak stimulated cortisol < 500 nmol/L confirms adrenal insufficiency; assay‑specific lower cut‑offs (e.g., ≈ 414 nmol/L for certain platforms) have been proposed but are not universally adopted【4】.
• The low‑dose (1 µg) cosyntropin test does not provide superior diagnostic accuracy and is not recommended for routine evaluation【4】.

Special Considerations in Acutely Ill Patients

• In critically ill patients with a strong clinical suspicion of adrenal insufficiency, glucocorticoid therapy should be initiated promptly without awaiting test results; blood for cortisol and ACTH should be drawn before treatment when feasible【4】.

Guidelines for Performing the ACTH (Cosyntropin) Stimulation Test

Test Protocol

• Administer 250 µg cosyntropin as a single IV or IM bolus and measure serum cortisol at baseline and again at 30 min (or 60 min) after injection. This is the validated standard dose that maximally stimulates the adrenal cortex and provides high diagnostic accuracy. 5
• Draw the baseline cortisol sample immediately before cosyntropin injection to establish the pre‑stimulus level. 5, 6
• Obtain a post‑stimulation cortisol sample at 30 minutes after the dose; this time point captures the typical peak response. 5, 6
• A 60‑minute cortisol measurement may be performed instead of, or in addition to, the 30‑minute sample to accommodate patients whose cortisol peaks later. 5, 6
• The test can be scheduled at any time of day because diurnal cortisol variation does not significantly affect the stimulated result. 5, 6

Interpretation of Results

• Peak stimulated cortisol > 500 nmol/L (≈18 µg/dL) traditionally indicates sufficient adrenal reserve. This threshold is based on historic assay performance. 5
• Baseline plasma ACTH should be measured concurrently with baseline cortisol to allow differentiation between primary and secondary adrenal insufficiency. 5
• Primary adrenal insufficiency is diagnosed when cortisol is low (<140 nmol/L or <5 µg/dL) and ACTH is >2 × the upper limit of normal (≈66 pmol/L). This pattern reflects adrenal failure with compensatory ACTH elevation. 5, 6

Special Populations & Acute Situations

• In suspected adrenal crisis, do not postpone treatment to perform the test. Immediate administration of 100 mg hydrocortisone IV is recommended. 5
• If feasible, obtain baseline cortisol and ACTH before giving hydrocortisone, but therapy should not be delayed for laboratory results. 5
• The ACTH stimulation test may be performed after emergency glucocorticoid therapy has begun, provided glucocorticoids are temporarily withheld for the sampling period. 5

Low‑Dose (1 µg) Cosyntropin Test

• The 1 µg cosyntropin test is not recommended for routine clinical use. Evidence from meta‑analysis shows no statistically significant improvement in diagnostic performance compared with the standard 250 µg test. 5, 6
• Limitations of the low‑dose test include lack of standardized commercial preparation, absence of consensus cut‑off values, and no superiority in accuracy over the standard dose. 5

Common Pitfalls to Avoid

• Never rely solely on the 250 µg test to distinguish primary from secondary adrenal insufficiency; baseline ACTH measurement is essential. 5
• Do not apply a universal 18 µg/dL cutoff; use assay‑specific reference ranges to prevent misclassification. 5
• Avoid performing the test in patients receiving supraphysiologic glucocorticoid doses, as results become uninterpretable. 5
• Do not delay emergency treatment for adrenal crisis to complete testing; immediate glucocorticoid therapy takes precedence. 5