What does a low‑normal adrenocorticotropic hormone (ACTH) level of 5.7 and a low‑normal morning cortisol of 8.6 µg/dL indicate, and what is the appropriate next step in evaluation?

Interpretation and Next Steps for Low-Normal ACTH and Morning Cortisol

Your patient's ACTH of 5.7 pg/mL with a morning cortisol of 8.6 µg/dL (approximately 237 nmol/L) represents an indeterminate result that requires a cosyntropin stimulation test to definitively rule in or rule out adrenal insufficiency. 1

Why These Values Are Indeterminate

• A morning cortisol of 8.6 µg/dL falls squarely in the "grey zone" where neither adrenal insufficiency nor normal function can be confidently diagnosed. 2, 1
• Morning cortisol >18–20 µg/dL (>500–550 nmol/L) would exclude adrenal insufficiency, while <9 µg/dL (<250 nmol/L) with markedly elevated ACTH (>300 pg/mL) would confirm primary adrenal insufficiency. 2
• Your patient's low-normal ACTH with borderline-low cortisol suggests possible secondary adrenal insufficiency (central ACTH deficiency), but this pattern alone is not diagnostic—it requires confirmatory dynamic testing. 1
• Low ACTH with normal-range cortisol cannot distinguish between adequate adrenal reserve and partial ACTH deficiency without stimulation testing. 1

Recommended Diagnostic Algorithm

Step 1: Perform High-Dose Cosyntropin Stimulation Test

• Administer 0.25 mg (250 µg) cosyntropin intravenously or intramuscularly. 2, 1
• Measure serum cortisol at baseline, 30 minutes, and 60 minutes post-administration. 2
• Interpretation thresholds:
  • Peak cortisol <18 µg/dL (<500 nmol/L) at 30 or 60 minutes confirms adrenal insufficiency. 2, 1
  • Peak cortisol >18–20 µg/dL (>500–550 nmol/L) excludes adrenal insufficiency. 2, 1
• The high-dose test is preferred over low-dose (1 µg) testing due to easier administration, comparable diagnostic accuracy, and FDA approval. 2

Step 2: Evaluate for Underlying Etiology if Adrenal Insufficiency Confirmed

If secondary adrenal insufficiency is confirmed (low ACTH, subnormal cortisol response):

• Assess other pituitary hormones to identify hypopituitarism: TSH, free T4, LH, FSH, testosterone (males) or estradiol (premenopausal females), and IGF-1. 1, 3
• Obtain MRI of the brain with pituitary/sellar cuts if multiple endocrine deficiencies are present, or if the patient has new severe headaches or vision changes. 3
• Review medication history for exogenous corticosteroids (prednisone, inhaled fluticasone), opioids, or immune checkpoint inhibitors that cause iatrogenic secondary adrenal insufficiency. 3

Step 3: Initiate Treatment if Confirmed

If cosyntropin test confirms adrenal insufficiency:

• Start hydrocortisone 10–20 mg orally in the morning and 5–10 mg in early afternoon to mimic physiological cortisol rhythm. 3
• Critical: If concurrent central hypothyroidism exists, start corticosteroids several days before initiating levothyroxine to prevent precipitating adrenal crisis. 1, 3
• Provide mandatory patient education on stress dosing (double or triple dose during illness), prescribe an emergency injectable hydrocortisone 100 mg IM kit with self-injection training, and ensure the patient wears a medical alert bracelet. 3

Important Clinical Caveats

Do Not Delay Treatment if Acute Crisis Suspected

• If your patient develops unexplained hypotension, collapse, severe vomiting, altered mental status, or inability to take oral medications, immediately administer IV hydrocortisone 100 mg bolus plus 0.9% saline infusion at 1 L/hour without waiting for test results. 2, 1, 3
• Draw blood for cortisol and ACTH before steroid administration if feasible, but never delay treatment for diagnostic procedures. 2, 3

Pitfalls to Avoid

• Do not rely on electrolyte abnormalities to make or exclude the diagnosis—hyponatremia occurs in 90% of adrenal insufficiency cases, but hyperkalemia is present in only ~50% and is typically absent in secondary adrenal insufficiency. 2, 3
• Do not diagnose SIADH in a patient with hyponatremia without first excluding adrenal insufficiency, as both conditions present with euvolemic hypo-osmolar hyponatremia and similar laboratory findings. 2, 1
• Avoid checking morning cortisol in patients actively taking corticosteroids—the assay measures both endogenous cortisol and therapeutic steroids, yielding uninterpretable results. 2

Alternative Diagnostic Thresholds from Recent Research

• A 2020 study found that a morning cortisol >234 nmol/L (8.5 µg/dL) predicted a normal ACTH stimulation test response with 80.6% sensitivity and 91.4% specificity. 4
• Your patient's value of 8.6 µg/dL sits just above this threshold, but guideline-based dynamic testing remains mandatory to definitively exclude adrenal insufficiency given the clinical context. 2, 1
• A 2017 study demonstrated that basal cortisol ≥450 nmol/L (16.3 µg/dL) had a 98.7% negative predictive value to rule out adrenal insufficiency, while ≤100 nmol/L (3.6 µg/dL) had a 93.2% positive predictive value to rule it in—your patient falls between these extremes. 5