When should serum adrenocorticotropic hormone (ACTH) be measured in a 21‑year‑old with persistent fatigue after infectious mononucleosis?

Timing of ACTH Measurement

Measure serum ACTH simultaneously with an 8 AM cortisol level when initially evaluating for adrenal insufficiency in this patient with persistent post-infectious fatigue.

Diagnostic Approach

The Endocrine Society guidelines establish that ACTH measurement should be obtained as part of the initial diagnostic workup when adrenal insufficiency is suspected 1. The key is to measure ACTH and cortisol together at baseline, ideally in the morning (8 AM), to establish the diagnosis before any confirmatory testing.

Initial Testing Strategy

• Draw baseline ACTH and cortisol simultaneously at 8 AM - this timing captures the physiologic peak of cortisol secretion and provides the most interpretable results 2
• Morning cortisol <140 nmol/L (5 μg/dL) combined with ACTH >2-fold above the upper limit of normal strongly suggests primary adrenal insufficiency 1
• ACTH >66 pmol/L represents maximal stimulus for cortisol secretion 1
• Low cortisol with normal-to-low ACTH indicates secondary (pituitary) adrenal insufficiency 2

Clinical Context for This Patient

Post-viral fatigue following infectious mononucleosis warrants consideration of adrenal insufficiency, particularly secondary adrenal insufficiency. Recent evidence demonstrates that viral infections, including COVID-19, can cause isolated ACTH deficiency presenting with persistent fatigue 3, 4. In one study, 27% of patients with severe viral pneumonia had hypocortisolism, with 91% having secondary (pituitary) causes 3.

The critical pitfall here is that secondary adrenal insufficiency may show normal ACTH stimulation test results despite true insufficiency 2. Therefore, if baseline ACTH and cortisol suggest secondary adrenal insufficiency but the standard cosyntropin test is normal, consider insulin hypoglycemia testing or metyrapone testing for confirmation 2.

Confirmatory Testing Algorithm

1. If baseline results are equivocal: Proceed to standard-dose (250 μg) cosyntropin stimulation test 1
2. The cosyntropin test is NOT affected by time of day - it can be performed at any time without time constraints 1
3. Measure cortisol at 0,30, and 60 minutes after cosyntropin administration
4. Peak cortisol response determines adequacy of adrenal function

Important Caveats

• Never delay treatment in acutely ill patients - if the patient develops hypotension, hyponatremia, or other concerning signs, draw baseline ACTH and cortisol immediately, then start IV hydrocortisone without waiting for results 1
• The low-dose (1 μg) cosyntropin test offers no diagnostic advantage over the standard 250 μg dose 1
• If the patient were on glucocorticoids, ACTH testing can still be performed within the first 72 hours of high-dose therapy 5

For this 21-year-old with persistent post-infectious fatigue, obtain simultaneous 8 AM ACTH and cortisol levels as the initial diagnostic step, recognizing that post-viral secondary adrenal insufficiency is an emerging clinical entity that may explain prolonged symptoms.