ACTH Stimulation Testing in Myasthenia Gravis Patients on Chronic Steroids
Yes, perform an ACTH stimulation test in this patient—the low ACTH (6) with low cortisol (4.4 mcg/dL) indicates secondary (central) adrenal insufficiency from chronic steroid use for myasthenia gravis, and stimulation testing is specifically recommended when morning cortisol falls in the indeterminate range between 3-15 mcg/dL to confirm the diagnosis and guide replacement therapy. 1
Understanding the Clinical Context
Your patient has secondary adrenal insufficiency, not primary adrenal insufficiency. The key distinguishing feature is the relationship between ACTH and cortisol:
- Low ACTH with low cortisol = secondary (central) adrenal insufficiency 1
- High ACTH with low cortisol = primary adrenal insufficiency 1
This patient's chronic steroid use for myasthenia gravis has suppressed their hypothalamic-pituitary-adrenal (HPA) axis, causing isolated central adrenal insufficiency. 1
When to Perform ACTH Stimulation Testing
Standard-dose ACTH stimulation testing (250 mcg cosyntropin) is specifically indicated when:
- Morning cortisol is >3 mcg/dL AND <15 mcg/dL (indeterminate range) 1
- Your patient's cortisol of 4.4 mcg/dL falls directly in this indeterminate zone
- A peak stimulated cortisol <18-20 mcg/dL (500 nmol/L) confirms adrenal insufficiency 1
Do NOT perform stimulation testing if:
- Morning cortisol <3 mcg/dL with low ACTH = diagnosis already confirmed 1
- Patient is currently on high-dose corticosteroids for other conditions = testing unreliable until steroids discontinued 1
Critical Timing Considerations
Before testing, ensure proper steroid withdrawal:
- Hydrocortisone must be held for 24 hours before testing 1
- Other steroids (prednisone, dexamethasone) require longer washout 1
- The cortisol assay may cross-react with therapeutic steroids, giving falsely elevated results 1
Important caveat: ACTH stimulation can give false-negative results early in hypophysitis or recent steroid withdrawal, as adrenal reserve declines slowly after pituitary stimulation is lost. In clinical uncertainty, opt for replacement therapy and retest for ongoing need at 3 months. 1
Management Algorithm Based on Results
If Stimulation Test Confirms Adrenal Insufficiency (Peak Cortisol <500 nmol/L or <18 mcg/dL):
Initiate physiologic glucocorticoid replacement: 1
- Hydrocortisone 15-20 mg daily in divided doses (preferred for recreating diurnal rhythm)
- Alternative: Prednisone 5 mg daily (equivalent to hydrocortisone 20 mg), though carries higher risk of over-replacement 1
Do NOT give fludrocortisone in secondary adrenal insufficiency—mineralocorticoid production remains intact because the renin-angiotensin-aldosterone system is independent of ACTH 1
Critical Patient Education Requirements:
All patients with confirmed adrenal insufficiency require: 1
- Stress-dose steroid protocol for illness/surgery
- Emergency injectable hydrocortisone supplies for self-administration
- Medical alert bracelet or necklace identifying adrenal insufficiency
- Instructions on when to seek emergency care for impending adrenal crisis
Special Considerations for Myasthenia Gravis
Avoid medications that worsen myasthenia gravis: 1, 2
- β-blockers, IV magnesium
- Fluoroquinolones, aminoglycosides, macrolides
- Anticholinergic medications 3
Coordinate with neurology before any surgical procedures or high-stress treatments, as stress-dose steroids will be needed 1
Monitor for myasthenia exacerbation when adjusting steroid doses—abrupt steroid reduction can precipitate myasthenic crisis 2, 4
Retesting Strategy
Plan HPA axis recovery assessment at 3 months if this is steroid-induced suppression without other pituitary hormone deficiencies. 1 Consider endocrinology consultation for recovery and weaning protocols using hydrocortisone in patients with symptoms of adrenal insufficiency after attempting to wean off corticosteroids. 1