Low cortisol and low acth what's the next step
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secondary adrenal insufficiency diagnosis workup
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Here is a complete breakdown of the workup when you find low cortisol + low (or low-normal) ACTH:
Low Cortisol + Low ACTH: Diagnostic Approach
This combination localizes the problem to the pituitary (secondary) or hypothalamus (tertiary) - NOT the adrenal gland itself. The adrenal gland is intact but under-stimulated.
Step 1 - Confirm the Pattern is Real
First, rule out the most common cause before expensive imaging:
- Exogenous glucocorticoid use (any steroid in the past 4+ weeks) suppresses CRH - ACTH - cortisol axis. This is the #1 cause of low ACTH + low cortisol.
- Review all medications, including inhaled, topical, and intra-articular steroids.
Step 2 - Cosyntropin (ACTH) Stimulation Test (CST)
This confirms adrenal insufficiency and helps characterize it:
- Administer 250 mcg cosyntropin (synthetic ACTH) IV or IM
- Measure cortisol at 60 minutes
- Abnormal = cortisol < 18 mcg/dL - confirms secondary adrenal insufficiency
Important caveat: In recent-onset secondary AI (e.g., after acute pituitary event), the adrenal glands have not yet atrophied. Exogenous ACTH can still stimulate them, giving a false-normal CST. If recent pituitary surgery or infarction is suspected, go straight to endocrine consultation.
Step 3 - Pituitary MRI (Most Important Next Step)
Once secondary/tertiary AI is confirmed:
"Low ACTH (<5 pg/mL) and cortisol levels suggest secondary or tertiary adrenal insufficiency, and pituitary CT or MRI is indicated."
- Textbook of Family Medicine 9e
Look for:
| Cause | Notes |
|---|---|
| Pituitary adenoma | Most common structural cause |
| Craniopharyngioma | Especially in younger patients |
| Metastatic tumor | Lung, breast most common |
| Sheehan syndrome | Postpartum pituitary necrosis |
| Lymphocytic hypophysitis | Autoimmune; can be postpartum |
| Infiltrative disease | Sarcoidosis, TB, histoplasmosis |
| Head trauma | History of TBI |
Step 4 - CRH Stimulation Test (to distinguish Secondary vs. Tertiary)
If needed to differentiate pituitary from hypothalamic cause:
- Secondary AI (pituitary failure): little or no rise in ACTH or cortisol after CRH
- Tertiary AI (hypothalamic failure): ACTH rises in an exaggerated, prolonged fashion (the pituitary is intact but un-stimulated)
Step 5 - Insulin Tolerance Test (ITT) - if CST is falsely normal
For recent-onset secondary AI where CST may be normal:
- Challenges the entire HPA axis by inducing hypoglycemia
- Requires endocrine specialist supervision (risk of severe hypoglycemia)
- Alternative: metyrapone test (inhibits cortisol synthesis, assesses ACTH response)
Step 6 - Evaluate for Panhypopituitarism
Because secondary AI often comes with other pituitary hormone deficiencies, check:
- TSH, free T4 (secondary hypothyroidism)
- LH, FSH, testosterone/estrogen (secondary hypogonadism)
- GH/IGF-1
- Prolactin
- ADH axis (diabetes insipidus)
Key Clinical Features That Distinguish Secondary from Primary AI
| Feature | Secondary/Tertiary (low ACTH) | Primary (high ACTH) |
|---|---|---|
| Skin hyperpigmentation | Absent | Present |
| Hyperkalemia | Absent | Present |
| Hyponatremia | May occur (from free-water retention) | Present |
| Mineralocorticoid need | Not required | Required (fludrocortisone) |
| Aldosterone | Normal (RAS intact) | Low |
Diagnostic Flowchart
Bottom Line: Next Steps in Order
- Exclude exogenous steroid use (most common cause)
- Cosyntropin stimulation test - confirm AI (may be false-negative if recent onset)
- Pituitary MRI - identify structural cause
- Assess all pituitary axes - look for panhypopituitarism
- CRH test or ITT if needed to distinguish pituitary vs. hypothalamic level
- Treat: glucocorticoid replacement (hydrocortisone); mineralocorticoid replacement is NOT needed
- Symptom to Diagnosis: An Evidence-Based Guide, 4th ed., p. 433-434
- Textbook of Family Medicine, 9th ed.
- Henry's Clinical Diagnosis and Management by Laboratory Methods
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