Morning Cortisol of 0.8 µg/dL After Overnight Dexamethasone Suppression Test Indicates Normal (Suppressed) Cortisol
A morning serum cortisol of 0.8 µg/dL after a 0.5 mg overnight dexamethasone dose represents normal, appropriate suppression and effectively rules out autonomous cortisol secretion or Cushing's syndrome. 1
Interpretation Based on Established Diagnostic Thresholds
Your result falls well below all accepted cutoffs for normal suppression:
- The standard threshold for normal suppression is <1.8 µg/dL (50 nmol/L) after dexamethasone administration, and your value of 0.8 µg/dL is less than half this cutoff 1, 2, 3
- Even the most stringent threshold of <2.0 µg/dL is satisfied by your result 4
- This level demonstrates intact negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis and confirms normal suppression by exogenous glucocorticoid 1
Clinical Significance of This Result
No further biochemical testing for hypercortisolism is warranted based on this result alone 1, 3. Your cortisol appropriately suppressed in response to dexamethasone, which means:
- Cushing's syndrome and autonomous cortisol secretion are excluded with high confidence 1
- Your adrenal glands respond normally to negative feedback signals 1
- If this test was performed as part of an adrenal incidentaloma workup, the mass can be classified as non-functioning from a cortisol perspective 1
Understanding the 0.5 mg Dose vs. Standard 1 mg Dose
While the standard overnight dexamethasone suppression test uses 1 mg, you received 0.5 mg—a lower dose. Despite this:
- Your cortisol still suppressed to 0.8 µg/dL, which would be considered normal even after the standard 1 mg dose 1, 2
- This demonstrates particularly robust HPA axis suppression, as you achieved excellent suppression with half the typical dexamethasone dose 2
What This Result Rules Out
This level of suppression definitively excludes:
- Cushing's syndrome (endogenous cortisol excess) 1, 3
- Autonomous cortisol secretion from an adrenal adenoma 1
- Any form of pathologic hypercortisolism 1, 3
Important Caveat: This Does NOT Diagnose Adrenal Insufficiency
While your cortisol is appropriately low after dexamethasone, this test cannot be used to diagnose adrenal insufficiency 5, 1. The dexamethasone itself suppresses cortisol production, so a low value is expected and desired 5, 1.
If adrenal insufficiency is suspected clinically, you would need:
- A morning cortisol measured WITHOUT prior dexamethasone (ideally between 8:00-9:00 AM) 1
- Values <10 µg/dL (<275 nmol/L) in the setting of symptoms would warrant an ACTH stimulation test 1
- A peak cortisol <18 µg/dL (<500 nmol/L) after 0.25 mg cosyntropin would confirm adrenal insufficiency 1
Bottom Line
Your result of 0.8 µg/dL represents normal, appropriate suppression—exactly what we want to see in someone without cortisol excess. 1, 2 This is a reassuring finding that rules out Cushing's syndrome and autonomous cortisol production.