Your cortisol level of 0.8 µg/dL after dexamethasone suppression is normal and effectively rules out Cushing's syndrome.
Interpretation Based on Guidelines
A post-dexamethasone cortisol of 0.8 µg/dL (approximately 22 nmol/L) represents excellent suppression and is well below all established diagnostic thresholds. This result indicates your hypothalamic-pituitary-adrenal axis is responding appropriately to dexamethasone, which excludes autonomous cortisol production 1, 2.
Established Cutoff Values
The most recent consensus guidelines define normal suppression as:
- Primary threshold: Cortisol < 1.8 µg/dL (50 nmol/L) after 1-mg overnight dexamethasone 1, 2
- Stricter threshold: Some experts use < 1.4 µg/dL (38 nmol/L) for improved specificity 2
- Historical threshold: The older cutoff of < 5 µg/dL (140 nmol/L) is now considered too permissive and may miss mild cases 2
Your result of 0.8 µg/dL falls comfortably below even the most stringent criteria, placing you in the clearly normal range.
Clinical Significance
With cortisol suppression to 0.8 µg/dL, you have demonstrated normal glucocorticoid feedback regulation. This means:
- No evidence of Cushing's syndrome or autonomous cortisol secretion
- Normal ACTH suppression in response to exogenous glucocorticoid
- No need for additional confirmatory testing (such as late-night salivary cortisol or 24-hour urinary free cortisol) 1, 2
Important Context
The 2021 Lancet consensus emphasizes that cortisol < 1.8 µg/dL "strongly predicts CS absence" 1. Research data from control populations shows that healthy individuals consistently suppress to undetectable levels (< 1 µg/dL) or very low levels after dexamethasone 3, 4. Your result aligns with this normal physiologic response.
One critical caveat: Ensure you actually took the dexamethasone as prescribed and that no medications interfering with dexamethasone metabolism were present (such as phenobarbital, carbamazepine, rifampin, or St. John's wort, which accelerate dexamethasone clearance and could cause false-normal results) 1, 5. If you were on such medications, the test interpretation becomes unreliable.
No Further Action Needed
Given this normal result, no additional endocrine evaluation for hypercortisolism is warranted unless new clinical features develop over time that would raise suspicion again 2.