How to Convert Cortisol µg/dL to nmol/L
Serum cortisol is reported in µg/dL in the US and in nmol/L across most of the world. The conversion factor is 27.59, from cortisol's molecular weight of 362.46 g/mol.
Cortisol nmol/L = cortisol µg/dL × 27.59 Cortisol µg/dL = cortisol nmol/L ÷ 27.59 Cortisol µg/dL to nmol/L Chart
| µg/dL | nmol/L | Clinical context |
|---|---|---|
| 1.8 µg/dL | 50 nmol/L | Dexamethasone-suppressed cutoff |
| 5 µg/dL | 138 nmol/L | Adrenal insufficiency threshold (AM) |
| 10 µg/dL | 276 nmol/L | Mid-normal morning |
| 18 µg/dL | 497 nmol/L | Upper morning |
| 25 µg/dL | 690 nmol/L | Upper limit (AM) |
| 30 µg/dL | 828 nmol/L | High |
nmol/L to µg/dL Reverse Chart
| nmol/L | µg/dL |
|---|---|
| 50 nmol/L | 1.8 µg/dL |
| 138 nmol/L | 5.0 µg/dL |
| 276 nmol/L | 10.0 µg/dL |
| 500 nmol/L | 18.1 µg/dL |
| 690 nmol/L | 25.0 µg/dL |
Cortisol Reference Ranges and the Daily Rhythm
This is what generic converters miss: cortisol has a strong diurnal rhythm. It peaks in the early morning and falls to its lowest around midnight, so the same number means different things at 8 AM and at night.
| Timing | µg/dL | nmol/L |
|---|---|---|
| Morning (around 8 AM) | 5–25 µg/dL | 138–690 nmol/L |
| Afternoon (around 4 PM) | 3–15 µg/dL | 83–414 nmol/L |
| Late night / midnight | Usually low | Low (nadir) |
| After dexamethasone (suppressed) | Below 1.8 µg/dL | Below 50 nmol/L |
Low Cortisol: Adrenal Insufficiency
A morning serum cortisol below 5 µg/dL (138 nmol/L) makes adrenal insufficiency likely, while a value above about 15–18 µg/dL (410–500 nmol/L) makes it unlikely. Borderline results are usually confirmed with a Synacthen (ACTH) stimulation test.
High Cortisol: Cushing's Screening
When screening for Cushing's syndrome with an overnight dexamethasone suppression test, a morning cortisol of 1.8 µg/dL (50 nmol/L) or below is considered adequately suppressed. Higher values suggest possible hypercortisolism and prompt further testing, such as late-night salivary cortisol or 24-hour urinary free cortisol.
Worked Example
Suppose a report shows 500 nmol/L on a morning sample and you want µg/dL:
500 ÷ 27.59 = 18.1 µg/dL 18.1 µg/dL is a reassuring morning value. Converting the other way, the 1.8 µg/dL suppression cutoff is exactly 50 nmol/L.
Frequently Asked Questions
How do you convert cortisol µg/dL to nmol/L?
Multiply µg/dL by 27.59. For example, 10 µg/dL is about 276 nmol/L.
What is a normal morning cortisol?
Roughly 5–25 µg/dL (138–690 nmol/L) around 8 AM.
Why does timing matter?
Cortisol peaks in the morning and bottoms out at midnight, so the draw time changes interpretation.
What level suggests adrenal insufficiency?
A morning cortisol below 5 µg/dL (138 nmol/L) makes it likely.
References
- UnitsLab — Cortisol unit conversion (µg/dL ↔ nmol/L).
- Cortisol factor 27.59 (MW 362.46 g/mol); morning reference range and adrenal insufficiency / dexamethasone suppression cut-offs from standard endocrinology references.
Conclusion
Converting cortisol between µg/dL and nmol/L is a ×27.59. But cortisol is the hormone where when matters as much as how much — always pair the result with the draw time and interpret it with your clinician.