Cortisol Unit Converter
Cortisol Unit Converter
Convert serum cortisol between µg/dL, nmol/L and ng/mL, and see why the sampling time and the patient's oestrogen status matter more than the number itself.
Cortisol converter
Mass ⇄ molarSerum cortisol 14 µg/dL, drawn 07:00–09:00
Formula and conversion factor
µg/dL = nmol/L ÷ 27.58925
- 27.58925
- derived from the molecular weight of cortisol, 362.46 Da
- ng/mL
- numerically ten times the µg/dL value
- total cortisol
- this converts the routinely measured total; only around 10% circulates free and biologically active
Worked example
Serum cortisol 14 µg/dL, drawn 07:00–09:00
14 × 27.58925 = 386 nmol/L
= 140 ng/mL
Why the sampling point matters
| Sampling point | What it is used for |
|---|---|
| 07:00–09:00 (peak) | Standard morning reference interval, shown above |
| Late-night (23:00), salivary | A raised value is a screening test for Cushing syndrome — loss of the normal nadir is often the earliest abnormality |
| Random, unknown time | Of little value except when very low or very high; cannot be judged against a reference interval |
Why timing and binding matter more than the number
Cortisol secretion follows a marked diurnal rhythm. It rises through the last hours of sleep, peaks roughly 30 to 45 minutes after waking, and falls to its lowest point close to midnight. Because the same total daily output produces very different concentrations depending on the clock, a value is essentially uninterpretable without knowing when it was drawn — a mid-afternoon result compared against a morning reference interval will look falsely low.
That rhythm is also the basis of a specific screening test. A raised late-night salivary cortisol is used to screen for Cushing syndrome precisely because loss of the normal midnight nadir is one of the earliest detectable abnormalities, often appearing before the daytime levels or the 24-hour output become obviously excessive.
The assay measures total cortisol, and oestrogen raises cortisol-binding globulin, which raises the total without raising the free, biologically active fraction. This is why total cortisol runs high in pregnancy and in anyone taking the combined oral contraceptive pill, with no true excess of cortisol at all — the pill should be stopped for six weeks before a cortisol result is used for diagnosis. Critical illness, depression, heavy alcohol use and shift work all disturb the rhythm independently of any adrenal disease. Taken together, a random cortisol is of limited diagnostic value except when it is very low or very high; everything in between needs a known sampling time, and often dynamic testing, to interpret.
Frequently asked questions
How do I convert cortisol from µg/dL to nmol/L?
Multiply by 27.589. A cortisol of 14 µg/dL is 386 nmol/L. Divide by 27.589 to go the other way.
Why does the sampling time matter so much for cortisol?
Cortisol has a marked diurnal rhythm — highest 30 to 45 minutes after waking, lowest around midnight — so the same underlying adrenal output produces very different numbers depending on the hour. A result without a known sampling time is close to uninterpretable.
Why is a late-night cortisol used to screen for Cushing syndrome?
Because loss of the normal midnight nadir is often the earliest detectable abnormality in cortisol excess, appearing before daytime levels are obviously raised. A raised late-night salivary cortisol is one of the standard first-line screening tests.
Why is my cortisol high on the combined pill or in pregnancy?
Oestrogen raises cortisol-binding globulin, which raises total cortisol — the fraction routinely measured — without raising the free, biologically active fraction. Stop the combined pill for six weeks before a cortisol test used for diagnosis.
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References
- Nieman LK et al. The diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2008;93(5):1526–40.
- Bornstein SR et al. Diagnosis and treatment of primary adrenal insufficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2016;101(2):364–89.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.