Urinary Free Cortisol Unit Converter
Urinary Free Cortisol Unit Converter
This is an amount excreted over 24 hours, not a concentration — so a bottle that was over- or under-collected changes the answer without anything changing in the patient. Convert µg/24 h to nmol/24 h here, but check the urine creatinine first, because that is what tells you whether the collection was complete.
Urinary Free Cortisol converter
Mass ⇄ molar, per 24 hUrinary free cortisol 30 µg/24 h, adult, complete collection
The conversion, and why the denominator is not a volume
µg/24 h = nmol/24 h ÷ 2.759
because 1 µg is 10⁻⁶ g, and 10⁻⁶ ÷ 362.46 g/mol = 2.7589 × 10⁻⁹ mol = 2.759 nmol
- MW 362.46
- cortisol. Urinary free cortisol is the same molecule as serum cortisol, so the molar mass and therefore the arithmetic are identical
- per 24 hours, not per litre
- the collection volume has already been multiplied in. That is why this page offers no mg/L or µg/dL: a concentration in the bottle is a statement about how much the patient drank, and means nothing on its own
- 2.759 vs 2.76
- Mayo prints the factor as 2.76 on its own test page. Derived from the molecular weight it is 2.7589, which rounds to the same thing — an independent check rather than a disagreement
- what ‘free’ means
- only unbound cortisol is filtered at the glomerulus, so the 24-hour excretion tracks the free fraction integrated over a day. That is the property that makes it useful, and it is also why it rises steeply once cortisol-binding globulin is saturated
Worked example
Urinary free cortisol 30 µg/24 h, adult, complete collection
30 × 2.759 = 83 nmol/24 h (82.8 before rounding)
That sits inside Mayo's adult interval of 3.5–45 µg/24 h, which is 10–124 nmol/24 h
Now the number that decides whether any of this means anything: the urine creatinine. A 70 kg adult man excreting well below about 1,000 mg of creatinine in the bottle has under-collected, and both this result and its verdict are then wrong in the same direction
And a caveat the arithmetic cannot show: the same urine run on an immunoassay would read higher, because cortisol metabolites and some synthetic steroids cross-react and mass spectrometry does not count them
Mayo reference intervals by age, in both units
| Age | µg/24 h | nmol/24 h |
|---|---|---|
| 18 years and over | 3.5–45 | 10–124 |
| 13 to 17 years | 4.0–56 | 11–155 |
| 9 to 12 years | 2.6–37 | 7–102 |
| 3 to 8 years | 1.4–20 | 4–55 |
| 0 to 2 years | Not established | Not established |
What moves a urinary free cortisol without moving cortisol production
| Cause | Direction | What to do |
|---|---|---|
| Incomplete collection — a missed void, a discarded first sample, a short window | Falsely low | Measure urine creatinine on the same bottle and compare it against expected excretion for sex and weight. Repeat rather than interpret |
| Over-collection — more than 24 hours in the bottle | Falsely high | Same check. The creatinine is high for the same reason the cortisol is |
| High fluid intake during the collection | No effect on the amount | This is the advantage of a timed collection: the total is independent of dilution, unlike a spot sample |
| Immunoassay rather than LC-MS/MS | Higher | Cortisol metabolites and synthetic corticosteroids cross-react. Mayo states its mass-spectrometry method eliminates interference from carbamazepine and synthetic corticosteroids. Compare only against the method’s own interval |
| Any exogenous corticosteroid, including topical and inhaled | Variable, often higher on immunoassay | Record what the patient is taking. Prednisolone and several other synthetic steroids are detected by some immunoassays |
| Renal impairment | Falsely low | Excretion falls with glomerular filtration rate, so the test loses sensitivity in chronic kidney disease |
An amount, not a concentration — and what that changes
Almost everything else on this site converts a concentration. A urinary free cortisol is not one. It is an amount of hormone excreted over a defined period, and the period is the measurement. The laboratory takes the concentration in the bottle and multiplies it by the total volume, so the number on the report already contains the collection. That has one large advantage and one large vulnerability. The advantage is that it is immune to dilution: a patient who drank four litres during the day produces a dilute urine with the same total cortisol in it, and the result does not move. The vulnerability is that the result is only as good as the collection, and an incomplete collection is invisible in the number itself.
This is why the urine creatinine matters more than the conversion factor. Creatinine excretion is reasonably constant from day to day for a given person, set mostly by muscle mass, so the amount of creatinine in the bottle is a measure of how much of the day actually made it into the bottle. A collection that is missing a third of its creatinine is missing a third of its cortisol, and the result will read low in a patient whose adrenal output is entirely normal — or, worse, read normal in a patient who has Cushing syndrome. Over-collection does the reverse. There is no way to recover the true value from an incomplete collection, because the relationship is not a correction factor but a missing sample, so the answer is to check and repeat rather than to adjust. Our 24-hour urine collection adequacy calculator does that check against expected creatinine excretion for sex and body weight.
The second thing this number carries is its method. Immunoassays for cortisol cross-react with cortisol metabolites and with several synthetic corticosteroids, and urine is full of metabolites, so an immunoassay urinary free cortisol reads systematically higher than a mass-spectrometry one on the same specimen. Mayo states directly that its LC-MS/MS method eliminates analytical interference from carbamazepine and from synthetic corticosteroids, which affect immunoassay-based cortisol results. That is not a small difference at the margin: it moves the reference interval, which is why the interval belongs to the method and why a result from one laboratory cannot be read against another laboratory’s limits. The intervals on this page are Mayo’s, by mass spectrometry, and they should be replaced by the ones printed on your own report.
Finally, what the test is for. Urinary free cortisol integrates the free, biologically active fraction over a full day, which makes it complementary to the two other first-line screening tests for Cushing syndrome: the overnight dexamethasone suppression test, which asks whether the axis can be switched off, and late-night salivary cortisol, which asks whether the normal midnight nadir has been lost. All three can be normal in mild or cyclical disease, and the Endocrine Society guideline therefore asks for at least two measurements and for a second test of a different type before concluding that cortisol excess is present. A single raised 24-hour collection, on its own, is a reason to repeat the test rather than a diagnosis.
Frequently asked questions
How do I convert urinary free cortisol from µg/24 h to nmol/24 h?
Multiply by 2.759. A urinary free cortisol of 30 µg/24 h is 83 nmol/24 h. Divide by 2.759 to go the other way. The factor comes from the molecular weight of cortisol, 362.46, and Mayo publishes it as 2.76.
Why does an incomplete 24-hour collection matter so much?
Because the result is an amount excreted over the period, not a concentration, so the collection is part of the measurement. A bottle missing a third of the day’s urine is missing a third of the day’s cortisol, and the result reads low in a patient with normal adrenal output. There is no correction factor — check the urine creatinine and repeat the collection.
How does urine creatinine tell you the collection was complete?
Creatinine excretion is fairly constant day to day for a given person, set mostly by muscle mass, so the total creatinine in the bottle indicates how much of the 24 hours it actually contains. Compare it against expected excretion for sex and body weight before interpreting anything else in the collection.
Why is immunoassay urinary free cortisol higher than LC-MS/MS?
Urine contains a large quantity of cortisol metabolites, and immunoassay antibodies cross-react with them and with several synthetic corticosteroids. Mass spectrometry separates cortisol from them and counts only cortisol. The reference interval therefore belongs to the method, and results from the two are not interchangeable.
Does one normal urinary free cortisol exclude Cushing syndrome?
No. Mild and cyclical disease can give a normal 24-hour collection. The Endocrine Society guideline asks for at least two collections and for a second, independent first-line test — an overnight dexamethasone suppression test or a late-night salivary cortisol — before cortisol excess is either diagnosed or excluded.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: CORTU — Cortisol, Free, 24 Hour, Urine. Liquid chromatography tandem mass spectrometry. Reference values: 0–2 years not established; 3–8 years 1.4–20 mcg/24 h; 9–12 years 2.6–37; 13–17 years 4.0–56; 18 years and older 3.5–45 mcg/24 h. The method “eliminates analytical interferences including carbamazepine (Tegretol) and synthetic corticosteroids, which can affect immunoassay-based cortisol results.”
- Nieman LK, Biller BMK, Findling JW, et al. The diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2008;93(5):1526–1540. doi:10.1210/jc.2008-0125
- Nieman LK, Biller BMK, Findling JW, et al. Treatment of Cushing’s Syndrome: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2015;100(8):2807–2831. doi:10.1210/jc.2015-1818
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.