Is 24 hour urinary cortisol better than 9am cortisol?

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mhco

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Hi,

I have asked this question in the USMLE forum, but did not get a reply, so thought it was worth asking it here. (NB I can't delete the old thread, apologies).

Anyhow I am studying cushing's at the moment and I'm a little confused about measuring cortisol levels.

From my understanding, correct me if I am wrong, you need to measure urine cortisols, either at a specific time during the day in all patients (eg 9am or midnight) or after collecting the urine for 24 hours. This is due to the circadian rhythm of cortisol release, which will mean different values at different times of the day.

However surely just measuring cortisol at one point during the day is quicker & easier? So why do we need to do a 24 hour urine test?

Thanks in advance for your help.

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Hi,

I have asked this question in the USMLE forum, but did not get a reply, so thought it was worth asking it here. (NB I can't delete the old thread, apologies).

Anyhow I am studying cushing's at the moment and I'm a little confused about measuring cortisol levels.

From my understanding, correct me if I am wrong, you need to measure urine cortisols, either at a specific time during the day in all patients (eg 9am or midnight) or after collecting the urine for 24 hours. This is due to the circadian rhythm of cortisol release, which will mean different values at different times of the day.

However surely just measuring cortisol at one point during the day is quicker & easier? So why do we need to do a 24 hour urine test?

Thanks in advance for your help.
They are used measure different things.

If you are worried about someone having *too little* cortisol (adrenal insufficiency), you measure it at the time of peak concentration, typically around 8am. If they have a decent value then but vary throughout the day, nbd, that's normal. If it's too low or intermediate, then we do further testing.

If you are worried about someone having *too much* cortisol (cushings), you measure either A) at the nadir (midnight, but here we use salivary cortisol b/c serum assays aren't that accurate with very low numbers) B) all day (24 hour urine collection) or C) after giving them a slug of dexamethasone (the dexamethasone suppression test should decrease your serum cortisol to near-zero, if not you have some kind of autonomous secretion).
 
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for Cushing's as a screening tool, all three are acceptable to be used...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2386281/

if abnormal, then a second confirmatory test needs to be done.

for AI, a 24 hr Urine cortisol is not the test of choice...AM ACTH and cortisol is enough to make a diagnosis of primary vs secondary, but usually at the ACTH is a send out and takes a while, so a Cosyntropin Stimulation test is usually done.

an AM cortisol >10 effectively r/o AI.
 
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