I found the following on Google...
Can you test cortisol levels while on prednisone?
If you are on prednisolone once daily, you can have a synacthen test in the morning before your next dose. The prednisolone from 24 hours previously will not be detectable, so the cortisol values will be accurate.
impendo.co.uk › pre...
Prednisolone in vasculitis and cortisol suppression
This sounds different than what I've been reading on this forum. Any comments?
My Endocrinologist is making me wait until I am down to 4 mgs to be accurate. I keep trying but I have had a Covid - like virus again. So I guess he believes that will give us optimal accuracy.
Think you have had your share of viruses! Take care…..
Don’t think you read the complete paragraph
Not sure what you mean. What did I miss?
Sorry hit reply button before completed and then been off line. will get back to you
Update - some of this page is a bit confusing-
-impendo.co.uk/prednisolone/...
1. Agree with 1st paragraph and its what is in my post on adrenals -but the last sentence of 2nd paragraph is strictly true - yes they may shrink [but not 100% convinced] and they certainly don't waste away otherwise all of us who have had GCA and been on high dose wouldn't have any adrenal response - and we do - I can testify to that...
What happens to your adrenal gland when you take your steroid treatment?
Human endocrinology has a system of negative feedback, so that normal adrenal glands produce exactly the right amount of cortisol under the control of the pituitary gland. So when you take a large dose of prednisolone for your vasculitis, the pituitary detects this and stops stimulating the adrenal gland to make cortisol (Fig 2). This doesn’t matter because you have a large excess of prednisolone, so you won’t have an Addisonian crisis.
If you stay on high dose prednisolone for several days the ACTH will be fully suppressed and the adrenal glands won’t make any cortisol for that whole period. If you stay on prednisolone for several weeks, the adrenal glands will start to shrink, and waste away.
2. What you quoted is true - not sure we've ever said the cortisol values won't be accurate - what we do say say it they have limited value - as you are still on steroids.
If you are on prednisolone once daily, you can have a synacthen test in the morning before your next dose. The prednisolone from 24 hours previously will not be detectable, so the cortisol values will be accurate.
But you didn't include this -
However there is no point doing this unless you are on less than 4mg once daily, and you are sure that your inflammatory disease is in remission.
From Article -
1.What happens to your adrenal gland when you take your steroid treatment?
Human endocrinology has a system of negative feedback, so that normal adrenal glands produce exactly the right amount of cortisol under the control of the pituitary gland. So when you take a large dose of prednisolone for your vasculitis, the pituitary detects this and stops stimulating the adrenal gland to make cortisol (Fig 2). This doesn’t matter because you have a large excess of prednisolone, so you won’t have an Addisonian crisis.
If you stay on high dose prednisolone for several days the ACTH will be fully suppressed and the adrenal glands won’t make any cortisol for that whole period. If you stay on prednisolone for several weeks, the adrenal glands will start to shrink, and waste away.
My response -
1st para okay, but 2nd para sentence in bold sounds as if everyone on high doses has no adrenal function once off steroids - that isn't true - as many on here will tell you - including myself. Unfortunately there will always be a few where adrenal function does not return.
2. How do you know whether your adrenals have recovered?
Probably the best test is how you feel. However many patients want to try and measure their cortisol to see if there has been recovery. It sounds easy, but it isn’t. The problem is that cortisol rises in response to stress, so if you are very calm on the day that the blood sample is taken, it might look like your cortisol is abnormally low, when in fact it is normal for the circumstances. The other problem is that cortisol levels vary during the day as cortisol is a diurnal hormone, with levels highest in the morning and lowest at night possibly because waking up is the most stressful part of the day.
There must be a test I can have to check my adrenal function?
Another method that some people use is a synacthen test. SynACTHen is synthetic ACTH which is a hormone that is produced from the pituitary gland (Fig. 2). When a patient is given a dose of ACTH, that dose rapidly stimulates the adrenal gland to make cortisol, and you can measure the cortisol response at the end. This tells us whether your adrenal glands have woken up.
If you are on prednisolone once daily, you can have a synacthen test in the morning before your next dose. The prednisolone from 24 hours previously will not be detectable, so the cortisol values will be accurate. However there is no point doing this unless you are on less than 4mg once daily, and you are sure that your inflammatory disease is in remission.
My response re bold -
Not disagreeing the cortisol level is accurate and not sure anybody has said that, but what we do say is they will be a scant value if patient is still on steroids. The bit you missed above is the second sentence in bold.
Thak you so much Dorset Lady. You were absolutely correct..... I did not see "the rest of the story. " Again, thank you for a thorough response once again....... smiles
No problem-wasn’t having a pop at you. But thought a few odd comments in the article which seemed strange considering it’s from the experts in all things adrenals..😊
Two slightly different replies have come up -thought the original (on laptop) had disappeared into the ether - So I rewrote it! On to find they are both there when I looked on iPhone.. 😊
😁😁😁
"This sounds different than what I've been reading on this forum."
Not entirely. What that says is that the presence of pred doesn't interfere with the actual assay. The true value of cortisol will be measured, not be artificially high because of the pred.
What it doesn't mean is that the pred isn't suppressing the production of cortisol by the adrenal glands. And that is where the innaccuracy of the synacthen test in a patient on steroids arises.
I understand, but do have a followup question. I absolutely feel worse in the morning, and don't start feeling better (fatigue wise) until 2 hours or so after taking my prednisone. It feels like the pred "wore off" overnight, almost as inferred in this article. Might that be happening do you think?
Well yes - pred as a substance is in the body for well under 24 hours, the halflife is 3 to 4 hours, the whole lot is effectively excreted in 5.5 half lives - 16 to 22 hours maximum. The antiinflammatory effect lasts 12 to 36 hours depending on the person. So the effects pred causes will be very faded by the time a next daily dose is due, whatever ones you are looking at.
Got it. Thanks again.
I just thought of another question.... so it seems then that people experiencing adrenal insufficiency issues will most likely feel worse in the AM prior to taking their prednisone dose?
Not necessarily…,
Same applies for PMR or GCA as well. But yes, the pred dose is also a replacement therapy for adrenal insufficiency but as DL says, not necessarily.
Cortisol test before or after Prednisolone
Morning cortisol test results have arrived.
cortisol blood test 
Cortisol test
Cortisol
