I was on a moderately high dose of prednisone for 13 years for PMR. When I was able to taper down to 3 mg of prednisone -- only a morning 8 a.m. cortisol level was checked. My morning cortisol level was low so my rheumatologist told me to stay on 3 mg and an endocrinologist was consulted. I stayed on 3 mg of prednisone for 3 months waiting for my appointment with the endocrinologist.

I asked the endocrinologist about a synacthen test but she said it wouldn't be needed. In fact, the endocrinologist "expected" my cortisol level to be low given I was on prednisone for a very long time. The only question was whether or not my adrenals would respond to a lower dose of prednisone and start producing cortisol again. The endocrinologist said only time would tell whether or not my adrenal function would improve.

My endocrinologist wasn't overly optimistic. She said I might need a "maintenance dose" of prednisone for the rest of my life. She explained that cortisol was a life essential hormone. A side effect of prednisone was that it replaced the cortisol that my adrenals produce. The adrenals lose their function when prednisone is taken for a long time. We eventually become "steroid dependent."

According to my endocrinologist, another reason I might still need prednisone was if I still needed prednisone for PMR. I didn't think I needed prednisone any longer for PMR because a biologic called Actemra (tocilizumab) was keeping PMR symptoms in check.

It took a long time for my cortisol level to improve. I was able to stay on a low dose of prednisone while doing Actemra injections. This biologic didn't suppress my adrenal function like prednisone did. When my endocrinologist said my cortisol level was "adequate" only then was I able to discontinue prednisone.

The following link is a good overview about how to manage adrenal insufficiency.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6297573/#:~:text=When%20uncertainty%20exists%20as%20to,and%2For%2060%20minutes%20later.
As the link suggests:
"When uncertainty exists as to whether a patient has adrenal insufficiency, a synacthen (ACTH1-24 stimulation)17 test should be used. In this test, 250 μg of synthetic ACTH is given parenterally and serum cortisol is measured 30 and/or 60 minutes later."

My endocrinolgist was certain about my symptoms of adrenal insufficiency in the context of long term prednisone use. She said the synacthen test wasn't needed in my case.
https://www.uptodate.com/contents/diagnosis-of-adrenal-insufficiency-in-adults#:~:text=Cortisol%20%E2%89%A43%20mcg%2FdL,needed%20to%20confirm%20the%20diagnosis.
"Cortisol ≤3 mcg/dL – A low early morning serum cortisol concentration (≤3 mcg/dL [80 nmol/L]) in the absence of CBG deficiency is consistent with adrenal insufficiency [13-15]. When multiple symptoms correlate with the low cortisol value, ACTH stimulation testing typically is not needed to confirm the diagnosis."

I should add that my being able to stay on 3 mg for an extended period of time was only possible because of Actemra. After my cortisol level improved, my endocrinologist said "it might be safe" to stop prednisone. She said it would be okay to go from 3 mg to zero without tapering. However, she allowed me to taper off if I wanted too. She said I could take prednisone again for any reason if I "felt the need."

I had the need to take prednisone again when I stopped prednisone the first time but that is another long story. I didn't have an adrenal crisis.

I was able to taper off prednisone a second time --- 60 mg to zero in a few months the second time. I have been off prednisone for more than two years. I still take Actemra as a monthly IV infusion.