You may not be able to get conclusive answers since these can be overlapping diseases/conditions. Lupus can cause adrenal glands to fail or reduce their output, as can other things. I'm fairly certain that as long as you remain on prednisone that the endocrinologist won't be able to do conclusive testing. Not sure how your other meds may also interact with the adrenals.
Sometimes, since several diseases can overlap so closely, getting treatment for the symptoms is much more important that stressing out trying to pinpoint a diagnosis -- our bodies are so complex a real diagnosis might even change from week to week!
Have you had a positive ANA? That might be the defining factor that says it's Lupus rather than a strictly adrenal issue.
Lynnwood, Lupus & Sjogren's ModeratorDIAGNOSING-LUPUS & LUPUS-RESOURCES
"Life is far too important to be taken seriously" - Oscar Wilde