Cortisol is what the adrenals produce. The glucocorticoid that is nearly identical to cortisol is hydrocortisone; Cortisol and hydrocortisone are essentially the same molecule --- when referring to cortisol as a medication, it is called hydrocortisone. That was why my endocrinologist would have preferred that I switch from prednisone to hydrocortisone when my cortisol level was low.

Prednisone and other glucocorticoids are not the same as cortisol. They all have different properties and different molecular structures. They are all similar to cortisol though. When we take prednisone the body thinks there is cortisol in abundance so the adrenals stop producing cortisol.

Over time, the adrenals "forget" what their function is because there is more than enough prednisone present. When we taper our prednisone dose lower, that is when there isn't enough prednisone and no cortisol. The adrenals don't immediately produce cortisol again and only in spurts. The adrenals have to resume their full cortisol production or otherwise our inflammation levels go up again.

If PMR is still "active" our inflammation levels go up rapidly and we "flare" if we aren't taking enough prednisone OR the adrenals can't produce enough cortisol.

There is no way to encourage the adrenals to produce more cortisol again EXCEPT a very low dose of prednisone. If we take prednisone for "too long" the adrenals may never produce cortisol again. What is "too long" depends on the person I guess. Many years of Prednisone wasn't too long for me but my endocrinologist said I got very lucky.