What are the indications, if any, for trending ACTH and cortisol levels in patients with adrenal insufficiency on steroid replacement therapy?

We don't usually monitor ACTH or cortisol levels for patients on steroid replacement. We follow blood pressure, electrolytes (if primary) and clinical symptoms.

The only indication is if the patient has steroid-induced adrenal insufficiency and you are planning to wean the patient off steroids. In that case, I usually measure early AM cortisol and ACTH every 8 weeks. An increase in ACTH will be the first sign of HPA axis recovery. During this process, ACTH ...

If the patient is on hydrocortisone, I hold it for 24 hours and check am cortisol and ACTH to assess the adequacy of replacement, and then also check cortisol 2-3 hours after the morning dose to avoid over-replacement.