It isn't unusual for a rheumatologist not to order a cortisol lab test. This test is hard to interpret when you are still taking prednisone. Even my endocrinologist was reluctant to check my cortisol level until I could maintain a very low dose of Prednisone without needing to increase my dose to control PMR.

My endocrinologist said if I still needed prednsisone to control my autoimmune conditions there was no point in checking my cortisol level. When I said how long I had taken Prednisone, my endocrinologist "expected" my cortisol level to be low. My endocrinologist referred me back to my rheumatologist when I was still needed 10 mg of Prednisone.

A year later when PMR was controlled and I could maintain a 3 mg dose of Prednisone without having a flare my cortisol level was checked. Because I had symptoms of adrenal insufficiency an 8 a.m. morning cortisol level was checked by my rheumatologist. When my cortisol level was low my rheumatologist told me not to decease my Prednisone dose any lower and referred me back to the endocrinologist.

My endocrinologist rechecked my cortisol level and an ACTH level and verified that my cortisol level was low but said there wasn't much she could do. She told me to stay on 3 mg of Prednisone. She offered to switch me to hydrocortisone because she felt hydrocortisone would allow me to regain adrenal function better than Prednisone would. She said it would be okay to take Prednisone if I preferred.

I asked about an Synacthen test but my endocrinologist said it wasn't necessary unless I was still having symptoms when I was off Prednisone. She said a Synacthen test might be needed to determine if I needed to go back on prednisone if I still had symptoms when I wasn't taking any Prednisone.

After my cortisol level improved I was able to discontinue Prednisone but it took 6 months for my cortisol level to improve. Only then did my endocrinologist say "it might be safe to discontinue Prednisone." I never needed a Synacthen test.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6260064/