I was referred to an endocrinologist who took control of my Prednisone dose. The endocrinologist was surprised I had a cortisol level after 12 years of daily prednisone at moderately high doses. However, she was optimistic that my cortisol level would improve if I could simply stay on 3 mg for an "extended period of time."

Keep in mind that I was taking Actemra and PMR was controlled for the time that was needed. I wouldn't have been able to stay on 3 mg of Prednisone without Actemra. Tapering from 30 mg to 10 mg of prednisone took me 12 years. After Actemra was started, I tapered from 10 mg to 3 mg in about four months.

When I reached 3 mg, I was instructed to stay on 3 mg for "as long as it takes" for my cortisol level to increase. My endocrinologist seemed to think that 3 mg was a low enough dose for my adrenals to get the message to make some cortisol. My adrenals didn't "kick in" all of a sudden. It was a gradual process that took about 6 months. I mostly stayed on 3 mg of Prednisone but I skipped some doses occasionally.

After 6 months, my endocrinologist said my cortisol level was "adequate" but she didn't know what would happen if I stopped taking prednisone. We discussed some of the possible things that could happen. She also said that 3 mg was a small dose and since my cortisol level was adequate that I didn't have to taper.

As surprising as it sounds, I could simply stop Prednisone because my cortisol level was adequate on the day it was tested. What my endocrinologist didn't know was whether or not I still needed Prednisone for PMR and my other autoimmune conditions. She talked to my rheumatologist who said Actemra was keeping things under control.

Another thing my endocrinologist didn't know was whether or not my cortisol level would remain at adequate levels for days in the future. She was mostly concerned about a stressful event like a car wreck or something of that nature.

I tapered Prednisone from 3 mg at a rate of 1 mg per day down to zero. Something happened about 2 weeks after I stopped taking Prednisone. My opthalmologist had to restart 60 mg of Prednisone again when I had a flare of uveitis.
https://www.mayoclinic.org/diseases-conditions/uveitis/symptoms-causes/syc-20378734
-----------------------
Uveitis was treated with a different biologic for 3 months while Actemra was stopped. Uveitis went back into remission but I was only able to get back to 15 mg of Prednisone again. I couldn't taper any lower than 15 mg because my PMR symptoms returned.

My rheumatologist apologized by saying that it would be impossible to adequately treat all my autoimmune conditions with one biologic. Since I couldn't be on two biologics, I had to choose the one that worked the best for me. I was given the option of restarting Actemra so I did. My rheumatologist, ophthalmologist and endocrinologist all had different opinions about what I should do but it was ultimately my choice.

When Actemra was restarted I tapered from 15 mg back to 3 mg in a month. My cortisol level was rechecked and it was still adequate so I stopped Prednisone a second time. So much for slow tapers!

I still experienced symptoms of adrenal insufficiency but they improved over time. Now that I have been off Prednisone for almost 5 years, adrenal insufficiency isn't a concern anymore. I would say it took about a year after being off Prednisone for all my symptoms of adrenal insufficiency to disappear.