SAI and switching prednisone to hydrocortisone

Hello all,

I was diagnosed with secondary adrenal insufficiency recently, but I had many of the symptoms for a year or two. I also have RA, osteopenia, and hypothyroidism. I'm 73 and have taken Prednisone for 40 years for my RA.
My endo first told me to take 7.5 mg of prednisone/day and to take 15 mg for two days, if I felt very sick. At that point, I was taking 7mg, and my rheumatologist wanted me to take only 5 mg. So, one day I felt really sick and took 15 mg for two days. Then, I started to taper the prednisone, very slowly. But I wasn't able to taper from 10 mg to 9 mg.
My endo recommended switching to hydrocortisone plus 2.5 mg of prednisone. The idea of going from 10 mg of prednisone to 2.5 with no tapering makes me afraid of an RA flare-up, prednisone withdrawal syndrome, or both. On top of that, there has been a heat wave; extreme weather worsens my RA, and heat worsens SAI.
I have an appointment with a second endo in two months. Meanwhile, I'm still taking 10 mg of prednisone and not hydrocortisone. I feel okay, as long as I stay home, mostly resting.