I was diagnosed with RSD in 1977 and it was treated with prednisone. Lucky for me that was the last I dealt with that. I also have MS and thyroid problems. It doesn't seem fair, but I think anyone can have a multitude of autoimmune related illness. Personally, I think if you have one, you are more prone to another one. (Just my own theory.)
I hope you get both/all conditions under control quickly.