I doubt that God is the cause of your illness. The Protestants have a perverse notion that God is the source of evil (they get it from the OT myths), and that He uses evil for greater good.
After all, we are the sick, and He is the physician. Doctors don't give you diseases. They cure them.
But I agree most sgtrongly that we should give thanks to God, no matter what happens to us in our fallen world. He didn't cause it be fallen. We did.
I didn’t say God WAS the cause of my illness, I’m saying I thank God for the illness because he says to thank Him in all circumstances. I know He didn’t cause it. He’s just giving me the grace to go through it all. Period.