Can a employer fire me for not providing medical records?

I work for a large company for about 8 yrs and for about 5 yrs of my employment I've had FMLA given to me for a illness that prevents me to work but sometimes i can exceed the time allotted because of my illness. Because of that it cause me to use time off that my employer gives us and sometimes causes me to get to the point of separation. For the past 5 years I have always spoke with our HR to help and they have always asked for me to release my medical information. I have always told them that I do not feel comfortable with releasing that information and after several battles I only provided doctor's note. Now almost every year I have to go thru this with my employer and it seems I have to fight with new faces with our HR dept. At one point I caved in to fill out the paper work with my primary doctor and because of the constant information my employer asked from my doctor, my doctor terminated service with me. Now after all of that I lost my doctor but the company decided to cover my lost time and clean my slate. Now I'm in the same ordeal and I refuse to sign and release my medical information. A doctor's note should be enough.