I just posted on another topic, but I thought this was something worth bringing up.
I never felt the need to share my medical problems with my boss until this past summer when I was having the vertigo and balance problems. My being off-balance some days makes it quite obvious that I appear drunk when I'm not so instead of letting the rumors start to fly about my "drinking problem" and having the boss send me for a drug test, I thought I better tell her what's going on. Although I still don't have an official dx, I told her I could have MS and that my neuro suspects it. She was cool with it and I don't have to worry about rumors.
But on the flip-side, since a few people know now that I'm not acting right sometimes and my potential MS, I don't want the sympathy either. So I guess that's something you have to consider when telling your boss but I felt like I didn't have a choice in my situation.