I've had some co-workers (including my boss) who overshare. My boss is very paranoid about her health, to take just one thing--it's more stuff like her eyes or her heart than any, er, other parts, but I still don't like to hear about it. She's one of those people who complains and speculates and over-dramatizes but won't actually go see a doctor about anything, or if she finally does go she doesn't trust what the doctor told her.
She really wants to hear how special she is for being so sensitive (as in, sensitive eyesight, sensitive hearing, etc.). Sorry, having secret constant migraines that don't always manifest with pain but make you feel seasick or unable to look at someone wearing red stinks... but it's not a superhero power.
Generally, I try the "little response" route. Since it's my boss, I feel like totally ignoring her isn't very smart, so I usually face her and nod or say vague supportive things like, "Oh yeah. Sure. Of course." But nothing more elaborate or emotional than that.
She keeps coming back, though... I've been trying a new thing lately with some success. It also worked on a previous co-worker who overshared and generally talked about himself all the time. I've found that if I actively contribute to the conversation, incorporating my own troubles or those of people I know, it seems to end faster.
Like when my co-worker would go on and on ad nauseum about the videogame he was playing, I would wait for a natural pause and then jump in with, "Oh yeah, I know exactly what you mean! I've been playing this game online that..." And believe me, I wouldn't get very far before he was visibly bored and ended the conversation himself. I mean, I wouldn't interrupt him or anything rude like that--I didn't talk for a quarter as long as he had. It could have been a nice conversation about our gaming hobbies... if he was actually interested in a conversation, which obviously he wasn't, he just wanted to hear the sound of his own voice.
I tried this a little bit with my boss the other day and it also seemed to bring the conversation to a close quicker. She said someone suggested she had allergies but she'd never had allergies, so I jumped in with a (true) story about how my grandma felt bad for a year before someone looked into her crawlspace and found all kinds of mold growing there, that was blowing up into the house. You don't have to be "allergic" to mold to get sick from that amount. My boss threw in a couple more lines about how she's had someone check her basement... then decided she was done with the conversation. Yay for me!