^Depression, side effects from drug-use, and possibly an illness he'll spend the next season of the episode trying to figure out himself.
It's quite fitting that the depression brought this all on, and how they played up the recent twists of him not knowing the answers, and he doesn't know his own answer.
They've set up incredible potential for next season, but it's going to be hard to top what they did this season. But I love that House finally humbled enough to seek for help, and you know he's going to try and figure out what is wrong with himself at the expense of nearly killing himself a few times next season.