Oracle Text

Target player reveals their hand. You choose a nonland card from it. That player discards that card. You lose 2 life.

Card Rulings

9/15/2013 The loss of life is part of the spell’s effect. It’s not an additional cost. If Thoughtseize doesn’t resolve, you won’t lose life.
2/1/2014 If you target yourself with this spell, you must reveal your entire hand to the other players just as any other player would.