If a creature has been dealt damage, that damage remains marked on it until the cleanup step. If Thresher Lizard has been dealt 2 or 3 damage while you have zero or one card in hand, and later in the turn two cards are in your hand after a spell or ability has finished resolving, Thresher Lizard will be destroyed. Similarly, if that causes it to have 0 toughness, it's put into its owner's graveyard.