Bush follows showrunner Matt Olmstead off of the Dick Wolf procedural, according to Deadline which first reported the news. The decision to leave was made by Bush, whose character Erin was toying with the idea of leaving her Chicago unit for the FBI in New York.
According to Deadline, Bush could return next season for guest-starring episodes to officially wrap up Erin's storyline and the show will make moves to replace her with another prominent female character.
Of course, the loss of such a big character isn't just a loss for Chicago P.D.. Bush has been instrumental in the one-Chicago crossovers, having appeared on 22 episodes of the three other Chicago shows including Law & Order: SVU.
Bush is leaving just as P.D. heads into its fifth season, which will see the cop show hit its landmark 100th episode. Chicago P.D. was Bush's first prominent role after wrapping up The CW drama One Tree Hill in 2012.
NBC declined to comment on the departure.