"Chicago Med" said goodbye to an original cast member on Wednesday's episode.

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Rachel DiPillo made her exit during the Season 4 premiere after three seasons as Dr. Sarah Reese.

Sarah left Gaffney Chicago Medical Center to continue her psychiatry residency at a different hospital after discovering her father is a serial killer and that Dr. Charles (Oliver Platt) nearly let him die after learning the truth.

Executive producers Diane Frolov and Andrew Schneider explained Sarah's decision in an interview with TVLine published Wednesday.

"She had such a close relationship to Charles, and she feels betrayed by him and just can't stay," Frolov said.

"[After] the trauma of her father doing such terrible things, she feels like she needs to go someplace else," Schneider added.

Schneider said the door is open for DiPillo to return as Sarah in the future.

"We would welcome Rachel coming back," he said. "We love the actress. She did a great job in the premiere. And if we can find, down the line, a storyline that would involve her, we would bring her back."

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"Chicago Med" confirmed DiPillo's departure on its official Twitter account.

"This is one hard goodbye. #ChicagoMed," the show wrote, adding a broken heart emoji.

"Chicago Med" stars Platt, Nick Gehlfuss, Yaya DaCosta, Torrey DeVitto and Colin Donnell.

The series is part of the "Chicago" franchise, which also includes "Chicago P.D." and "Chicago Fire."

Photo credit: Chicago Med/NBC