"She is deeply concerned about the direction the show has been taking," a source tells PEOPLE. "She is turned off by the mean-spirited story lines and poor care of the contestants."
The revelation comes just months after the controversial season finale after which many speculated that winner Rachel Frederickson (who worked with Dolvett Quince during the show, and continued losing weight at home) had dieted herself down to a dangerously low weight – an accusation the show awkwardly side-stepped after the finale.
Now, "Michaels wants to distance herself from the Biggest Loser brand," says the source. "Especially considering what happened with Rachel, her feeling is that there isn't proper attention paid to the contestants' health or wellness."
Michaels, 40, had already left the series once, returning in 2013 for season 14.
"They asked her to come back to boost the ratings and they needed her, but the Rachel thing shook her up," says the source. At the finale, Michaels seemed visibly distraught to see Frederickson's emaciated appearance.
(NBC has not publicly announced any policies that would prevent an underweight contestant from winning the show.)
The trainer has also apparently grown weary of the "bad guy" persona she has earned as a result of her tough approach on the series.
"The show uses her as the bad guy, and anyone who knows Jillian knows she's not the bad guy at all. She's a loving mother of two kids. She's not the negative person they portray her as," says the source. "She wants to be in a place where she has more respect."
Reps for the network and for Michaels declined to comment.