According to the 36-year-old personal trainer, she is leaving the NBC weight-loss show for personal reasons that include starting a family.
"I want to take a year off TV and focus on becoming a mommy and doing more charity work," Michaels told her fans.
Michaels said she has been wanting to have children for a couple of years and this year's production of Losing It with Jillian -- which featured Michaels moving in with families and helping them lose weight and adopt healthier lifestyles -- accelerated her feelings.
"Shooting [Losing It with Jillian] also had big impact on me. Living with kids I saw firsthand what I was missing," Michaels said.
While Michaels -- who has previously disclosed she suffers from health conditions that would make getting pregnant difficult -- didn't detail her motherhood plans, her subsequent tweets suggested she may be adopting.
"It's scary. I haven't even met this little person & I'm already talking about my fear of messing them up in therapy," she wrote.
According to Michaels, The Biggest Loser has already found a new female trainer to replace her and she is learning the ropes from Michaels during the show's eleventh season, which is currently in production and will premiere in January.
Michaels' departure will mark her second exit from The Biggest Loser. In 2006, she left the series following the second season due to a dispute about the show's creative direction. While she was replaced by trainer Kim Lyons for the show's third edition, she subsequently returned for its fourth season and has remained with it since.