The 32-year-old center said in a statement he's been experiencing symptoms that have left him feeling "drained and lethargic."
"I am working with doctors so I can better understand my condition," Toews said. "Until I can get my health back to a place where I feel I can perform at an elite level and help my team, I will not be joining the Blackhawks for training camp.
"I do not have a timetable for when I will rejoin the team. I am extremely disappointed, but it wouldn't be fair to myself or my teammates to attempt to play in my current condition."
Toews, a three-time Stanley Cup champion, scored 60 points (18 goals, 42 assists) in 70 games last season and nine points (five goals, four assists) in nine post-season games.
A shortened season -- in the wake of the pandemic-disrupted 2019-20 campaign -- is scheduled to begin on January 13, with NHL teams due to start formal training camps on January 3.
The seven teams that did not participate in the 2019-20 return to play in the league's quarantine hubs in Canada can open their camps as early as Thursday.
"Jonathan's health is our top priority as he deals with this medical issue," Blackhawks general manager Stan Bowman said. "The Chicago Blackhawks organization and our medical staff will provide all necessary resources to help him return to playing hockey."
The Blackhawks are scheduled to begin the 56-game regular season at the Stanley Cup champion Tampa Bay Lightning on January 13.