Thomas left the Bears on Sunday and later announced his retirement. The former Washington Redskins and New York Giants player offered no immediate explanation, saying on Twitter, "I have decided to retire from the NFL."
Thomas was trying to make the squad as a reserve receiver and special teams player.
A 2008 second-round draft pick by the Redskins, he was cut in 2010 and made three catches last season with the Giants. He had 43 career receptions for 482 yards and three touchdowns and averaged 24.1 yards per kick return.
Thomas signed a one-year deal with the Bears in March.
The Bears have Brandon Marshall, Devin Hester, Earl Bennett, rookie Alshon Jeffery and free agent acquisition Eric Weems at receiver, so Thomas appeared to be battling with several inexperienced players for a sixth receiver spot.