Thornton is eligible to return to the active roster following the team's Oct. 15 game against the New Orleans Saints. The defensive lineman is allowed to participate in all offseason practices and preseason games.
The 27-year-old backup defensive tackle signed a two-year, $3.3 million contract this offseason, including a $325,000 signing bonus.
Thornton appeared in 13 games in 2016, including starting six due to injuries along the Lions' defensive line. He compiled 18 tackles and one sack.
Thornton was originally drafted in the third round in 2014 by the Green Bay Packers before injury wiped away his rookie year, and he bounced around before landing in Detroit in 2015.
Thornton entered the offseason as an interior backup and on the roster bubble heading into training camp. Due to the suspension, Thornton would not take up a spot on the Lions' 53-man roster through the length of his absence -- if he makes the team. Detroit could decide to part ways after the suspension.