In a corresponding move, the team has recalled right-hander Jacob Waguespack, who was optioned earlier Thursday along with infielder Santiago Espinal as the Blue Jays cut their active roster from 30 players to 28.
Thornton, 26, has started one game for the Blue Jays so far this season, pitching four innings in a 4-1 win over the Washington Nationals on July 27. He currently sports an ERA of 2.25.
Waguespack has made three appearances this season, recording a 1.25 WHIP and four strikeouts without surrendering any earned runs.
The Blue Jays wrap up a three-game series against the Atlanta Braves on Thursday night with the first pitch at 7:10 p.m. ET/4:10 p.m. PT.