Thornton was optioned to Triple-A Buffalo on Wednesday, Shi Davidi of Sportsnet.ca reports.
Thornton was called up by Toronto on Aug. 7 and made four relief appearances for the big-league club since then. He struck out six while posting a 4.50 ERA and 1.17 WHIP in six innings but will head back to the minors after Ross Stripling (hip) was reinstated from the 15-day injured list Wednesday.