Thomson, the last of six eligible Royals, filed for free agency on Thursday.
He was signed as a free agent last June 22 when the Royals were scrambling for a starter. He replaced Scott Elarton, who went on the disabled list.
Thomson's first start was an impressive seven innings against the Angels. He won, 5-3, using just 81 pitches in a no-walk, no-strikeout performance. However, in his next start against the White Sox, he made 87 pitches in just 3 2/3 innings of a 3-1 loss.
That was his last appearance for the Royals. He went on the disabled list in mid-July with back spasms and missed the rest of the season.
Royals who filed previously were designated hitter Mike Sweeney, catcher Jason LaRue, outfielder Reggie Sanders and pitchers David Riske and Odalis Perez.
The Royals have exclusive negotiating rights on those players until Nov. 12, and then the market doors fly open.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.