The 21-year-old will now remain at Whaddon Road until the end of the season having originally signed a one-month loan deal two weeks ago.
Due to Cheltenham's postponed fixture at home to Darlington on Saturday Thornhill has only played in the 4-1 defeat by Rochdale.
He has been at the City Ground since 2007, making 38 league appearances.
Source: BBC Sport
Source: BBC Sport