Thorne spent the latter stages of the 2013-14 campaign with Derby and helped them to the play-off final, where they lost out to QPR.
Derby subsequently confirmed their intention to sign Thorne permanently, with the 21-year-old having failed to establish himself in West Brom's first team.
However, the Premier League club, who are currently without a manager, confirmed on Monday they had rejected an offer for Thorne, adding they are not entertaining the sale of any squad members.
Recently appointed technical director Terry Burton told the club's official website: "George has progressed through the academy into the first-team squad and we have high hopes for him.
"He underlined his potential while on loan at Derby and we have no intentions of selling him. We have told Derby he is not for sale and that is the end of the matter as far as we're concerned.
"The same goes for all of our players. We are not looking to sell anyone. Our only aim is to strengthen our squad this summer - not weaken it."
Thorne has been loaned out to four different clubs since coming through the West Brom youth set-up, featuring for Portsmouth, Peterborough United, Watford and Derby.
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