The 21-year-old has not featured for the Baggies in the Premier League during this campaign, and Derby boss Steve McClaren believes that it is the "ideal opportunity" for his club and the player to make the temporary switch.
The former England manager told BBC Radio Derby: "He's a very talented young player. He went to Watford and we kept a close eye on him. He has been back at West Brom for a month now and we feel at this stage, we need back-up in that midfield area.
"It's an ideal opportunity to bring him here to help us, and also for us to help him."
Thorne made eight appearances during his spell at Vicarage Road at the end of 2013.