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The 22-year-old’s contract at Heerenveen expires in the summer and he was given permission by the club to negotiate a move to Italy.
The Norwegian has now signed a four-year deal with Sampdoria which will start from next summer.
Thorsby has had a good season so far, contributing five goals and two assists so far. Heerenveen were hoping to receive a fee for the midfielder, who has been with the club since 2014, but he will now leave on a free.