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Walter Thurmond has decided to retire at 28. USATSI

Not many NFL players decide to retire when they have a multimillion-dollar offer sitting on the table, but that's what Walter Thurmond has decided to do. According to ESPN.com, Thurmond has decided to call it quits after six seasons.

The 28-year-old now joins a long list of players who have decided to retire early.

Over the past two years, Calvin Johnson, Marshawn Lynch, Patrick Willis, Chris Borland, Husain Abdullah, Jerod Mayo and D'Brickshaw Ferguson all decided to call it quits even though they could have conceivably contributed to an NFL team for a few more seasons.

For Thurmond, the decision to retire means leaving millions of dollars on the table. According to NFL.com, at least one team offered the defensive back a total of $4 million to sign for the 2016 season.

Thurmond's decision to retire comes just months after he finished the best year of his career, even after changing positions. The defensive back was originally drafted as a corner but was moved to safety in 2015 after signing a one-year, $3.25 million deal with the Eagles.

At safety, Thurmond started in all 16 games for Philly and led the team with three interceptions.

Before 2015, Thurmond had only started nine games in his career, and they were all at corner.

Thurmond's first year in the NFL came in Seattle, where he was a fourth-round draft pick in 2010. The defensive back spent four years with the Seahawks and won a Super Bowl before moving on and signing with the Giants prior to the 2014 season.

It's not completely clear why Thurmond decided to retire, but it's possible that concussions played a part in his decision.

Thurmond has spent the past few days tweeting stories and pictures about concussions, including the one you see above.