PITTSBURGH -- More than 1,000 days after suffering a severe spinal cord injury that initially left him unable to walk, Pittsburgh Steelers linebacker Ryan Shazier is officially retiring from football.

Shazier, 28, made the announcement in a video posted to Twitter on Wednesday morning.

"When I was 5 years old, I made the greatest discovery of my life," Shazier said in the Twitter video. "I discovered the game that I love -- the game of football. Ever since then, I've given my life to the game. I love everything about it ...

"Football gave me everything I ever wanted and more. It taught me about hard work, dedication, teamwork. It took me to college and the NFL. It made me money and gave me a life most people could only dream about. I'm here today to make sure the world knows how much I still love football, how grateful I am for everything football gave me. And I'm here to let the world know that today I am officially retiring from the game I love so much."

The Steelers placed Shazier, their first-round pick in 2014, on the reserve/retired list earlier this season in an administrative move that kept him a member of the organization.

"Ryan's placement on the reserve/retired list serves as a matter of protocol to ensure his continued inclusion within our organization moving forward in his professional career," general manager Kevin Colbert said at the time.

In nearly four seasons with the team, Shazier was a two-time Pro Bowl inside linebacker, finishing his career as a full-time starter in his final three seasons, with seven interceptions, seven forced fumbles, seven sacks and 299 tackles.

Shazier was rushed to a University of Cincinnati hospital on Dec. 4, 2017, after a tackle attempt against the Cincinnati Bengals on Monday Night Football left him struggling to move. He went headfirst into rookie Josh Malone and immediately collapsed. Shazier grabbed his back as he rolled onto his side, unable to move his legs.

He underwent spinal stabilization surgery a few days later, and he didn't regain movement in his legs until the following February.

"To lose the game in a way I never envisioned has not been easy," Shazier said in his video. "When you play the game of football the way I did, you convince yourself you're Superman, that nothing can stop you. But then the moment I got hurt, I stopped being Superman. That was difficult to make sense."

After months of rehab, Shazier walked across the stage with the help of his then-fiancée at AT&T Stadium in Arlington, Texas, to announce the Steelers' first-round pick at the 2018 draft.

Absolutely incredible moment as @ryanshazier announces the @steelers' 1st round pick in the 2018 #NFLDraft: #TerrellEdmunds (@rell_island6).#SHALIEVE pic.twitter.com/bXeJ6lDSyk

— NFL (@NFL) April 27, 2018

He hasn't been around the team during training camp this year because of COVID-19 protocols concerning the number of personnel with access to the team, but he continues to focus on his daily rehab, and he's taking one final online class to complete his undergraduate degree in psychology at Ohio State.

Shazier isn't sure whether his next steps include a return to the football field as a coach.

"It's kind of tough not being able to be around the team as much, just help the young guys or be around the guys that I worked around before," Shazier, who turns 28 on Sunday, said last week. "It's really tough. I'm constantly putting one foot in front of the other, trying to learn different things, trying to figure out my next step, trying to take it one day at a time."