Former Baltimore Ravens left tackle Eugene Monroe announce his retirement Thursday morning on The Players Tribune.

He wrote: "Today, I am making my exit from the game of football. I'm leaving the sport I love -- the sport that has consumed my existence for the last 18 years -- to start a new life. I'm nervous, but I feel like I'm ready for whatever comes next. I have three beautiful children and an amazing wife, and they will be with me as I take these next steps."

Monroe mentioned the "never-ending cycle: injury, shake it off, 'recover,' repeat" as a factor in his decision. And he cited concerns about head trauma in this chilling passage:

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I'm only 29 and I still have the physical ability to play at a very high level, so I know that my decision to retire may be puzzling to some. But I am thinking of my family first right now -- and my health and my future. The last 18 years have been full of traumatic injuries to both my head and my body. I'm not complaining, just stating a fact. Has the damage to my brain already been done? Do I have CTE? I hope I don't, but over 90% of the brains of former NFL players that have been examined showed signs of the disease. I am terrified. My wife used to joke about the "little things I forget," but now she's more concerned about things like me putting my phone in the freezer and then tearing up our house looking for it. Things like that were just a joke around the house until this past winter, when my four-year-old daughter said, "Daddy you don't remember anything!" Since then, she's said it a few more times. That's why one of the first things I'm planning to do is to go to the doctor. I need to take stock of my current health, and I don't want to miss a thing. I'm going to get brain and body imaging scans, mental health assessments -- anything that might help me get a handle on the state of my body and my mind. My health is critical to the future of my family.

Monroe, 29, had season-ending shoulder surgery in December and was released by the Ravens in June.

Following his release from the Ravens, Monroe speculated that his advocacy for medical marijuana factored into the organization's decision to part ways with him. He has campaigned for medical marijuana to be taken off the NFL's banned substances list and be cleared as an alternative to medication for pain management.

— Eugene Monroe (@MrEugeneMonroe) June 16, 2016

Monroe was the eighth overall pick in 2009 by the Jacksonville Jaguars, for whom he started until he was traded to the Ravens in October 2013.