Thomas could not say, though, whether playing in the NFL has caused the problem.
"I definitely expect memory loss," Thomas said on "In Depth With Graham Bensinger." "I'm already seeing memory loss, and maybe that's just because of my old age or maybe it's football, it's hard to tell.
"I mean, there's no double-blind studies when it comes to people's life. It's just a part, I think, of sometimes getting older. And it's hard to tell it's because of football or because you're 32 and you're not 21 anymore and you have a lot of stuff going in your life."
What kind of memory loss? Bensinger asked.
"Short-term memory loss -- hard times remembering things that have happened recently," he said. "Like, you walk to the grocery store and you're like, 'Huh, I can't remember what I needed to get.'
"Just little stuff like that, and I think if you let it really bug you, I think it can make you depressed and feel sad. Like, 'Wow, I don't have the memory I used to have.' But, you know, I try to be relatively good-natured about it at this point."
Thomas is entering his 11th season with the Browns. He has not missed a snap since being drafted third overall in 2007. He said he is well aware of the risks of playing in terms of long-term brain damage, but they are risks he's willing to accept.
"There's definitely a concern," Thomas said. "But the way I look at it is just about every profession in our society: There's some lasting effects. It's just the way that our society is set up. People have to work. If I was a stone mason or if I was a painter or building bridges or whatever, there's going to be some wear and tear on your body and your brain. And that's just the way it is.
"To be able to live the lifestyle and provide for my family the way that football has been able to do, to me it's a trade-off that I'm willing to accept."
The NFL has agreed to a settlement of a lawsuit filed by 5,000 former players citing the damage to their brains caused by the constant trauma of NFL play. Thomas mentioned other long-term issues believed to be linked to brain trauma.
"You look at guys with significant Alzheimer's and dementia and the mood swings and the suicides that unfortunately NFL players have been faced with," Thomas said. "And depression. Lou Gehrig's disease. These are all things that have kind of been linked to the brain damage from football.
"Those are obviously scary and frightening things, but I think from my perspective, I can't do anything about it. This was the profession that I have already chosen, and most of the damage has probably been done already. So what are the things that I can do to try to minimize my chances of having those negative effects down the line, and then do everything I possibly can. Then I can't worry about it. I have to accept it.
"But I do hope that medicine continues to improve and, in 10 years maybe, they'll be able to fix my body better than they did for the poor guys who are crippled up from playing in the NFL in the '60s and the '70s."