The 35-year-old Thomas, who will have to sit out the first four games of the 2018 season, claimed it was an honest mistake.
"This is by far one of the saddest days of my NFL career," Thomas wrote. "I never thought that this would happen to me. I've worked extremely hard to do things the right way on and off the field. Panther Nation please know that I am not a cheater.
"I was informed that I tested positive for a banned substance. I was completely caught off guard by this. I've never in any way done anything to try to intentionally cheat the game. It's one of those situations where the NFL rules are clear. They state that you are responsible as a player for what you put in your body.
"I've taken the same supplements for the last seven or eight years and never had any issues. Been tested numerous times over the years while taking the same stuff. And you know, unfortunately, these are some of the things that happen when you take supplements. I've never tested positive for a steroid or HGH. It's not one of those situations. It ended up being an estrogen blocker that triggered a positive test for me."
Davis told NFL.com in January that this will be his last season, although he has hinted lately that it might not be.
A three-time Pro Bowl selection, Davis appeared in 15 of 16 regular-season games for the Panthers last season. He was suspended one game for an illegal hit on wide receiver Davante Adams of the Green Bay Packers.