The Welshman, who ruptured his Achilles tendon in December, has had seven operations in an attempt to return to fitness. However, the former Lyon centre has now been forced into an early retirement.
"The decision becomes a lot easier once it is taken out of your hands but it is still very difficult to accept," Thomas told reporters at a press conference.
"I would like to thank everyone I have worked with over the years, including coaches, supporters, players and my agent Christian Abt."
Thomas has also represented Cardiff, Celtic Warriors and Sale Sharks.