The back-rower, who represented Wales 67 times and played in the 2003 and 2007 World Cups, helped guide Worcester Warriors to promotion to England's top flight last season but has hung up his boots after medical advice.
"It's with real sadness I have to announce my retirement from the game with immediate effect," he said in a statement from his club.
"I've done everything in my power to keep playing, however, there comes a point when you realise you need to listen to medical experts and also do what's best for your wellbeing.
"I'm keen to stress there are many different types of epilepsy and I'm fortunate to only suffer from it in a mild way compared to some. Naturally though, it has proved too difficult to continue as a professional athlete."
Thomas played the majority of his club career at Ospreys before joining Worcester in 2013.
The effects of concussion are a hot topic with its long-term impact on players continuing to cast a shadow over the game.
World Rugby has said player welfare is its number one priority at the World Cup which gets underway on Friday.
(Reporting by Justin Palmer; editing by Sudipto Ganguly)
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