Thomas has joined Arsenal and he sent a message to Atletico Madrid on social media.
Thomas wrote: "Dear Atletico fans, the only thing I want to say today is gratitude. In recent years, Atletico has been my home, and a part of this family will always be with me. I want to thank the club from the very beginning. Trust me, but most importantly, I want to thank you fans. I have felt your support from the beginning, no matter when things go wrong, and you will never stop believing. Atletico Madrid is more than just a family The club is still a big family. Thank you."
Thomas entered the first team of Atletico Madrid in 2015, representing the team in nearly 200 games.