He is one of three players in the squad with more than 200 appearances alongside Stephen Jones (315 appearances) and fellow prop Phil John (229).
Vice-captain of the Scarlets team this season, he has been the cornerstone of the Scarlets pack for the past decade. Thomas started his rugby career with his hometown club of Pontypool RFC then moving to top division Ebbw Vale which was then under former Wales Head Coach Mike Ruddock.
He joined the Scarlets in 2002 and has an impressive has an impressive line of rugby honours to his name which includes 33 caps for Wales after making his debut in this country's victory against Samoa in the Autumn of 2000. He also played in Wales' 2003 Rugby World Cup squad.
His first competitive game for Llanelli Scarlets was in 2002 against Neath scoring a try on his competitive debut.
He played in his last game on the 9th of February 2012 after 18 games for the region this season. He sustained a neck injury that has kept him off the field since the early part of this year and will now force him to retire from the game, on medical advice.
He has been one of the region's most consistent performers over the past two seasons and a well-known face in European battles for the Scarlets, Iestyn has 62 European caps to his name and has huge respect in the game as one of the Elite 50 Heineken Cup players.
His form over the last two years was recognised when he was named in a League dream team in 2011 as well as being last year's Scarlets' players' player of the year with many expecting the experienced Prop to get a place in the World Cup this season. He was handed the honour of wearing the Barbarians jersey in June 2011.
Speaking about his retirement Iestyn said: "It's not easy to have to give up my rugby career but my neck injury is such now that it really is impossible to carry on - with a series of medical opinions telling me that it would be difficult for me to recover full strength even if I opted for surgery again.
"So, the advice I have been given by the surgeons is that I have to retire from the game.
"Ironically for me, I have been really enjoying playing some of the best rugby of my career in the past two seasons and it's really difficult to let go to that now.
Iestyn Thomas, who is celebrating a benefit year in 2011/2012 said: "It was a great honour to be named as the vice-captain for the Scarlets this season and stand in for Matthew while he was injured at the start of the season.
"I have also been fortunate in being awarded a benefit year this season which has kept me occupied during the past few months while the final decisions were being made about my career in discussion with the medical teams and with my family."
Thomas recalled his greatest memories. He said: "I am grateful for all the great opportunities and experiences I have had in my rugby career and there's a long list of brilliant memories. Playing for my country against New Zealand in 2003 will stay with me always, that was a great game. And then there's the win away in Toulouse in 2006 on my 30th birthday and winning the Celtic League title in 2003/4."
"I won't miss the training and conditioning on a wet Tuesday afternoon as much though," he smiled; "but the thrill of the game and competitiveness of match day has always given me the challenge to keep going despite the battering the body takes.
"I have been privileged to be part of Scarlets for such a long time and been part of the team through some real highs and played alongside some great players.
"That spirit in Scarlets rugby is still really burning - the young boys coming through have been an inspiration and energised us as senior players. It's been great to be part of and watch some of these new generation of Scarlets push forward this season both in a Scarlets jersey and in the international set-up.
"I'd like to express my thanks all my Scarlets colleagues players and coaches, past and present for what they have done to support me in my career and to the medical staff who have looked after me.
"My thanks to the great Scarlets crowds and loyal supporters who are the lifeblood of our region and follow us at Parc y Scarlets and on our away games and match the spirit of this team with their great vocal support from the stands."
As for life after rugby, Thomas says he'll concentrate on making the rest of his testimonial events a huge success and then take some time to think about what lies ahead.
"It's too early to make firm decisions now, I just want to take time to accept that I have had to retire, spend time with my wife and family and chill out a little bit this summer," he said.
"It's important to give yourself time to consider things in the future and after such an intense last few years in rugby I feel I owe that to myself and my family and friends."
Nigel Davies, Scarlets Head Coach said: "Iestyn has been immense for us - and to have such a experienced and trusted professional in your ranks with so many younger players is something that as a coaching set-up you really appreciate and rely on.
"He has given so much to our region and in recent seasons the accolades he has achieved reflect the level of consistency he has delivered in his performances. We thank him for all he has given and wish him well now in the future. Let's hope that he has a great send-off now from the game and its supporters over the reminder of his testimonial events this year."