Thomas had returned to his Bridgend home after recording a rugby programme, Scrum V, for BBC Wales which was to air later that evening. The Wales fullback was sitting watching his interview with the former Wales captain Eddie Butler when he complained of blurred vision, pain in his chest and a numbness down his left arm.
An ambulance was called and he was taken to the Princess of Wales Hospital in Bridgend. Thomas said yesterday: "I thought I was having a heart attack. I thought I was going to die."
Thomas was told by physicians at the hospital that he had suffered a migraine which had brought on a temporary numbness. Professor John Williams, the Wales team doctor, said last night: "He was assessed by the Wales medical team this morning, and he is continuing to rest at home for the remainder of the day. His progress is being monitored, and he will be examined again in the morning."
The worry for Wales is that Thomas could be suffering from stress in the aftermath of the resignation last week by the coach Mike Ruddock, who appears before the 17-man board of directors of the Welsh Rugby Union this afternoon to explain his side of his shock departure.
Thomas has been accused of fomenting unrest among the squad and there were accusations of player power - and Thomas was seen as the ringleader of a player revolt against Ruddock - which led to the demise of the Wales coach.
During his 30-minute Scrum 5 interview Thomas admitted to having had sleepless nights following Ruddock's departure, which is no way to prepare for an international match. During the interview Thomas confessed: "I've not slept for five nights worrying about the things that I've been accused of."
It was in that state of mind that he returned home to watch the programme with family and friends and suffered the migraine. He is not Wales' only fitness worry: the winger Shane Williams is also a doubt for Dublin after suffering a dead leg. But Gavin Henson, back for Ospreys after serving a seven-week ban, has officially been reinstated to the national squad and the centre Matthew Watkins has recovered from a dead leg.