"Following examination on Sunday night Gareth was allowed home to rest," said Wales team doctor, professor John Williams.
"He was assessed by the Wales medical team this morning (Monday) and is continuing to rest at home for the remainder of the day.
"His progress is being monitored and he will be reviewed again in the morning (Tuesday)."
Thomas had earlier admitted to feeling under "extra pressure" following the departure of coach Mike Ruddock.
Thomas and company tackle Ireland in Dublin on Sunday, with caretaker coach Scott Johnson at the helm following Ruddock's shock resignation.
"Of course, there is extra pressure," full-back Thomas told BBC Wales' Scrum V programme.
"If we are truthful, I think all the press and everyone else would like us to go there and lose because that is what they want.
"If we go there and win, everything is happy and rosy again. I've done nothing but worry for the last five nights about what I am coming back to and what I have been accused of.
"As a group of players and people we went out and won the Grand Slam. Yes, Mike was an important part in the machine, but ultimately everyone played a part in winning it, and it's like all of a sudden everyone has forgotten that.
"If we have the support we've always had and the backing we've always had, then we are capable of it (retaining the Six Nations title). If we keep fuelling the fire, I guarantee we are not."
Meanwhile, Johnson has been boosted by the return to full fitness of Llanelli Scarlets centre Matthew Watkins.
Watkins suffered a dead leg during the victory over Scotland at the Millennium Stadium, but has returned to full fitness as Johnson prepares his side to travel to Ireland.
However, Ospreys wing Shane Williams, who suffered a similar injury, has yet to return to full training and Dragons fly-half Ceri Sweeney (groin) has been ruled out by the Wales medical team for selection against Ireland.
"Matthew is taking a full part in training, but Shane will not get fully involved for a further couple of days," said Wales team physio Mark Davies.
"Ceri (Sweeney) is almost three weeks down the road to recovery from his groin injury, but is also unable to participate in full team training at the moment and he has been ruled out for the weekend's match."