The airline posted a net loss of 5.65 billion baht for October-December against a profit of 792 million a year earlier and a loss of 6.2 billion in the previous quarter.
The result compared with a mean forecast of a 7-billion-baht loss from five analysts surveyed by Reuters.
The fourth-quarter figure brought the airline's full-year net loss to 12 billion baht versus a profit of 6.2 billion in 2012.
Shares in Thai Airways closed down 1.5% to 13 baht ahead of the earnings release, compared with a 0.2% rise in the benchmark index.
Thai Airways expects to remain in the red in 2014 due to a drop in passenger numbers as a result of prolonged political unrest, a board member has told Reuters.