The airline posted a net loss of 6.19 billion baht ($195.82 million) for the July-September period, versus a profit of 1.75 billion baht a year earlier, and a 8.4 billion baht loss in the previous quarter.
The loss was higher than the 5.6 billion baht forecast by five analysts surveyed by Reuters.
Thai Airways, facing regional competition from bigger rivals such as Singapore Airlines and Cathay Pacific Airways , has cut its 2013 revenue and profit growth target due to weak performace earlier this year.