The Index lost 31.82 points or 2.14 per cent on turnover of Bt81.6 billion.
Local institutional investor net-sells were valued at Bt3.24 billion. Foreign investors remained net-buyers, at Bt626 million. Year to date, foreign net-buys were tuned at Bt3.2 billion while local institutional investors' net-sells at Bt8.59 billion.
Most of Asian markets, but Hong Kong and Shanghai, also closed lower.
Regional shares have enjoyed a strong run since the start of the month, fuelled by optimism about the state of the US economy and the prospects of borrowing costs being kept at ultra-low levels until at least December.
But the rally petered out Thursday as investors put their foot on the brake.
Asia Plus Securities reckoned in a research note that the SET Index could fall further, if plunging below 1,460 points.