Thai king enters hospital for tests
The official declined to say how long the 81-year-old king, the world's longest-reigning monarch and highly revered by his people, planned to stay at the hospital.
He arrived late on Wednesday accompanied by Queen Sirikit and a few members of the royal family.
Pictures carried by newspapers showed him looking healthy as he walked into the hospital aided only by a walking stick.
"This is a personal visit by His Majesty to the hospital and that is why there has been no announcement by the Royal Household. I don't know how long he will stay here," the hospital official said.
On Monday the monarch appeared healthy as he hosted a formal reception at the Royal Palace for the visiting Malaysian king, Sultan Mizan Zainal Abidin.
In late February, he attended a function at his seaside palace at Hua Hin to welcome government leaders attending a summit of the Association of South East Asian Nations (Asean).
The quietly spoken, bespectacled king missed his traditional birthday eve address to government leaders and diplomats last December – when it had been eagerly awaited because of political turmoil in the country – because he was suffering from a throat infection.