A hospital source, speaking on condition of anonymity, confirmed that King Bhumibol was still being treated at Siriraj but said the royal household was expected to release further details later.
The household statement on Sunday said that the king had "fever, fatigue and loss of appetite. The medical team invited him to go to Siriraj hospital to find the cause and get medical treatment."
"His Majesty has been given intravenous fluids and been provided with antibiotics," it added.
Abhisit Vejjajiva, the Thai prime minister, said on Sunday that he was aware the king had gone to hospital but insisted it was only for a check-up, adding: "His Majesty's condition is not a problem."
King Bhumibol has long projected an image as the only unifying figure in a nation that has seen 23 prime ministers, 17 constitutions and several coups during his 63-year reign.
While remaining a constitutional monarch, he waded into politics during the 1970s and in 1992, when he ordered military dictatorships to end bloody crackdowns.
Thai children are taught that respect for the king, patriotism and religious devotion are closely intertwined, giving him an almost divine status in Thailand's Buddhist society.
But the monarchy is also protected by strict laws under which anyone who insults the royal family can be jailed for up to 15 years.
Last month the king issued the latest in a series of warnings, saying the country could "collapse" if Thailand's feuding political factions do not unite.