The royal household bureau said that the 81-year-old king received an intravenous saline drip and antibiotics.
Dozens of followers went to Siriraj hospital to know about the king's health condition.
Some signed a book wishing a speedy recovery to the king while others kept flowers for the monarch.
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," said a royal household bureau statement.
However, Prime Minister Abhisit Vejjajiva insisted that the hospital visit was only for a check-up.
"His Majesty's condition is not a problem," said he.