The king is the object of an intense personality cult and his frail health is a subject of significant public concern.
He has spent most of the past two years hospitalised for a series of ailments, with very few public appearances and speeches.
The Royal Household Bureau said in a statement today that he had a fever, low blood pressure, and inflammation in his right lung and knees earlier this week.
His temperature and blood pressure have since returned to normal but doctors are "still administering antibiotics to mitigate the inflammation in his knees, as well as medicine to dissolve phlegm," the statement said.
Bhumibol is the world's longest-reigning monarch and most Thais have never known life under another king.
Anxiety over what will happen after his reign comes to an end is considered an aggravating factor in the country's past decade of tumultuous politics.
Bhumibol last left Bangkok's Siriraj hospital in early January for an hour-long trip by car to visit his palace and other royal projects, according to palace officials.
He was last seen in an official photograph distributed in mid-December.
The kingdom has one of the world's strictest lese majeste laws, punishing any criticism of the monarchy with up to 15 years in prison.