The world's longest-serving monarch has been in hospital since the end of May and received treatment for several illnesses. Nervousness over his health and the succession has played into a decade of bitter political division in the Southeast Asian nation.
The king has had fever for more than two weeks and a recent X-ray showed swelling in his lung and right knee, the palace statement said. He has also received antibiotics for a blood infection, it said.
His temperature has fallen but he has had quickened breathing for some time, the statement said. The monarch was treated for similar illnesses in September.
The revered and ailing king has spent most of the last six years in hospital. He turned 88 on December 5 but was not seen publicly during nationwide birthday celebrations, although he did make a rare public appearance on December 14.
The king's health and the succession are of huge public interest but cannot be openly discussed in the kingdom because the royal family is shielded by one of the world's toughest lese-majeste laws. Crown Prince Maha Vajiralongkorn does not command the same devotion as his father.
Prosecutions of those accused of defaming the royals have increased dramatically and sentences have become harsher under the military government that seized power in a coup in May 2014.
The palace issues notices on the king's health and keeps tight control over news about the royal family.