Thailand's 88-year-old King Bhumobol Adulyadej, who has been in hospital for a year, has a swollen lung and knees and is recovering from a fever, the Royal Household Bureau says.

King Bhumibol, the world's longest-serving monarch, has been treated for various ailments during hospitalisation in the Thai capital.

He was last seen in public on January 11, when he spent a few hours visiting his Bangkok palace.

Nervousness over his health and the succession has formed the backdrop to more than a decade of political crisis in Thailand, where the military took power in a coup two years ago.

On May 11, King Bhumibol suffered a fever, low blood pressure and swollen knee joints, the palace said on Saturday.

An X-ray taken the following day also showed swelling in his right lung.

His temperature, pulse and blood pressure returned to normal on Friday with the administration of antibiotics and oxygen, the palace statement said.

News about the royals is tightly controlled in Thailand, where laws protecting the royal family from insult make it a crime to defame, insult or threaten the king, queen, heir to the throne or regent.

Updates on King Bhumibol's health are typically made public once he is recovering.

The palace last issued a statement on his health three months ago.