BANGKOK (REUTERS) - Thailand's King Bhumibol Adulyadej, the world's longest-reigning monarch, has been treated for a swollen intestine and remains in hospital after surgery to remove his gall bladder in October, the palace said on Tuesday.

The health of the King, 86, is a subject of keen public concern as he is revered by many Thais as a pillar of national unity and morality especially in times of crisis. His portrait hangs in every government building and in many shops and homes.

Doctors found the swelling in a scan after the King suffered stomach pain, a fever and his breathing quickened, the palace said in a statement.

The King received antibiotics and his fever has come down, it said.

The King, who was admitted to Siriraj Hospital in Bangkok on Oct 3, may continue to experience occasional swelling, the statement added. He was first diagnosed with the condition that causes the swelling in 1999, it said, and has had several recurrences since then.

Gall bladder removal is a common surgical procedure, usually carried out to relieve painful gallstones, and does not cause complications for most patients.

The King's health has formed part of the backdrop to a complex crisis driven by Thailand's rival business and political elites. He is widely seen as a moral arbiter among Thais.

Bhumibol, a constitutional monarch, made several interventions in the 1970s and 1990s to call for calm during political upheaval. He was silent during the latest crisis, which began in November and culminated in a coup in May.

The King was discharged from hospital in September after nearly five weeks of treatment for stomach inflammation.

He left the same hospital in 2013 after spending nearly four years there in a special suite after being admitted in 2009 for a lung infection.