The 86-year-old king, revered by Thai people, had developed a fever and experienced pain in the abdomen, while his breathing was slightly faster, a statement issued last night Royal Household Bureau said.
"Results from the blood tests and a stomach X-ray found inflammation to his colon," the statement said.
"Doctors treating the monarch at a Bangkok's Siriraj hospital prescribed antibiotics to reduce an accompanying fever", it added.
Doctors conducted an electrocardiogram which showed no irregularities.
The doctors then prescribed antibiotics to treat the swelling, the statement said adding the monarch's fever had eased.
The king was rushed to the hospital in early October from his coastal palace. He underwent an operation to remove his gall bladder after tests showed the organ was swollen.