Bhumibol, the world's longest-serving living monarch, is considered to be a unifying figure for his country, which usually turns to him in times of crisis even though - strictly speaking - his powers are limited as a constitutional monarch.
Selina Downes, Al Jazeera's correspondent in Bangkok, said the king had been visited by the Surayud Chulanont, the prime minister, and that many people had gathered outside the hospital.
Concern over Bhumibol's health and a potential successor has intensified in recent years as the king has suffered many medical ailments, including heart problems.
He underwent spinal surgery in July after years of back pain from lumbar spinal stenosis, a narrowing of the spinal canal that can result from aging.
Bhumibol also suffered injuries including a fractured rib in June, when he fell during a walk outside the palace.
He had a hernia operation in 2003 and an operation on his heart in 1995 to open up constricted arteries. He was treated in 1997 for an irregular heartbeat.
Bhumibol celebrated his 60th year on the throne last year. More celebrations are planned for this December, when he turns 80.
In recent years, the king has reduced his appearances in public and travelled very little.