The king has initially gone into hospital last october to have his gall bladder removed.
Since then he has rarely been seen in public, although he did leave last week to celebrate the 65th anniversary of his coronation at a ceremony at the royal palace in Bangkok.
It is not first time he has had a long stay in hospital-he spent four years getting treatment before being released in 2013
The king's popularity stems partly from his long regin-he ascended the throne in 1946. He is also seen as a pillar of stability in thailand