The health of Thailand's ailing 88-year-old king has deteriorated and his condition is unstable, the palace said, adding to concerns about the eventual succession of the monarchy.

Doctors performed a hemodialysis on King Bhumibol Adulyadej to purify his blood and changed a tube that drains excess cerebrospinal fluid during a 2-hour procedure, the palace said in a statement late on Sunday.

"The medical team are watching his symptoms and giving treatments carefully because the overall symptoms of his sickness are still not stable," the statement said.