King Bhumibol Adulyadej’s health has “overall not yet stabilized”, the Thai palace has said.
On October 9, the palace announced that King Bhumibol was in an unstable condition after receiving haemodialysis treatment.
The 88-year-old monarch is widely revered and a crowd of well-wishers have gathered outside Siriraj hospital.
A palace statement late on October 12 said the king’s “blood pressure lowered and breath quickened”.
It said King Bhumibol’s liver and kidneys were not working properly and he remained on a ventilator.
The unprecedented concern over the king’s health has caused stocks and Thailand’s currency, the baht, to tumble.
The Stock Exchange of Thailand closed down 2.5%, its lowest since the end of May.
A government spokesman urged people to rely on official announcements for updates on the situation, rather than on “uncorroborated information in social media circles”.
King Bhumibol is the world’s longest reigning monarch, having acceded to the throne when his brother died in 1946.
During his seven decades on the throne King Bhumibol, who is seen as a unifying force, has intervened when events threatened to plunge Thailand into crisis.
Amid increasing concerns about his health in recent months, the Royal Household Bureau has issued more frequent bulletins.
Earlier this month, a statement said King Bhumibol was recovering from a respiratory infection.
Thailand’s strict lese majeste laws mean public discussion of the king’s health and succession plans are not permitted, and are punishable by lengthy jail terms.