The health of Thailand's King Bhumibol Adulyadej, the world's longest reigning monarch, has "overall not yet stabilised", the palace has said.
On Sunday the palace announced that the king was in an unstable condition after receiving haemodialysis treatment.
King Bhumibol, 88, is widely revered, and a crowd of well-wishers have gathered outside Siriraj hospital.
Prime Minister Prayuth Chan-ocha cancelled an official engagement to meet the Crown Prince.
A palace statement late on Wednesday said the king's "blood pressure lowered and breath quickened".
It said his liver and kidneys were not working properly and he remained on a ventilator.
The unprecedented concern over his 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".