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.
