Reports say the 87-year-old Koch was in good spirits at a press conference Friday afternoon at the hospital. He said he was going straight back to work at a law firm.
He later told the Daily News doctors checked his heart and arteries and found he's "in good shape."
The Democrat felt weak over the weekend while staying with friends in North Carolina. He was admitted to NewYork-Presbyterian Hospital on Tuesday.
A spokesman says Koch kept tabs on developments at the Democratic National Convention in Charlotte, N.C.
Koch served three terms from 1978 to 1989. He's famous for asking constituents, "How'm I doing?"