It was his second hospitalization of the year, and he has ulcerative colitis, an inflammatory disease of the lining of the large intestine, The Hollywood Reporter said. He is at Cedars Sinai Medical Center in Los Angeles.
As the show opened, host Ryan Seacrest said, "You'll notice that Casey is missing from the group. He is unfortunately sick and in the hospital right now. Wave to him. ... We know you're watching buddy, feel better. Shout out to the nurses. Get back here soon, OK?"
The show and Fox gave no details.
Later Thursday night, Abrams tweeted: "hey guys, thanks for the well wishes, I got some nice fresh blood in me and feeling better. People at Idol & Cedars are treatin me real good."
Abrams gave a well-received performance of "With a Little Help From My Friends" on Wednesday's show.