A spokesman for Ali sent out a release earlier Thursday saying the former heavyweight champion is being treated at an unidentified hospital for a respiratory issue. Spokesman Bob Gunnell said that the 74-year-old was in fair condition, and that a brief hospital stay was expected.
Ali's longtime Parkinson's doctor declined comment when reached by the AP on Thursday night.
"I can't really say much more than what's in the papers," said Dr. Abraham Lieberman of the Barrow Neurological Institute in Phoenix.
Ali has been hospitalized several times in recent years, most recently in early 2015 when he was treated for a severe urinary tract infection initially diagnosed as pneumonia.
Ali has looked increasingly frail in public appearances in recent years, including on April 9 when he wore sunglasses and was hunched over at the annual Celebrity Fight Night dinner in Phoenix, which raises funds for treatment of Parkinson's. His last formal public appearance before that was in October when he appeared at the Sports Illustrated Tribute to Muhammad Ali at The Muhammad Ali Center in his hometown of Louisville, Kentucky, along with former opponents George Foreman\ and Larry Holmes.