No details were offered on Glenn's illness or condition, but spokesman Hank Wilson of the John Glenn College of Public Affairs told AFP yesterday that the former astronaut had been admitted more than a week ago to the university's James Cancer Hospital.
"I caution that even though Sen. Glenn is at The James that does not necessarily mean he has cancer," Wilson said.
Glenn is 95 years old and has been in declining health, undergoing heart-valve replacement surgery in 2014 and suffering a stroke, according to the Cleveland Plain Dealer which broke the story of his hospitalisation.
The former astronaut served as a Democratic senator from Ohio for 24 years after his 23-year career in the US military and space program.
More From This Section
In 1962 he was the first US astronaut to orbit the Earth in a flight lasting just under five hours, and returned to space decades later at the age of 77 -- becoming the oldest astronaut in space.
"On the morning that John Glenn blasted off into space, America stood still," Obama said during the medal ceremony.
"The first American to orbit the Earth, John Glenn became a hero in every sense of the word," the president said.
Glenn served in both World War II and the Korean War, and in 1957 he made the first nonstop supersonic flight from Los Angeles to New York.