Howe was missing in the weekend's victory match against Manchester United, but why he was at the hospital was not disclosed.
Now the club writes that he has been afflicted with pneumonia.
"A big thank you to everyone connected to Newcastle United and the broader football community for your messages and warm wishes. They have meant a lot to me and my family," writes Howe in a message on the website.
He also thanks the hospital staff for the care he receives.
When Howe can return to duty is unclear. Jason Tindall and Graeme Jones will lead the team against Crystal Palace and Aston Villa in the Premier League this week.
