Reports in Australia suggest Thorpe was admitted to hospital in Sydney on Wednesday following a fall and subsequently sought treatment for depression and issues with alcohol at a rehabilitation centre.
Alan Jones, a radio presenter and friend of Thorpe's, told Australia's Daily Telegraph newspaper: "Yes, it's serious but there's not a lot I can or want to add. Ian is a beautiful person but he has difficulty recognising his problems."
Thorpe spends much of his time based overseas but had been back in Australia to spend the Christmas period with family and attend a friend's wedding.
The 31-year-old has previously detailed his experiences with "crippling depression" in his autobiography, as well as his use of alcohol "to rid [his] head of terrible thoughts".
Thorpe won five Olympic gold medals in 2000 and 2004 but a comeback attempt ahead of the London Games was unsuccessful.
Instead he worked as an analyst BBC TV and developed a big following in the UK for his charismatic and informed analysis.
TV presenter Clare Balding, who worked alongside Thorpe in 2012, told the Guardian: "As well as being a sporting icon, Ian is such a warm, thoughtful and sensitive man. I very much hope we'll see him back to his best very soon."
Thorpe announced last year he had given up hope of competing at the Commonwealth Games and at the 2016 Olympics in Rio because of a shoulder injury.