The swimming great has been living in Switzerland for 18 months, but returned to Sydney to spend the Christmas period with his family at his parents' Panania home.

The 31-year-old was injured in a fall earlier in the week, according to media reports on Friday, and was admitted to hospital on Wednesday night.

Friends say Thorpe has struggled to adjust to life after swimming and with the disappointment of a failed comeback at the London Olympics in 2012.

Thorpe, who has been seen at social events over the New Year, including the Australian Open in Melbourne, has spoken of constant pain from an old back injury.

He released an autobiography last year in which he said he'd struggled with depression and alcohol abuse.

"Not even my family is aware that I've spent a lot of my life battling what I can only describe as crippling depression," the five-time Olympic gold medallist wrote.

He said he had also used alcohol as a way of dealing with hard times in his life.

A close friend of Thorpe said on Thursday night that they were thinking of the swimming legend.

"He had an accident at home," the friend said.

"He slipped and hurt himself.

"We're hoping he's going to be OK and make a full recovery."

Broadcaster Alan Jones, a close friend of Thorpe's, also said the swimmer had been going through some problems recently.

"Yes, it's serious but there's not a lot I can or want to add," Jones said on Thursday night.

"Ian is a beautiful person but he has difficulty recognising his problems."

Readers seeking support and information about depression can contact Lifeline on 13 11 14 or MensLine Australia on 1300 78 99 78.

AAP