Michael Kopelman, a psychiatrist who has interviewed Assange around 20 times, said the former hacker would be a "very high" suicide risk if he were extradited to the United States for leaking military secrets.
He cited as evidence Assange's "severe depression" and "psychotic symptoms", which included auditory hallucinations while in solitary confinement in his cell at the high-security Belmarsh Prison in southwest London.
Kopelman told the Old Bailey court in central London that Assange said he hallucinated music and voices saying "you are dust, you are dead, we are coming to get you".
Assange's suicidal impulses "arise out of clinical factors... but it is the imminence of extradition that will trigger the attempt," he added, warning "he will deteriorate substantially" if extradited.
Assange's partner Stella Moris has previously said she feared he would take his own life, leaving their two young sons without a father.