11:48 AM PT -- According to the autopsy report -- obtained by TMZ -- St. John was discharged from a mental health facility just 2 days before his death. The report says he was admitted after threatening to hurt himself.

Kristoff St. John died from heart disease and alcohol abuse ... according to a report from the L.A. coroner's office.

The 'Young and the Restless' star's official cause of death is listed as hypertrophic heart disease and is categorized as accidental, according to the report obtained by TMZ. The disease makes it difficult for the heart to pump blood and often goes undiagnosed.

As for the "accidental" label -- that's because Kristoff was apparently on an alcohol binge at the time of his death.

We broke the story ... 52-year-old Kristoff was found dead last month at his home in the San Fernando Valley. Law enforcement sources say one of the actor's friends went to check on him and found his body. Police and paramedics responded, and we're told Kristoff was pronounced dead on the scene.

As we reported ... a source close to Kristoff tells us the actor checked himself into UCLA Medical Center in January for depression issues, and he had just been released a few days before his death. We're told Kristoff's depression was partially linked to his son, Julian's suicide in 2014. It also seems to be the likely trigger for Kristoff's heavy drinking at the time of his death.

As you know, around the 3-year anniversary of Julian's suicide in 2017, Kristoff allegedly threatened to take his own life with a gun ... and cops placed him under a 72-hour hold for psychiatric evaluation.

Kristoff had played Neil Winters on 'Y&R' since 1991 -- earning 9 Daytime Emmy Award nominations for the role. He also won 10 NAACP Image Awards.