Under Ontario law, privacy exemptions keep autopsy information from public view, but Stout revealed the results of the Monday autopsy to The London Free Press, an Ontario newspaper.
"I don't know what to say," Stout told the paper. "It's not supposed to be happen to a healthy 37-year-old person. Who would think he had to get checked out for that?"
Stout also told the newspaper that funeral arrangements would be made on Tuesday.
Meanwhile, Tompkins' management team, AMR Group, is setting up a Shawn Tompkins Memorial Fund, which is accepting contributions to help Tompkins' widow Emilie with burial costs.
Tompkins shot to fame in the MMA world with a core group of three fighters who were also from Ontario: Stout, Mark Hominick and Chris Horodecki.
He also spent time training former champions like Randy Couture and Vitor Belfort.
At the time of his death, the 37-year-old Tompkins was in Ontario working with some of his younger fighters. According to those that were with him, he went to sleep after an event and never woke up.