The 31-year-old was found dead in his Vancouver hotel room Saturday. No foul play is suspected.
The BC Coroners Service is performing the autopsy. There was no obvious cause of death noted at the scene, so autopsy and toxicology tests are necessary to help determine the cause of death, BC Coroners Service said.
The results are not expected for several days.
Monteith spoke openly many times about his substance abuse problems. In 2011, he told Parade magazine that he was "lucky to be alive." He checked himself into rehab in April. It was not Monteith's first time in rehab. He also received treatment when he was 19.
Outside the Vancouver hotel where Monteith's body was discovered, a makeshift memorial popped up where fans left flowers and notes commemorating the actor. #RipCoryMonteith and #StayStrongLea became trending topics on Twitter.
Meantime, authorities continue to talk to his friends, trying to piece together the last hours of his life.
The Associated Press contributed to this report.