The entertainment site quoted the Park County Coroner as saying that Kidder died from an intentional drug and alcohol overdose.
Kidder was found dead at her home in Livingston, Montana, on May 13 at the age of 69.
Her daughter Maggie McGuane said on Wednesday that it was a "big relief" to have the truth out. She believes it is important to be open and honest about suicide and not hide it. She also noted Montana's high suicide rate and the empathy she feels for the people there going through the same thing.
She continued to act in her later years but her popularity waned after she suffered a highly publicised mental breakdown in 1996. Her most recent role was in 2017 with the indie The Neighborhood.
Kidder was born in Canada and spent decades acting in movies and television shows before and after Superman, including Some Kind of Hero, The L Word and Smallville.
Both Kidder and co-star Christopher Reeve, who played Superman, were relative unknowns when they got their leading parts, and neither saw many major roles afterwards.
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A joint statement from the coroner's office and her family urged "those suffering from mental illnesses, addiction and/or suicidal thoughts to seek appropriate counselling and treatment". She was also a political activist who was arrested in Washington in 2011 at a protest over the proposed Keystone XL pipeline.
"In her last months, she was herself - same kind of love, same kind of energy", Kesich said. She wants to avoid any "cloud of shame".
"The reality of my life has been grand and wonderful, punctuated by these odd blips and burps of madness", she said.
A serious vehicle accident in 1990 left Kidder temporarily unable to work, putting her in debt.
If you're anxious someone you know may be at risk of suicide, you should talk to them about it, says the Canadian Association for Suicide Prevention.