Kidder, who starred as Superman's girlfriend Lois Lane opposite the late Christopher Reeve, was announced to have died as a result of a self-inflicted drug and alcohol overdose.
"Ms. Kidder's family urges those suffering from mental illnesses, addiction and/or suicidal thoughts to seek appropriate counseling and treatment," Park County coroner Richard Wood said in a statement.
"The public is encouraged to allow the family to grieve privately."
Kidder died in May at the age of 69 at her home in Livingston, Mont.
The actress suffered from mental health issues throughout her life that left her homeless in 1996.
She was diagnosed with bipolar disorder and was taken to Olive View-UCLA Medical Center after going missing for several days.
She famously portrayed Lois Lane, the determined and strong-willed Daily Planet reporter who falls in love with Superman, starting with 1978's "Superman."
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