Park County coroner Richard Wood said in a statement released today that Kidder "died as a result of a self-inflicted drug and alcohol overdose" and that no further details would be released.
But she didn't say anything until the coroner released an official statement.
Her obituary in the New York Times did not specify a cause of death.
Her daughter Maggie McGuane, 41, told the wire service that she already knew that her mother, who had battled bipolar disorder during her lifetime, had committed suicide.
Canadian-born Kidder had been a mental health advocate during her life with her own struggles well-documented in the media. Her obituary at the time said simply that she passed away at her home, and does not mention her cause of death. She wants to avoid any "cloud of shame". Additionally, ambulances were called to Kidder's home five times in seven months, including at the time of her death.
McGuane noted that Montana has one of the highest suicide rates in the nation and she urged people with mental illness to seek help.
The rumors that she had died from flu complications began because of her phone interview with The Drew and Mike Show in Detroit, just four days before she died.
McGuane continued, "It's a very unique sort of grief and pain".
Kidder struggled with mental illness for a big chunk of her life, with events getting worse following a vehicle accident in 1990 that left her in debt and led her to using a wheelchair.
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.