The state medical examiner's office determined the cause of death Thursday after an autopsy. No further details were revealed, reported USA Today.
Thaddeus, 27, was found gravely injured in his car on Wednesday in Roxbury "suffering from life-threatening injuries," according to the state police report.
He was taken to Danbury Hospital, where he was pronounced dead. There was "no criminal aspect" to the death.
The "Rosemary's Baby" star broke her silence about Thaddeus' death with a post on Twitter in which she asked fans to lend their support to the American Foundation for Suicide Prevention.
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"Thank you for the outpouring of condolences and words of kindness. If you or someone you love needs help, or if you want to support a group doing life saving work, please visit: https://afsp.Org/."
Thaddeus, a paraplegic due to polio, was adopted by Farrow in 1994 from an orphanage in Calcutta, India, following her divorce from Woody Allen.
Farrow has nine other adopted children. She also has four biological children, including Ronan Farrow whom she had with Allen as well as Fletcher Previn and twins Matthew and Sascha Previn with her second husband Andre Previn.