Nora Quoirin died from intestinal damage likely caused by starvation and stress, and there was no foul play involved, say Malaysian police.
There was no evidence the 15-year-old schoolgirl was kidnapped or raped, police said as post mortem results were revealed on Thursday.
When Nora was still missing her family said they believed she was abducted, but police insisted she had likely climbed out of a window, wandered off from a holiday cottage and got lost in the dense jungle.
The Franco-Irish teen, who has special needs, was found dead on Tuesday, more than a week after she disappeared during a two-week family holiday at a resort south of Kuala Lumpur.
The results of a post-mortem, conducted at a hospital in the nearby town of Seremban, were announced on Thursday, and they were in line with the police theory.
Nora, who was born with the brain defect holoprosencephaly and was described by her family as "vulnerable", disappeared on August 4, a day after her family checked into the Dusun resort.
The Quoirins said Nora's condition meant she was not independent and had difficulty walking.
Describing her as a "very special person", they said she would not have gone anywhere alone.
Her unclothed body was discovered on Tuesday beside a small stream about 1.6 miles from the cottage where she was staying with her parents, Meabh and Sebastien, and her younger brother and sister.
Police said the body was found in an area that had previously been searched by rescuers.
In their most recent statement, Nora's devastated family said their "hearts are broken" and paid tribute to her as "the truest, most precious girl".
They made an emotional statement released through the trust, describing how the teenager, who lived in London and was the daughter of French-Irish parents, had "truly touched the world".
They said: "Nora is at the heart of our family. She is the truest, most precious girl and we love her infinitely.
"The cruelty of her being taken away is unbearable. Our hearts are broken."
They offered thanks to those involved in the search for the teenager, adding: "Nora has brought people together, especially from France, Ireland, Britain and Malaysia, united in their love and support for her and her family."
Hundreds of people were involved in the search operation and it was volunteer helpers who found the teenager's body.
Search and rescue teams had used helicopters and drones with thermal imaging cameras, sniffer dogs, local shamans who summoned spirits during rituals and indigenous villagers during the effort involving more than 350 personnel.
Mrs Quoirin, originally from Belfast, had previously thanked search teams in an emotional address, and her voice was played over loudspeakers as trackers went through the jungle looking for Nora, hoping she would hear it and emerge.
Nora's parents had offered a €10,800 reward for information leading to her return.
Online fundraisers had collected more than £100,000 to support the family from Balham, south-west London.
The head teacher at Nora's school in Wandsworth paid tribute to her as "a delight to work with" and said her fellow pupils and others who knew her will be supported during a difficult time.
