The family shared a statement online saying: "We would love to thank everyone for their amazing donations and messages.
"Sadly our warrior passed away yesterday morning and is now at peace.
"We understand if anyone would like their donations back but the money raised will now be used to give our baby girl the send off she deserves and the rest of [the] money will be donated to a charity.
"Thank you for all the support."
Tributes for Alix
Those who knew the teen have sent well wishes to the "beautiful" schoolgirl who died last weekend.
On the GoFundMe page one user commented: "So sorry to read your sad news, Alix touched the hearts of many people with her story. Thoughts and condolences are with you all. The stars will shine a little brighter tonight in her memory. Rest in peace sweetheart x."
Another said: "Totally gutted to hear of alix passing, truly devastating. Thinking of you all at this sad time."
"Heartbreaking thinking of the family at this very sad time. Life can be so cruel. Fly high beautiful Alix. You touched the hearts of the nation. RIP — heaven has gained a beautiful Angel," wrote another.
One wrote: "So many people were touched by Alix and your obvious love for her. So sorry about the sad news."
What went wrong?
Last year, in October, Cassidy lost movement in her fingers.
But it was an agonising six months before doctors worked out that she had a cancerous tumour.
Her mother told The Sun that until then, the family had no clue what was happening to Cassidy.
While doctors initially said she could have arthritis, after three months on painkillers and what Cassidy's mother says was little communication, her daughter went on to see a private surgeon.
She said: "Alix's fingers went numb. She couldn't even open her Christmas presents, I had to open them, she couldn't pet her dog.
"Within 10 minutes the private doctor did a flex test of her arm and her leg and said 'that is not arthritis — that is her spine, I'm going to admit her right away'."
The teen was admitted to hospital, but in a few days lost control of her left leg. She was transported to the neurological department at Queen Elizabeth Hospital in Glasgow.
Her mother explained: "We got there at 5pm, she went for a scan at 6pm, and by 8pm I was being told it was a tumour.
"Within 48 hours of me taking her to a private doctor we were told it was a tumour on her spinal cord."
But after Cassidy underwent a biopsy, surgeons deduced it wasn't a tumour, rather it was probably an inflammatory disease.
Cassidy was then discharged and allowed to go home. But in the few weeks that followed, the rest of her body continued shutting down.
Her mother explained: "Both legs went from under her. I phoned the ambulance and within 24 hours both her arms and legs were gone, she couldn't use either of them.
"Her muscles were shutting down. The doctor was still saying 'it's not a tumour.' So neurology came back in, they blasted her through an IV with a really strong steroid for three days that had no effect.
"It's absolutely horrendous," she told The Sun.
It wasn't until May 2nd, when Cassidy went back into hospital for a biopsy, that surgeons diagnosed a cancerous tumour on her spine.
Her mother said the news broke her, and she turned to family for support: "I lost it a bit and my family had to step in and take me away until I calmed down.
"I had this oncologist come in, who we've never met before in my life, and say 'that's it, there's nothing more we can do. I can't offer you treatment. We can't offer you radiotherapy, the tumour has grown and we can't do anything for you but make you comfortable'."
Her mother explained how she had "trusted this surgeon", that they had "guaranteed" her daughter was battling an inflammation disease. "And he's spent 14 weeks battering my child's life about."
She believes a simple reflex test could have led to earlier detection of her daughter's tumour.
"There's no words for it. These are the people you put your faith and trust in and they're just, I have no words," she told The Sun.
A spokesman at NHS Greater Glasgow and Clyde told The Sun: "This is an incredibly complex case with a very difficult diagnostic process."
