Audrina Patridge's Niece Dead Following Her 15th Birthday: 'It Is the Hardest to Say Goodbye'

Audrina Patridge is mourning the death of her teenage niece.

On Tuesday, the Hills alum, 37, posted a carousel that featured special moments shared between her and her niece, Sadie Raine Loza, and their family over the years.

In the caption, Patridge wrote, "My heart hurts to even write this. My beautiful niece is now in heaven."

She continued, "I know [it's] not goodbye forever, but it is the hardest to say goodbye for now."

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Ending with a message for Sadie, Patridge said, "We will miss you and cherish every single moment we had with you. Rest in peace Say Say!! We love you forever & ever! ✨🤍🙏."

The reality star received support in the comments section from her The Hills costars. Brody Jenner wrote, "So so sorry for your loss Audrina. This is heartbreaking. Sending love to you and the family."

Kristin Cavallari commented, "Omgggg this kills me."

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Audrina Patridge/instagram

Actress Rachel Bilson told the star, "Audrina! I'm so so sorry! ❤️🙏 sending all the love and light to you and your family."

Ashley Greene commented, "Sending you and your family lots of love ❤️," while Kaitlyn Bristowe added, "I can't even imagine that sort of pain. I'm so sorry."

Shortly before Patridge's post, her sister — Sadie's mother, Casey Loza — shared the news of Sadie's death.

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"Sadie Raine Loza has left this Earth for the heavens," she captioned a selfie of her late daughter. "Writing this was the hardest thing I ever have had to do, not knowing the right words I'll leave you with this, Sadie liked to remind me that the first law of thermodynamics aka (Law of Conservation of Energy) works in all spheres of life…..'Energy cannot be created or destroyed, it can only be changed from one form to another.' "

She shared how Sadie's legacy will live on, writing: "Transformation is a better word than death. Her story will save countless lives, her memory will never be forgotten. Rest In Peace you brilliant starseed. You're with your creator. We will miss you dearly. I love you."

"❤️❤️😢😢," Patridge wrote in the comments section.

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Sadie's death comes shortly after the family celebrated her 15th birthday, as seen in a post shared last week.

Casey marked the occasion with a video of Sadie holding a candle-filled cake as loved ones sang happy birthday. The video ends with Sadie blowing out the candles.

"I cant believe youre 15. it feels like the years flew by and we still have so many more adventures to go on!" Loza captioned the footage. "Aliens in sedona, portals, learning to drive in the desert hahah i love you so much my little clone."

She called her daughter "so beautiful, kind, smart and funny," adding: "That dryyy af british humor tho."

"I'm so proud to be your mom and i cannot wait to see the insane ancient civilization architecture you're going to construct someday. i love you so so much my sadie raine. ❤️," she concluded.

Sadie's cause of death has not been made public.