Audrina Patridge Reveals Her Niece Died Shortly After 15th Birthday

On Tuesday, the reality star shared the heartbreaking news on Instagram, mourning Sadie's tragic passing in an emotional tribute. Patridge, 37, posted a series of photos and videos of her niece throughout the years, including several sweet shots of herself and Sadie together.

"My heart hurts to even write this. My beautiful niece is now in heaven," she began in the post's caption. "I know [it's] not goodbye forever, but it is the hardest to say goodbye for now. We will miss you and cherish every single moment we had with you."

"Rest in peace Say Say!!" Patridge added. "We love you forever & ever! ✨🤍🙏."

"The Choices" author did not reveal the cause of her niece's death.

Many of Patridge's followers and celebrity friends, including several of her "Hills" co-stars, took to the comments section to offer their condolences.

"So so sorry for your loss Audrina. This is heartbreaking," Brody Jenner commented. "Sending love to you and the family."

"Omgggg this kills me," Kristin Cavallari wrote, while Kaitlynn Carter said, "oh my goodness, I’m so sorry 😢 love you."

"🙏🏼 rest in paradise ..😢," Frankie Delgado added.

Paris Hilton also expressed her sympathies, writing, "😢 I'm sorry for your loss💔."

Rachel Bilson commented, "Audrina! I'm so so sorry! ❤️🙏 sending all the love and light to you and your family," as Kaitlyn Bristowe said, "I can’t even imagine that sort of pain. I’m so sorry."

Earlier on Tuesday, Patridge's sister and Sadie's mother, Casey Loza, announced on Instagram that her daughter had sadly passed away.

Alongside a photo of Sadie, Loza wrote, "Sadie Raine Loza has left this Earth for the heavens. 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.'"

"Transformation is a better word than death," she added of her daughter. "Her story will save countless lives, her memory will never be forgotten. Rest In Peace you brilliant starseed. Youre with your creator. We will miss you dearly. I love you."

Less than two weeks ago, Sadie celebrated her 15th birthday, according to an Instagram post Loza shared on February 5 in honor of her daughter's special day.

"i cant believe youre 15," Loza wrote alongside a video of loved ones singing happy birthday to Sadie, who blew out the candles on her birthday cake. "it feels like the years flew by and we still have so many more adventures to go on! aliens in sedona, portals, learning to drive in the desert hahah i love you so much my little clone. youre so beautiful, kind, smart and funny!! that dryyy af british humor tho."

"im so proud to be your mom and i cannot wait to see the insane ancient civilization architecture you're going to construct someday," she added, before ending her post, "i love you so so much my sadie raine. ❤️"