Audrina Patridge is mourning the loss of a family member.
In a Feb. 14 Instagram, The Hills alum shared the devastating news that her 15-year-old niece Sadie died. A cause of death has not been revealed.
"My heart hurts to even write this," Audrina began her message, which was paired with a collection of photos of Sadie over the years. "My beautiful niece is now in heaven. I know Its not goodbye forever, but it is the hardest to say goodbye for now."
Sadie's mom—Audrina's sister—Casey Patridge Loza also mourned the heartbreaking loss.
"Sadie Raine Loza has left this Earth for the heavens," she wrote. "Writing this was the hardest thing I ever have had to do."
Admitting she doesn't have the right words, Casey shared a lesson Sadie liked to remind her of often.
"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 wrote. "Transformation is a better word than death."
Casey added, "Her story will save countless lives, her memory will never be forgotten. Rest In Peace you brilliant starseed."
News of Sadie's passing comes a week after Casey shared a look into her daughter's 15th birthday celebration. At the time, she posted space a video of Sadie blowing out her candles while surrounded by loved ones.
"i cant believe youre 15," Casey wrote on Instagram Feb. 5. "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."
Not only did Casey paint a photo of their dreams together, but also of Sadie's legacy.
"Youre so beautiful, kind, smart and funny!! that dryyy af british humor tho," she continued. "im so proud to be your mom."