Kristin Cavallari is heartbroken.

The former Laguna Beach star mourned the death of her brother, 30-year-old Michael Cavallari, with a sweet throwback photo and heart-wrenching caption on her Instagram account.

WATCH: Kristin Cavallari's Brother Michael Found Dead Days After Reported Missing 

"My heart is in a million pieces," the 28-year-old wrote. "Mikey, I could always count on you to make me laugh. You had such a good heart and will be forever missed. I'm at a loss for words but I know u are in a better place and finally at peace. I love u so much RIP."

WATCH: Kristin Cavallari's Brother: A Timeline 

Michael's body was discovered on Thursday around 10:30 a.m. in Grand County, Utah, according to the local Sheriff's Office, in an area with "steep and very rocky" terrain. Amid a missing person investigation earlier this week, an officer from the Grand County Sheriff's Office told ET that a 2014 Honda Civic registered in Michael's name had been found abandoned on Nov. 27.

Two days prior, on Nov. 25, Michael had been released from jail after making bail. The Orange County Sheriff's Department in California told ET that he was arrested for making criminal threats with a shotgun on Nov. 23. Coincidentally, Kristin and her husband, Chicago Bears quarterback Jay Cutler, welcomed their third child -- daughter Saylor -- on that same day.

WATCH: Kristin Cavallari's Brother Was Arrested With a Shotgun Prior to Disappearance 

"We want to thank everyone for their love, support and prayers during this very difficult time. We have just been informed by the authorities my brother's body has been found," Kristin said in a statement to ET on Thursday. "This is a very painful time and we are still processing it all. We kindly ask everyone please respect our privacy during our time of grieving."