Kristin Cavallari on Missing Brother Michael: 'We Remain Hopeful'

— -- Reality star Kristin Cavallari posted to Instagram and Twitter on Monday morning about her missing brother Michael.

"I appreciate all the concern, thoughts, and prayers that have been extended to me and my family," the former "Hills" star, 28, wrote next to a throwback photo of her and her brother. "We remain hopeful and I will share any news that we receive. I have such gratitude for all those that have shared their support."

According to the Associated Press, Utah authorities are looking for Michael Cavallari, who went missing shortly after Thanksgiving. Cavallari's car was found abandoned on Nov. 27, the Grand County Sheriff's Office reports.

Kristin Cavallari is the mother of three sons with her NFL husband Jay Cutler. She became a reality star more than a decade ago when MTV featured her in "Laguna Beach" and then "The Hills."

The Associated Press contributed to this report.