The 27-year-old singer took to her Instagram Story on Wednesday to share the news that her friend Thomas had passed away.
"Devastated. Please hold your loved ones tight," she captioned a photo of Thomas. "Tell them they are special and that you love them. Make sure they know it. RIP to my boo @sirtruss."
She went on to give her fans some insight into what led to her friend's death, writing, "Addiction is NO joke.. heaven gained this beautiful angel last night because of that terrible disease. I'm crushed and will always miss you @sirtruss. If you or someone you know is struggling please know it's okay to ask for help."
Lovato has been struggling with addiction for all of her adult life. In addition to several rehab stints, the "Skyscraper" singer also suffered a near-fatal overdose in summer 2018.
On the one-year anniversary of her overdose in July, Lovato is "grateful for her sobriety and taking it very seriously," a source previously told ET.
Lovato has also teased to fans that she's working on new music, though she hasn't revealed when she plans to drop the songs.