Hilary Duff's "heart aches" for her former Lizzie McGuire co-star Robert Carradine

In a post on Instagram, she wrote: "This one hurts. It's really hard to face this reality about an old friend.

"There was so much warmth in the McGuire family and I always felt so cared for by my on-screen parents. I'll be forever grateful for that.

"I'm deeply sad to learn Bobby was suffering. My heart aches for him, his family, and everyone who loved him."

She concluded her post with a broken heart emoji.

Robert's family confirmed his passing in a statement to Deadline, which read: "It is with profound sadness that we must share that our beloved father, grandfather, uncle, and brother Robert Carradine has passed away.

"In a world that can feel so dark, Bobby was always a beacon on light to everyone around him.

"We are bereft at the loss of this beautiful soul and want to acknowledge Bobby's valiant struggle against his nearly two-decade battle with Bipolar Disorder.

"We hope his journey can shine a light and encourage addressing the stigma that attaches to mental illness.

"At this time we ask for the privacy to grieve this unfathomable loss. With gratitude for your understanding and compassion."

Keith admitted Robert's bipolar "got the best of him", but insisted there is "no shame" in mental illness.

He added: "We want people to know it, and there is no shame in it. It is an illness that got the best of him, and I want to celebrate him for his struggle with it, and celebrate his beautiful soul.

"He was profoundly gifted, and we will miss him every day. We will take solace in how funny he could be, how wise and utterly accepting and tolerant he was. That's who my baby brother was."