The actress reflected on her complicated friendship with Garth on The Howard Stern Show on Monday (July 30).
Thiessen told Stern that she did not get on with any of her female Beverly Hills 90210 co-workers initially, but forged a close bond with Garth when her co-star "actually apologised" for her behaviour.
"Jennie and I became very good friends… I saw her give birth to her first child," she recalled.
The relationship eventually soured, but Thiessen was reluctant to discuss the specifics of the pair's clash.
"We had a falling-out and we grew apart… I was the one who was hurt, let's just say that. I was the one who was hurt," she insisted.
Stern pressed for further details, prompting Thiessen to reply: "It's funny, our relationship started really rough and ended the way it started.
"It's water under the bridge... I've moved on. I wish her well."
Thiessen also revealed that her friendship with Tori Spelling ended when Spelling divorced ex-husband Charlie Shanian in 2006.
"It was a hard situation for everybody. We were close with Charlie... my best friend was Charlie's roommate. It was hard," she said.
Thiessen starred on Beverly Hills 90210 as bad girl Valerie Malone from 1994 to 1998.
Watch Thiessen seducing Brian Austin Green on the show below: