The former Beverly Hills 90210 stars are thought to be the only actresses among the group of main female cast members that have not re-formed a friendship following an ongoing feud.
Thiessen joined the teenage soap in 1994 after the departure of Shannen Doherty and quickly forged a friendship with Garth.
However an insider told Us Weekly that the two no longer speak, recalling an incident where Garth refused to acknowledge Thiessen at a party in 2009.
The guest said: "Finally, Jennie snapped, 'Okay, you got me?' Tiffani looked sheepish and Jennie walked away. It was very awkward."
Garth recently made up with 41-year-old Doherty as well as their former co-star Tori Spelling, and she is said to be enjoying the company of her ex-colleagues during her difficult separation from estranged husband Peter Facinelli.
Facinelli filed for divorce in March after 11 years of marriage, saying that their relationship broke down as a result of irreconcilable differences.
A second insider compared their friendship to "like high school friends that you fought with, but now love. They're a big part of your past!"