The big lock initially joined Glasgow in August 2010 on a two-month loan from Saracens.
Tom returned to Saracens after impressing in his loan period at Firhill but returned north in February 2011 after agreeing a two-year contract with the Warriors.
The 25-year-old has represented England at every age-group level up to U21s but is also Scottish qualified through his Glaswegian father Paul. He made hi Scotland A debut in 2010.
A product of the Leicester Tigers academy who has also played provincial rugby in New Zealand with Taranaki, the lock forward joined Saracens five years ago and quickly established himself as a regular member of their squad. He has made close to 100 appearances for the club.