The 26-year-old actress has played nurse Maria Kendall on the BBC One medical drama since late 2006.
Reflecting on her time with the programme, Thomas commented: "I've loved every minute, it's a fantastic show to be on. The cast are wonderful and you get to work with some fantastic directors.
"The executive producer has really taken the show in a great direction, but I think you do have to move on eventually."
The role of Maria has been Thomas's most high-profile part, however she has also appeared in Teachers, Family Affairs and The Bill.
Thomas's character will remain on screen in Holby City until the summer.