A transgender person was someone whose gender did not align with that which they were assigned at birth.
Mickey Smith had a trans friend called Sally, who was part of his Bad Wolf band. (PROSE: Rose)
Cassandra O'Brien.Δ17 hinted to Rose Tyler that she was previously a male, stating that she was once "a little boy". (TV: The End of the World)
Jack Harkness once had a transgender co-worker, who went away and came back with a new name, Vanessa. (TV: Greeks Bearing Gifts)