The character of Captain Jack, when we first meet him in the Doctor Who episode "The Empty Child", is pretending to be an American, but is in reality a Time Agent from another planet. The character later stars in his own series, Torchwood.
From time to time, Captain Jack will pronounce words in the British way instead of the American way, such as "oestrogen" instead of "estrogen" in Torchwood's first episode. In-universe, this could be explained by the fact that Jack isn't really an American, but out-of-universe, is it intended that way, or is it simply an accidental consequence of Barrowman's Scottish roots?