CHI & Partners, which had been handling Heart's advertising, has ceased working for the station.
CHI has worked for Heart for three years and was responsible for its current £1.5m TV, digital and cinema campaign, which features inflatable people springing to life whenever they hear the station's music.
The review follows the departure of Heart managing director Barnaby Dawe and head of marketing Tina Finch, following a restructure. The group is in the process of appointing a head of marketing across the station.
Global Radio, which is chaired by former ITV chief executive Charles Allen, has also axed management in some of its other stations, including Galaxy and LBC.