Mckee, 29, has also penned hits for the pop star like California Gurls', 'Wide Awake' and 'Part of Me', said she found it difficult to part with the titular song on Perry's second album, reported Contactmusic.
"I loved 'Teenage Dream'. I think that was my favourite one and that was a hard one for me to give up because I've had that teenage theme kind of throughout my music," she said.
The songwriter, who also wrote song for Britney Spears and Leona Lewis, feels that 'Teenage Dream' would not have been a hit if if Perry had not lent her voice to it.