In the pronunciation of non-standard speakers of American English /θ/ and /ð/ may be fronted, i.e. realized as the labio-dental fricatives [f] and [v] rather than as dental fricatives.
Medial and final /θ/, as in Cathy, month, are frequently realized as [f] by speakers of popular Black English and in lower-class Southern speech generally. Speakers of these accents will also use [v] for medial and final /ð/, as in mother, breathe, smooth.
Th-fronting is also common in the speech of young children in many parts of the English-speaking world, as well as in Cockney, the popular London accent.