I'm unsure what you mean by this... English <th> is inherited from Proto-Germanic, not Greek.

Also, did you mean Greek θ (theta)? θ started as aspirated /tʰ/, and it's a pretty common sound change for aspiration to become a fricative, in this case /θ/. φ also made a similar journey from /pʰ/ to /ɸ/ to /f/.