Quote Originally Posted by Remyisme
There's a rule for the TH sound in Russian, it gets exchanged for the letter Ф most of times, because Russian doesn't have this sound.
As far as I know it's more often replaced with "T": theory (теория), theology (теология), theta (тета), theodolite (теодолит), etc.

С заменой на "Ф" приходят в голову только имена собственные: Фома (экв. Томас), Федор (экв. Теодор), и т.п.