Oestrogen hormone therapy given to people transitioning into female will not have any effect on the vocal cords or the vocal tract. This can sometimes be disappointing news to some transgender females because they really hope (and understandably so) that it would help to make their voice higher or a bit lighter. Unfortunately though, it simply doesn’t have that effect.
It is for this reason that trans women are really dependent on having voice therapy if they want to change or work on modifying their voice. This really does depend on the individual though as some don’t wish to work on their voice at all. The vast majority of people that come to see me however are very keen to make modifications to their voice, and so voice therapy is really the solution that you’re stuck with. There are voice surgeries too of course (and that is a subject for another blog post I think) but my general advice is always to try therapy first.