For many years, I was aware of a seasonal change, in sex drive; so I am well aware of the complex of things, affecting hormone levels.
To answer the question of, what is TS, you need to look at a totaly different thing.
All TS have had an overwhelming feeling, from early childhood, that they were born the wrong gender. It is not hormone driven, although the hugely powerful effect of T, may contribute to suppressing it. As with most conditions, some are effected more strongly, than others and we all cope in different ways. The simple truth, however, is that ultimately, it will become so dominant, that we have to accept it, or be destroyed by it. The fall in tostesterone levels, that come in middle age, seem to be important; they, in my experience, remove the only real defence, you have, in fighting your transsexuality; a fear of becoming assexual.
In short, TS has nothing to do with hormones, but they may play a role, in suppressing it. Lberated, from that trap, as a MTF TS, oestrogen, in the form of PM, is the best thing that has happened, in my life.