Therefore, the aim of this trial is to evaluate the effects of selective steroid replacement on different physiological functions. In particular in this study we will evaluate the relative role of testosterone, DHT and estradiol on the different physiological functions. For these purposes, we will administer testosterone undecanoate alone (Group A) or in combination with letrozole, an aromatase inhibitor (Group B). A third group of FtM subjects will be treated with TU plus dutasteride (Group C).