Bone Density: An important factor to consider
In general, the maintenance of healthy bone density in all people is partly dependent on both estrogen and testosterone levels. When an individual's body produces estrogen as its main sex hormone (as in the case of female-bodied people), that estrogen in healthy levels protects against bone loss. If a female-bodied person were to begin testosterone therapy, there would be a time of transition in the body while hormone levels adjust. While testosterone would soon become a more dominant presence in the body of a trans man, he would still retain some estrogen in his system, both through the presence of his ovaries (if he has not had an oophorectomy) and/or via the natural aromatization of testosterone into estrogen (which takes place whether or not the ovaries are still present and functional).