Because steroids are lipophilic, they diffuse easily through the cell membranes, and therefore have a very large distribution volume. In their target tissues, steroids are concentrated by an uptake mechanism which relies on their binding to intracellular proteins (or " receptors ", see below). High concentration of steroids are also found in adipose tissue, although this is not a target for hormone action. In the human male, adipose tissue contains aromatase activity, and seems to be the main source of androgen-derived estrogens found in the circulation. But most of the peripheral metabolism occurs in the liver and to some extent in the kidneys, which are the major sites of hormone inactivation and elimination, or catabolism (see below).
Finally, there are instances when a progesterone-based hormone therapy might be used in the treatment of trans men. Progesterone may be used in some instances to help stop menstrual flow if testosterone therapy alone does not adequately stop the cycle after a reasonable period of treatment. A short course of progesterone may also be prescribed to induce a shedding of the uterine lining after testosterone therapy has progressed, in the event that there is any unusual buildup of the endometrium. This may help prevent spot bleeding as well as potentially decreasing the risk of uterine cancer. To learn more specifics about FTM testosterone therapy, please read the sections " FTM Testosterone Therapy Basics " and " FTM Testosterone Therapy and General Health ."