The multiple actions of testosterone in men: nature knows best

In male hormone replacement therapy Finkelstein et al. show that testosterone rather than synthetic "pure" androgens should be prescribed. Testosterone is converted to the superactive androgen dihydrotestosterone and to estradiol, and thus has actions via androgen receptors and both estrogen receptors (ERα, ERβ). Although muscle strength is androgen dependent, estradiol has major physiologic effects in men-on bone, cartilage, and together with androgens, on sexual functioning. Neither dihydrotestosterone nor 'pure' synthetic androgens can be converted to estradiol; those so treated thus risk missing out on the beneficial (and necessary) effects of estrogens in men.