Although men with low testosterone are recommended for testosterone replacement to build bone mass, the science has not yet shown to what extent the bone-building benefit is a direct testosterone effect or the result of turning testosterone into estrogen. It is important, therefore, not to minimize the role of testosterone-to-estrogen conversion. Estrogen preserves bone density in both men and women. Men normally convert testosterone to estrogen in a slow rate to build bone mass.2
Men who genetically lack an enzyme to make even small amounts of estrogen are prone to osteoporosis; this is shown by the fact that if they receive estrogen, their osteoporosis improves. So even though estrogen is not present in very high concentrations in the blood of men, it is a critical factor for bone health.