Estrogen deficiency and the pathogenesis of osteoporosis

Estrogen deficiency is a pivotal cause of postmenopausal bone loss and contributes to age-related bone loss in both sexes. At the cellular level, estrogen acts on multiple cell types, including T cells. Estrogen deficiency is associated with increased production of tumor necrosis factor by bone marrow T cells, which augments receptor activator of nuclear factor kappa-B ligand-induced osteoclastogenesis. Moreover, estrogen deficiency alters the regulatory cross talk between T cells and stromal cells, resulting in enhanced production of osteoclastogenic cytokines by stromal cells. Estrogen also has indirect positive effects on calcium metabolism and its deficiency reduces both intestinal and renal calcium absorption. In addition, estrogen is a major regulator of bone metabolism in men, and low estrogen levels are associated with increased fracture risk, even in the setting of normal testosterone levels. Thus although the causes of osteoporotic fractures are multifactorial, estrogen deficiency plays a prominent role in the pathogenesis of osteoporosis in both sexes.