Reproductive Hormones and Skeletal Health in Young Women

In young women, low estrogen levels complicate a wide variety of diseases, including premature ovarian failure, anorexia nervosa, athletic amenorrhea, prolactinoma, hypopituitarism, and chronic kidney disease. Hypoestrogenemia may also result from therapy with GnRH agonists, glucocorticosteroids, chemotherapy, and especially aromatase inhibitors. All of these situations are associated with bone loss, because estrogen chronically suppresses bone resorption. Several mechanisms are involved: 1) estrogen increases osteoclast apoptosis; 2) by suppressing interleukins and proinflammatory cytokine expression in bone marrow cells, estrogen decreases the number of osteoclasts; 3) by inhibiting the production of receptor activator of nuclear factor-κB ligand, estrogen reduces the number and activity of osteoclasts; and 4) by increasing stromal cell/osteoblast cell expression of TGFβ, estrogen inhibits osteoclast activity. Estrogen also has some positive effects on bone formation by acting as a mitogen to cells early in the osteoblast line, reducing apoptosis of osteoblasts, and increasing expression of TGFβ, bone morphogenetic proteins, and IGF-I (1). In these pathological conditions, the cause of the bone loss is frequently multifactorial. For example, women with anorexia nervosa also have low IGF levels. Nonspecific factors such as poor diet, reduced exercise, or chronic inflammation may further increase bone resorption. Low estrogen levels are also seen in healthy young women who are using depo-medroxyprogesterone acetate (DMPA) for contraception. Bone loss is a side effect of this medication (2). The bone mineral density (BMD) decreases at a more rapid rate during the first years of therapy and then continues to decrease at a slower rate (3). After 5 yr, the loss is about 6% (0.5 sd). The DMPA effects are worse in teenagers; combined results from four prospective studies found an average loss of spine BMD of 3.1% over 2 yr in DMPA users compared with a gain of 7.2% in untreated controls (4). A cross-sectional study in this issue by Walsh and colleagues (5) focused on the age of starting DMPA. One group aged 18–25 started DMPA before age 20, and another group aged 35–45 started after age 34. Each group had matched controls. Importantly, the average use was 37 months in both groups. This avoids the confounding between age and duration of use, which is encountered in other studies. The BMD was significantly lower than controls when DMPA was started in teenagers but not when it was started after age 34. Estrogen levels were lower in the DMPA users than the non-hormone users and were also lower in the younger women than in the older women. The authors concluded that estrogen levels mediated the effect of DMPA on bone markers because adding DMPA to a regression model reduced the correlation between markers and estrogen. These correlations were not very strong (r = −0.17 between N-telopeptide and estradiol-treated), so this is a tenuous conclusion, but it is bolstered by the findings from two other studies that adding estrogen to DMPA abolishes the negative effect on the skeleton (6,7). The age range of the older group of women is underrepresented in most of the previous studies of skeletal effects of DMPA. These results lead to several questions about basic bone biology. How are adolescent bones different from mature bones? How does DMPA use compare to the natural state of low estrogen seen in healthy lactating women? Is bone completely restored in these situations? There are also important clinical questions about potential risk of fractures and the skeletal risks from other forms of contraception.