Introduction

The steady decline of serum androgen levels across the lifespan of an adult woman has been well documented. Comparisons between early reproductive age women and women aged 65–75 show mean total testosterone (T) serum levels decreased by 55% and similar reductions are seen for mean values of free T (49%), dehydroepiandrosterone sulfate (DHEA-S) (77%), and androstenedione (A) (77%) [1]. While ovarian aging and the complete cessation of ovarian estradiol (E₂) synthesis produces the dramatic fall in E₂ levels that defines menopause, there is no evidence that the onset of natural menopause influences androgen levels. Rather, the postmenopausal ovarian hilar and stromal cells continue to respond to luteinizing hormone and secrete androgens. A 15-fold difference for T and a fourfold difference for A were noted between ovarian and peripheral veins in postmenopausal women [2], and in four out of five women a gradient for T still appeared more than 10 years after onset of menopause [3]. Additionally, women who underwent surgical menopause with hysterectomy and bilateral oophorectomy had a 40–50% lower serum level of total T and free T than those women with intact ovaries [4]. Other cross-sectional studies have added to the observation that women who had bilateral oophorectomies have significantly lower free T levels [1].

The mechanism of adrenal aging is less well understood, but the age-associated gradual, but marked decline in circulating dehydroepiandrosterone (DHEA) and androgen metabolites has been confirmed by Labrie et al. [5]. DHEA combined with its sulfated conjugate, DHEA-S, is present in the highest amount of all synthesized steroids and is almost totally supplied from the adrenals. While no specific action of its own has been proven, in peripheral tissue DHEA can be converted into active forms of both estrogens and androgens. This conversion plays an important role for postmenopausal women, because it is estimated that 90% of estrogen in these women is derived from peripheral conversion of DHEA. While there is no abrupt cessation of DHEA production mirroring estrogen levels and the onset of menopause, an age-related decline in adrenocorticotropic hormone (ACTH) levels reduces both the amplitude and frequency of DHEA release from the adrenal glands. The result is DHEA levels decline substantially with age as women transition through menopause and beyond. Additionally, the efficiency of peripherally converting exogenously supplied DHEA to androgens and estrogens also decreases after menopause. The approximately 60% or greater decline in serum DHEA and DHEA-S levels associated with menopause may contribute significantly to the development of conditions generally associated with the aging process in women.

Androgen receptors in women have been identified in the ovary, breast, brain, liver, muscle, bone, fat, skin, vulva, and vagina; and for over 60 years the potential benefits of androgen therapy in women’s healthcare have been studied [6]. A primary focus has been the treatment of postmenopausal women suffering from low libido and other sexual dysfunctions, but other considerations include the preservation of bone mass, muscle strength, vitality, and the alleviation of vasomotor symptoms and vulvovaginal atrophy. In 2002, a panel comprised of experts from the USA and Australia produced the Princeton consensus statement, which supported the diagnosis of female androgen deficiency syndrome, identified by the concurrent presence of low androgen levels and low libido, and to a lesser extent diminished vitality and general well-being [7]. Controversies arose, as clinical and scientific challenges to this diagnosis exist based on the uncertainty of what are the normal androgen values by age, the lack of a diagnostic lower limit for any of the androgens that can be defined as androgen deficient, the inconsistency amongst the various assays, and the insensitivity of most assays when detecting low levels of hormones. Additionally, there is great variability in sex hormone-binding globulin (SHBG) levels and their impact on bioavailable or free T. Guidelines published in 2006 by the Endocrine Society recommended against making a diagnosis of androgen deficiency in women for many of these reasons [8].

The anabolic effects of androgens are well established, and gender-based variations in serum levels likely play a significant role in the marked sexual dimorphism that exists in the development and function of muscle, fat, and bone. While the greatest interest for androgen therapy has been to improve or restore libido and reduce symptoms of sexual dysfunction, additional benefits for postmenopausal women may include preservation of bone mass, lean body mass, muscle strength, and general quality of life. Scientific research, including randomized and observational clinical trials and real-world experience with the use of various formulations of testosterone and DHEA for postmenopausal women, has added to our understanding of the therapeutic role for the androgens, and also provided some much needed safety data to address concerns regarding these treatments, including their potential impact on the endometrium and breast.

Bone

Androgens clearly play an important part in bone physiology. Androgen receptors are found in osteoclasts, osteocytes, and most abundantly in osteoblasts [9]. Postmenopausal women with hip fracture were found to have significantly lower free T levels and higher SHBG than age-matched [10] women not in menopause. A large cohort study also showed that low free T increased the risk of hip fractures in women 65 years or older [11]. Barrett-Conner et al. reported that adding T to estrogen increased bone mineral density (BMD) in hip and spine [12]. It is worth noting that the greatest effect of this treatment was in women with surgical menopause.

Most studies of the use of T treatment in postmenopausal women have incorporated this therapy with estrogen support. This addition of T consistently increased bone density. Both Watts et al. [13] in 1995 and Raisz et al. [14] in 1996 showed in double-blinded, placebo-controlled trials that the addition of 2.5 mg per day of methyl T improved bone formation [14] and lumbar spine BMD [13] greater than the treatment of oral conjugated estrogens alone.

Tibolone, a compound simulating estrogen, progesterone, and T with androgenic activity, is not available in the United States but is widely used in Europe. In all studies it prevented bone loss, and even increased bone density in postmenopausal women. In addition, it had excellent suppression of hot flashes. In a study of symptomatic women post-breast cancer, tibolone significantly decreased flushes compared with placebo with a P value of 0.004 at 4 weeks, and a value of 0.0001 at 8 weeks. In a study of osteoporotic women, compared with placebo, tibolone increased both spine and hip bone density. Of great interest is that in the study of the breast cancer survivors, there were more breast cancer recurrences than with placebo. However, in the osteoporotic study, there were fewer new breast cancer cases in the tibolone group than in the placebo group [15].

It had previously been reported in placebo-controlled studies that 50 mg/day of DHEA given to postmenopausal women increased BMD in both the hip and spine [16]. However, the carefully done study by Jancowsky et al. showed these significant increases in hip BMD in older adults undergoing DHEA replacement were mediated primarily by increases in serum E₂, rather than direct effects of DHEA-S [17]. Whatever the mechanism, DHEA does support bone.

Muscle and lean body mass

As important as bone preservation is in postmenopausal woman, there increasing attention is being paid to sarcopenia, the loss of muscle and muscle strength that often begins after menopause and leads to many of the physical disabilities associated with aging. Decreasing levels of T and DHEA add to the absence of estrogen in causing the onset of sarcopenia. DHEA may affect muscle performance as skeletal muscle can convert it to active androgens and estrogens. DHEA also induces the formation of insulin-like growth factor-1, which is important in muscle growth and recovery [18]. There have been several studies of the treatment of postmenopausal women with DHEA to increase their muscle strength. Only when exercise was added to the regimen did muscle strength improve. However, DHEA plus exercise was more successful than exercise alone.

Surprisingly, there have been few studies of T supplementation for muscle development in women; and similar to bone, these studies have been mostly limited to combination estrogen and T treatment. Several randomized controlled studies have confirmed an improvement in lean body mass [19], but contrary to this, in a previously cited study by Davis et al., the addition of T reversed the estrogen effect of decrease in central body fat and resulted in a decrease of total body fat-free mass [20]. The mechanism of this action may be similar to that of progestins, which decrease estrogen’s ability to increase insulin sensitivity.

Recent interventions with intermittent T therapy rather than continuous treatment have been shown to be effective in increasing muscle protein synthesis and lean body mass without an increase in side effects in both young athletes and older men [21]. Perhaps this paradigm should be tried in the prevention of sarcopenia in older postmenopausal women.

General quality of life

Androgen receptors have been identified throughout the brain, and the effects of androgens on the brain are mediated through these receptors, but also via the aromatization to E₂ and subsequent E₂ receptor-mediated actions, notably in the hypothalamic and limbic systems. Androgen effects in the brain influence sexual behavior, libido, temperature control, sleep control, assertiveness, cognitive function, and learning capacities, including visual-spatial skills and language fluency. The impact of the postmenopausal hypoestrogenic state on the brain has been more extensively studied, but many brain functions also suffer from the falling androgens levels associated with advancing age. Clinical studies, mostly conducted prior to the initial findings of the Women’s Health Initiative (WHI), showed that hormone therapy with estrogen plus androgens provided greater improvement in psychological and sexual symptoms than did estrogen therapy alone for both naturally and surgically menopausal women [22].

The objective of the Study of Women’s Health Across the Nation (the SWAN study), a community-based cohort study of 2961 women aged 42–52 years, was to “assess the association between androgens and a variety of end points thought to be affected by androgens.” Depressive symptoms were assessed using the 20-item Center for Epidemiologic Studies Depression Scale (CES-D). The Ladder of Life scale was used to assess global quality of life, and physical functioning was measured using the SF-36, a subscale of the Medical Outcome Scale (MOS-SF-36). Circulating testosterone, DHEA-S, and SHBG were measured and a free androgen index (FAI) was calculated. A positive association, at times achieving statistical significance, was noted between levels of T, DHEA-S, and the FAI for higher physical functioning, higher quality of life, and better self-reported health, but a negative association was also noted for CES-D depressive symptomatology [23].

T is almost never used as a primary therapy for symptomatic postmenopausal women, and rarely are general quality of life parameters a primary objective of the studies. In 2011, Glaser and her group reported on the treatment of 300 women, 163 of whom were postmenopausal. They used T pellet implants for one year in doses dependent on weight. The dose range was 75–160 mg with a mean subcutaneous dose of 121 mg. They measured symptom improvement using both the health-related quality of life scale (HRQOL) and the menopause rating scale (MRS). Remarkably, significant improvement was shown for all patients for all 11 categories rated. These included hot flushes, sweating, vaginal dryness, and urogenital complaints, and the higher the dose, the greater the improvement. Although there was no placebo control, the correlation of improvement in symptoms relative to the dose argues against placebo effect. It is also noteworthy that 98% returned for treatment after the study was complete [24].

Many recent studies have focused on the possible beneficial impact of androgen therapy on women with hypoactive sexual desire disorder and related psychological distress. A consistent improvement has been noted for diminished personal distress, but it may be difficult to extrapolate these findings to a more general population and a broader beneficial impact on health-related quality of life.

Endometrium

Androgens have two theoretical pathways to influence the endometrium; directly through androgen receptors and indirectly through peripheral enzymatic conversion by aromatase into E₂. While the ability of the endometrium to respond to either of these pathways has been questioned or minimized, in vitro research supports both as possibly of concern. Immunostaining of androgen receptors in both stromal and endometrial cells has been demonstrated [25].

Tuckerman et al. conducted in vitro studies in fertile women, investigating the effects of A, T, dihydrotestosterone (DHT) and DHEA [26]. A was shown to have a direct inhibitory effect on the endometrium, a finding that was further supported by the ability of the androgen receptor antagonist cyproterone acetate to reverse this inhibition [26]. No significant effect on the endometrium was noted for T, A, and DHT. The ability of aromatase to convert circulating androgens into E₂ and the stimulatory effect of estrogens on the endometrium are both well recognized. In the endometrium of disease-free patients, gene transcription of aromatase is inhibited and this enzyme is not expressed, eliminating concerns of a pathway toward hyperplasia [27]. However, increased endometrial aromatase activity has been confirmed in gynecologic conditions, including endometriosis, adenomyosis, and fibroids and is thought to play a significant role in the pathophysiology and progression of these diseases. Additionally, the conversion of A to estrone and E₂ was significantly increased in endometrial neoplastic cells [28]. However, the aromatization of androgens appears to have minimal growth-promoting impact on well-differentiated endometrial carcinoma [29].

Several studies have investigated the possible association in postmenopausal women between endogenous serum levels of the various androgens and the risk for endometrial hyperplasia/carcinoma, and have produced conflicting data. Potischman et al. reported significantly higher levels of A and lower levels of SHBG in women with endometrial cancer [30]. In a case-controlled study of 124 postmenopausal women not on hormonal therapy, Lukanova et al. first reported a weak link between androgen levels and endometrial cancer, but after adjustments were made to account for E₂ and estrone levels, the risk was no longer significant [31]. Boman et al. also showed no relationship between serum A levels and proliferation activity in women with endometrial cancer [32].

Various formulations of T have undergone clinical development, either for vasomotor symptom relief or for the treatment of hypoactive sexual desire disorder (HSDD), and their impact on the endometrium has been evaluated. A 6-month, double-blinded randomized trial comparing the effect on endometrial histology of 0.625 mg of oral esterified estrogen, with or without 1.25 mg of methyltestosterone (without progestins), showed estrogen stimulated proliferation in both groups [33]. Zang et al. conducted a randomized, unblinded trial with women receiving either oral T undecanoate (40 mg every second day), E₂ valerate (2 mg daily), or both for 3 months [34]. Endometrial thickness was measured by ultrasound and endometrial proliferation was assessed by both histology and the expression of Ki-67, a nuclear antigen only expressed by proliferating cells. Endometrial thickness was significantly increased in both the estrogen alone and estrogen plus T arm, but no increase was noted in the T alone arm. Similar findings were noted for endometrial histology, where there was a statistically significant 50% increase in evidence of proliferation in the estrogen alone arm, a non-significant 28% increase for the combination therapy, and no increase for those treated with only T. In the 52-week APHRODITE study, 814 women with HSDD were treated with either 150 µg or 300 µg of patches of T or placebo [35]. While 10.6% of the women on the higher dose of T experienced vaginal bleeding, there were no reported cases of hyperplasia or carcinoma in any of the subjects. The INTIMATE study of the T patch reported no increased risk for vaginal bleeding [36].