Wiebke Arlt
Division of Medical Sciences, Institute of Biomedical Research, Endocrinology, Room 233, University of Birmingham, Birmingham, B15 2TT, UK
Correspondence should be addressed to WArlt; E-mail: [email protected].
Abstract
Androgens in women either derive from direct ovarian production or from peripheral conversion of the adrenal sex steroid precursor, dehydroepiandrosterone, towards active androgens. Therefore, loss of adrenal or ovarian function, caused by Addison’s disease or consequent to bilateral oophorectomy, results in severe androgen deficiency, clinically often associated with a loss of libido and energy. Importantly, physiological menopause does not necessarily lead to androgen deficiency, as androgen synthesis in the ovaries may persist despite the decline in estrogen production. However, the definition of female androgen deficiency, as recently provided by the Princeton consensus statement, is not precise enough and may lead to over-diagnosis due to the high prevalence of its diagnostic criteria: androgen levels below or within the lower quartile of the normal range and concurrent sexual dysfunction. Importantly, physiological menopause is not necessarily associated with androgen deficiency and therefore does not routinely require androgen therapy. Current replacement options include transdermal testosterone administration or dehydroepiandrosterone treatment, both of which have been shown to result in significant improvements, in particular in libido and mood, while effects on body composition and muscular function are not well documented. It is important to keep in mind that the number of randomized controlled trials is still limited and that currently none of the available preparations is officially approved for use in women. Currently, androgen replacement should be reserved for women with severe androgen deficiency due to an established cause and matching clinical signs and symptoms.
Introduction
The apparent link between male hormones and female sexuality has recently fuelled interest in androgen therapy in women. Female androgen deficiency has quickly evolved as a center-stage topic, both in the perception of the scientific community and, importantly, of the lay audience. This development comes shortly after the results of the Women’s Heatlh Initiative have brought the concept of postmenopausal estrogen/progestin replacement to a standstill (1). However, in contrast to the proven and definitive loss of ovarian estrogen synthesis in physiological menopause, it is far from certain that the ovary invariably loses its androgenic capacity during the menopausal transition. The gap between fact and fiction in female androgen physiology is still broad and fashion terms like ‘female androgen deficiency syndrome’ and ‘hypoactive sexual desire disorder’ have provided little help with a precise definition of female androgen deficiency, but rather have contributed to a blurring of the lines. Clinical experience with androgen therapy in women is still limited, in particular with regard to randomized controlled trials.
The federal drug agency (FDA) recently denied approval of an androgen replacement tool for women because of concerns about the paucity of long-term safety data. This review seeks to summarize currently available information on androgen therapy in women and admits from the outset that there are several hitherto unresolved issues.
Physiology of female androgen production
In humans, the adrenal glands and the ovaries represent the main sources of circulating androgens in women. The adrenal steroid dehydroepiandrosterone (DHEA) represents the crucial precursor of human sex steroid biosynthesis. DHEA and its sulfate ester (DHEAS) are the most abundant steroids in the human circulation. DHEA is mainly released from the adrenal zona reticularis (2) and only desulfated DHEA, but not DHEAS, can be converted downstream towards sex steroids. Recent evidence suggests that hydrolysis of DHEAS to DHEA may be restricted to some peripheral tissues including prostate and mammary gland while the rate-limiting step regulating the equilibrium between DHEA and DHEAS will be DHEA sulfotransferase activity, converting DHEA to DHEAS (3). A significant amount of the total androgenic pool derives from androgen synthesis within peripheral target cells of androgen action. In addition, DHEA may serve as a prohormone for ovarian androgen synthesis (4). Figure 1 represents a schematic overview of androgen synthesis in women. Transient adrenal suppression by dexamethasone in healthy young women leads to a 90% decrease in circulating DHEA and DHEAS and also reduces circulating testosterone and dihydrotestosterone (DHT) levels to 3040% of their respective baseline levels (5). DHEA, DHEAS and androstenedione do not have androgenic activity unless they are converted to testosterone and DHT, which can both bind and activate the androgen receptor. Testosterone can be converted either to DHT, which has a five times higher binding affinity to the androgen receptor or it can be aromatized towards estrogens. DHT cannot be aromatized. Therefore, it is important to realize that an increase in the circulating testosterone pool will invariably be associated with increased estrogen generation within peripheral target tissues of sex steroid action. In women, significant androgen production physiologically starts during adrenarche, i.e. the increase in adrenal DHEA and DHEAS production from previously non-detectable levels occurring between the 6th and 10th year of age. This leads to an increased conversion of DHEA towards active androgens in peripheral target cells like the skin, and characteristically results in the first appearance of pubic hair (‘pubarche’). Adrenarche is independent of the onset of menarche. In girls who do not undergo adrenarche, ovarian maturation and folliculogenesis are not affected, but circulating androgen levels invariably remain low. The intraindividual maximum of DHEA and DHEAS production is reached in early adulthood, followed by a steady decline throughout adult life, eventually decreasing to 10 - 20% of previous maximum levels by 70 to 80 years of age (6-8). This age-associated decrease has been termed ‘adrenopause,’ in spite of the fact that adrenal glucocorticoid and mineralocorticoid secretion rates are maintained without change throughout one’s lifetime. Adrenopause is independent of menopause, and it occurs in both sexes. The degree to which naturally occurring menopause affects circulating androgen levels has been a matter of debate. Studies in subjects with dexamethasone-induced adrenal suppression indicate that the resulting decrease in serum androgen levels seems to be more pronounced in postmenopausal (9) than in premenopausal women (5). This suggests that the ovarian contribution to the total androgenic pool may be lower during menopause. However, DHEA of adrenal origin may still be converted to testosterone within the postmenopausal ovary. Cross-sectional and longitudinal studies during the menopause transition found no evidence of a significant decrease in circulating androgens (10, 11). This was confirmed by a recent study from Australia studying a carefully selected, population-based reference sample of 595 women in whom all potential confounding variables impacting on androgen levels had been excluded (12). This extremely well designed and adequately powered study describes a gradual but modest decline in circu-lating androgens (testosterone, free testosterone, androstenedione and DHEAS) with age (12) (Fig. 2). However, menopause did not significantly impact on serum androgens. Thus, the ability of ovarian theca cells to synthesize androgens apparently persists after menopause, despite the loss of estrogen production in granulosa cells. This is also illustrated by the finding that bilateral oophorectomy in postmenopausal women leads to a significant decrease in circulating levels of androgens (12, 13).
Figure 1. Schematic overview of the generation of androgen precursors and their conversion towards active androgens in women. The figure depicts the contributions of adrenal, ovary and peripheral target cells to this process. DHEA, dehydroepiandrosterone; DHEAS, DHEA sulfate; A’dione, androstenedione; T, testosterone; DHT, dihydrotestosterone; E1, estrone; E2, 17beta-estradiol.
Figure 2. Serum androgens (medians) across life span in a healthy female reference cohort (n = 595) according to Davison et al. (12).
Definition of female androgen deficiency
According to the recently published Princeton consensus statement issued by an expert panel from the United States and Australia, ‘female androgen deficiency syn-drome’ (FADS) may be diagnosed in women who meet all of the following three criteria: first, impaired well-being or libido; secondly, adequate estrogenization (i.e. either normal ovarian function or established estrogen replacement therapy (ERT)); and thirdly, serum androgen concentrations below or within the lower quartile of the female normal range (14). However, this seems to be a rather loose definition as impaired mood and libido are multi-factorial in origin and thus cannot be considered specific indicators of androgen deficiency. It does not seem feasible to consider androgen replacement for every woman with self-perceived impaired well-being, who also happens to have serum androgen concentrations within the lower quarter of the normal range. An even more problematic development is to consider the diagnostic term ‘hypoactive sexual desire disorder’ (HSDD) as a sufficient justification for the initiation of androgen replacement therapy. According to the consensus of the Sexual Function Health Council of the American Foundation for Urologic Disease (AFUD), comprising 19 experts from five countries, HSDD is defined by the concurrent presence of the following two criteria: first, persistent or recurrent deficiency (or absence) of sexual fantasies, thoughts, and/or desire for, or receptivity to, sexual activity and secondly, personal distress caused by the sexual dysfunction described in the first criterion (15). HSDD is obviously of multi-factorial origin and androgen deficiency is not part of its definition. If we were to base the indication for androgen therapy in women on the presence of HSDD, we would start to treat a large proportion of the general population, with recent figures indicating a 50% incidence of female sexual dysfunction based on the AFUD criteria in gynecologic and uro-gynecologic out-patient cohorts (16). The presence of HSDD alone is certainly no justification for the initiation of androgen treatment. Importantly, a recently published cross-sectional study in a large cohort of Australian women (n = 1423) found no significant correlation of circulating androgen levels with self-reported perception of sexual desire and sexual satisfaction (17). Interestingly, although the majority of women with a low DHEAS did not have low sexual function, a low DHEAS (10th percentile) was still significantly associated with higher odds of impaired sexual function (17). However, no data were provided on circulating levels of biologically active, desulfated DHEA in this cohort. It is well described that circulating DHEAS can be decreased in chronic disease or stress and this may also contribute to the observed decrease in libido in those women. The data from the Australian cohort were further corroborated by the results of the recently published Study of Women’s Health Across the Nation (SWAN) (18) that studied circulating androgens in a community-based cohort of 42- to 52-year-old women (n = 2961). Only modest or minimal associations of testosterone with increased sexual desire and DHEAS with functional status and self-reported health were found, but androgens were most strongly associated with markers of metabolic syndrome (body mass index, waist circumference, and waist-hip ratio) (18). The currently available facts on female androgen physiology clearly suggest that women with significant, near-total depletion of androgens are the most suitable candidates for androgen replacement. Women invariably develop severe androgen deficiency following bilateral oophorectomy, confirming the important role of the ovaries as a source of active androgens. Similarly, women with adrenal insufficiency usually present with significant androgen deficiency due to the pathological loss or decrease in adrenal DHEA synthesis. Pharmacological glucocorticoid treatment, e.g. for asthma or rheumatic diseases, invariably results in suppression of adrenal DHEA synthesis following feedback inhibition of adrenocorticotropin release, and therefore is also associated with androgen deficiency. Women with one of these established causes of severe androgen deficiency and concurrent complaints of impaired well-being and libido are likely to benefit from androgen replacement therapy. Women with premature ovarian failure of autoimmune origin may also have pathologically decreased androgen levels. However, DHEA production in premature ovarian failure persists (19) and therefore androgen deficiency is less pronounced. Furthermore, premature ovarian failure may, in some cases, be associated with even increased androgen levels, possibly as a consequence of a considerable variability in the extent of destruction of androgen-producing ovarian theca cells (20). Women with Turner’s syndrome may also suffer from significant androgen deficiency (21). It is important to consider the assays employed for assessment of androgen deficiency when establishing a diagnosis of severe androgen deficiency based on hormone measurements. Some luminometric assays used for determination of free testosterone levels may be problematic whereas radioimmunoassays (RIAs) generally are reliable. Concurrent measurement of total testosterone and sex hormone-binding globulin (SHBG) concentrations may represent an alternative when no RIA for free testosterone is available. Testosterone and SHBG levels can be used to calculate the free androgen index (FAI), where FAI = (testosterone nmol/l X 100)/SHBG nmol/l (22). Importantly, estrogens increase SHBG concentrations, and thus decrease the FAI. Consequently, if assessing androgen deficiency in a woman treated with an oral contraceptive or estrogen replacement therapy, one should always consider first reducing the estrogen dose rather than immediately initiating androgen therapy. Furthermore, the progestin component of oral contraceptives e.g. cypro- terone acetate or drospirenone, may exert anti-androgenic properties and should be exchanged for another progestin if problems like loss of libido occur.
Testosterone therapy in women - pharmacokinetics and delivery tools
Androgen replacement therapy is a challenge even when treating men, but adjusting androgen levels to the normal range in women has proven to be even more difficult. Oral testosterone preparations show a broad variability with regard to resorption. Methyltestosterone and testosterone undecanoate have short half-lives and require repeated administration. Their pharmacokinetic properties lead to supraphysiological androgen levels shortly after resorption, followed by rapid decline. Not surprisingly, some studies in which a single daily dose of methyltestosterone was used for androgen therapy in women did not detect any increase in circulating androgen levels (23). Subcutaneous testosterone depot implants have the advantage that they only need to be administered every four to six months. However, even the smallest available dose (100 mg) induces supraphysiological androgen levels for several weeks to several months after implantation (24). Recently introduced transdermal androgen patches are more convenient to use but may still have some unfavorable pharmacokinetic properties (24), sometimes resulting in supraphysiological active androgen levels in treated women (25). Optimization of a transdermal delivery system is underway which will include testosterone gel preparations already in use for men. However, it is important to bear in mind that none of the currently available options is officially approved for use in women (Table 1). Some studies have used the synthetic testosterone analog, oxandro- lone, to examine the effects of androgen replacement in women with Turner’s syndrome (26). This synthetic androgen has previously been used to induce growth- promoting effects in Turner’s syndrome patients. Oxan- drolone is a low-affinity androgen receptor agonist with 10-100 times lower activity than testosterone and DHT (27). Oxandrolone cannot be aromatized, thus its effect is mediated via the androgen receptor only. However, one disadvantage of treatment with synthetic anabolic steroids is that drug monitoring is more difficult as serum testosterone levels cannot serve as a parameter for treatment surveillance.
Table 1. Delivery tools for androgen replacement used in published studies on androgen treatment in women. Note that to date none of these preparations has been officially approved for use in women.
Clinical experience with testosterone therapy in women
Most, if not all, of the studies published to date concen-trated on potential effects on female libido and wellbeing and recorded androgenic skin effects while data on lipids, insulin sensitivity, and body composition are much more scarce, and often preliminary. When interpreting the results of studies on the effects of androgen treatment in women, several methodological issues have to be considered. First, due to the pharmacokinetic properties of available testosterone preparations most published studies on the effects of treatments are associated with supraphysiological androgen levels. Secondly, several studies were not carried out in a double-blind fashion, thus precluding proper assessment of the effects of androgens on self-perceived mood and libido. Thirdly, most of the earlier studies in this field compared the effects of conventional estro- gen/progestin hormone replacement therapy (HRT) with the effects of HRT + androgens in previously untreated, symptomatic postmenopausal women. The effect of initiating estrogen treatment in these women was so dramatic that any additional benefit of androgen treatment was almost non-detectable (28). Table 2 summarizes the published studies employing testosterone treatment in women. Results of two studies that employed testosterone implants and oral methyltestosterone in addition to HRT indicated the significant beneficial effects on bone mineral density (29, 30). However, the degree to which these effects were due to aromatization, thus representing estrogen rather than androgen effects, is unclear. The effects on body composition are inconsistent, and while mostly a gain in lean body mass was reported (31, 32), there were reports of both a gain (32) and a loss (31) in fat mass. The small number of participants and the lack of longer-term studies currently prevent proper evaluation of the effects of testosterone treatment on body composition, bone mineral density or insulin sensitivity. Study results showing androgen effects on lipids are also heterogeneous, but androgens are consistently reported to induce a decrease in high-density lipoprotein (HDL) cholesterol levels (29, 32, 33). Since early on, the effects of androgens on female sexuality have been the main focus of research. Beneficial effects on libido and mood were reported in both double-blind (34, 35) and single-blind (30) studies on testosterone replacement in surgically menopausal women, i.e. women with an established cause of significant androgen deficiency. However, in these studies, testosterone administration resulted in supraphysiological serum androgen concentrations. Administration of low-dose oral testosterone did not induce significant additional improvements compared with conventional ERT after 2 years of treatment in oophorectomized women (29). Shifren et al. conducted a landmark study helping to define the impact of androgens on sexuality (25). They studied the effects of transdermal testosterone replacement in 75 oophorectomized women who had impaired sexual function at baseline. Results showed that testosterone therapy had statistically significant beneficial effects on various aspects of sexuality. However, these results were only statistically significant for patients in the higher dose group (300 mg/day) associated with circulating androgen levels at or above the upper limit of the normal range (25). Most recently, three large studies on a transdermal testosterone delivery system in women with bilateral oophorectomy and concurrent sexual desire disorder have been published (36-38). The study by Braunstein et al. (36) compared 24 weeks of treatment with three doses of transdermal testosterone (150, 300 and 450 mg/day) with placebo treatment and established that 300 mg/day significantly increased the frequency of sat-isfactory sexual activity and sexual desire, while 150 mg/day did not result in significant improvements and 450 mg/day was associated with a significantly higher incidence of androgenic skin effects. Following up on this, Buster et al. (37) and Simon et al. (38) published the results of phase III trials comprising 24 weeks of treatment with transdermal testosterone (300 mg/day) or placebo in two studies with parallel design. These studies included 533 (37) and 562 women (38) respectively suffering from sexual dysfunction after bilateral oophorectomy. Both studies confirmed the efficacy of 300 mg/day on sexual activity and desire; side effects included an increased frequency of mostly mild skin effects and no serious adverse events were noted. However, in a significant proportion of these women testosterone treatment resulted in supra- physiological serum concentrations of testosterone and DHT (Fig. 3A).
Table 2. Randomized controlled studies on testosterone treatment in women.
DHEA as an alternative tool for androgen replacement in women
DHEA may represent an elegant alternative tool for treatment of androgen deficiency in women (2, 39), as it is a crucial sex steroid precursor and is rapidly converted downstream towards androgens (5, 40). Following oral administration of DHEA to women with adrenal insufficiency who suffer from invariably low or even nondetectable androgen levels (11, 41), circulating levels of androgens have increased from subnormal levels to the lower end of the normal range (40, 41). Daily administration of 50 mg DHEA has been shown to increase DHEA and androstenedione levels to the mid normal range, while testosterone and DHT were only increased to the lower limit of the normal range (41) (Fig. 3B). However, circulating levels of androstanediol glucoronide (ADG), an androgen metabolite and useful marker of androgen generation within peripheral cells, increased to the upper limit of the normal range (41) (Fig. 3B). This indicates that DHEA replacement may not affect circulating androgens as much as testosterone administration, but the androgenic effect may be similar subsequent to DHEA conversion within peripheral cells. However, the current use of DHEA is still hampered by the lack of preparations produced according to Good Pharmaceutical Practice (GPP). Several different over-the-counter preparations marketed in the USA that claim to contain 25 mg have been shown to contain everything between 0 and 140 mg (42). There are ongoing efforts to account for DHEA as a drug and not as a food supplement. However, recent legislatory changes affecting the over-the-counter sale of anabolic steroids in the USA did specifically exclude DHEA, most likely following lobbying by companies with monetary inter-ests in the huge ‘anti-aging’ market sales in the United States. Women with adrenal insufficiency suffer from pronounced impairment of well-being and mood (43), and DHEA replacement in these patients has been shown to improve libido (41) as well as mood and well-being (41, 44, 45). Table 3 summarizes the published studies employing DHEA therapy in women. By contrast, women with physiological menopause and intact adrenals did not show improvement in mood or self-perceived well-being following DHEA therapy (46). In general, the effects of DHEA replacement on mood may be more complex than those of testosterone replacement, as DHEA also potentially exerts direct neurosteroidal effects at gamma-aminobutyric acid and N-methyl-D-aspartate receptors, suggestive of a potential anti-depressive action (39), which has been demonstrated in women with midlife-onset dys- thymia, significantly benefiting from DHEA replacement (47, 48). In the largest study published on the use of DHEA in elderly women (60 to 79 years old), comparing 12 months of treatment with DHEA (n = 70) vs placebo (n = 70), modest increases in libido and bone mineral density were observed, but only in the subgroup of women older than 70 years of age (49). There are very consistent reports of significant decreases in HDL cholesterol following DHEA replacement. Two recent studies reported an improvement in some parameters of insulin sensitivity following 12 weeks of DHEA replacement in adrenal insufficiency (50) and six months of DHEA in elderly women (51). The latter study also reported a significant decrease in fat mass as assessed by magnetic resonance imaging (MRI) in 14 women treated for six months (51).
Table 3. Randomized controlled trials on DHEA replacement in women.
Side effects of androgen treatment in women
The FDA has recently denied the approval of a trans-dermal androgen delivery system for use in women, based on its concerns about the lack of long-term safety data. Most commonly reported side effects are androgen skin effects (increased sebum secretion, greasy skin and hair, scalp itching, alopecia, hirsutism) (Tables 2 and 3). The long-term impact of unfavorable changes in cardiovascular risk markers such as the decrease in HDL cholesterol cannot be properly judged based on currently available data. This would also have to be weighed against potential beneficial effects on body composition and insulin sensitivity. However, convincing data on these effects are still lacking. There is clearly the urgent need for more long-term trials to allow for more appropriate assessment not only of the aspect of long-term safety in female androgen replacement but also on its potential beneficial effects.
Conclusions
Choosing both a convenient and efficient mode of androgen administration in women remains a challenge and currently none of the available preparations is officially approved for use in women, although this is likely to change in the near future. It will be key to achieving a more precise diagnostic consenus for female androgen deficiency and to provide answers to the questions ‘whom to treat, why, when and for how long’. Androgen replacement seems to be a promising option for the treatment of women with established causes of severe androgen deficiency including surgical menopause or adrenal insufficiency, if they concurrently suffer from symptoms of impaired mood and libido. In addition, the therapeutic potential of androgen replacement in women receiving chronic pharmacological glucocorticoid treatment and women with Turner’s syndrome may deserve further exploration. Importantly, impairment of libido is multi-factorial in origin and in the majority of cases is not associated with evidence of androgen deficiency. Therefore, the diagnosis of hypoactive sexual desire disorder does not automatically lead to justification of androgen replac- ment, as androgen deficiency is not necessarily associated with this condition. It is important to acknowledge that physiological menopause in women with intact ovaries is not associated with a sudden loss of androgen synthesis, unlike the steep drop in ovarian estrogen production. Therefore, postmenopausal women do not routinely require androgen replacement. The slow, age-associated decline in DHEA, DHEAS and active androgens observed over a woman’s lifetime does not represent an indication for replacement per se but may well represent a physiological, protective mechanism e.g. preventing increased sex steroid action in breast tissue. More long-term studies in larger cohorts of women with severe androgen deficiency are needed comprehensively to assess both potential beneficial and adverse effects.
Acknowledgements
This work was supported by the Medical Research Coun-cil UK (Senior Clinical Fellowship G116/172, to WA).
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