Woraluk Somboonporna,✸, Susan R Davisb
a Department of Obstetrics and Gynaecology, Khon Kaen University, Khon Kaen, Thailand b NHMRC Centre of Clinical Research Excellence, Department of Medicine (CECS) Monash University, Melbourne, Australia. ✸ Corresponding author.
Keywords: Testosterone therapy; Menopause; Breast cancer.
Abstract
Background: Testosterone therapy is being increasingly used in the management of postmenopausal women. However, as clinical trials have demonstrated a significantly increased risk of breast cancer with oral combined estrogen-progestin therapy, there is a need to ascertain the risk of including testosterone in such regimens. Objective: Evaluation of experimental and epidemiological studies pertaining to the role of testosterone in breast cancer. Design: Literature review. Setting: The Jean Hailes Foundation, Research Unit. Main Outcome Measures: Mammary epithelial proliferation, apoptosis and breast cancer. Results: In experimental studies, testosterone action is anti-proliferative and pro-apoptotic, and mediated via the AR, despite the potential for testosterone to be aromatized to estrogen. Animal studies suggest that testosterone may serve as a natural, endogenous protector of the breast and limit mitogenic and cancer promoting effects of estrogen on mammary epithelium. In premenopausal women, elevated testosterone is not associated with greater breast cancer risk. The risk of breast cancer is also not increased in women with polycystic ovary syndrome who have chronic estrogen exposure and androgen excess. However, in postmenopausal women, who are oestrogen deplete and have increased adipose aromatase activity, higher testosterone has been associated with greater breast cancer risk. Conclusion: Available data indicate the inclusion of testosterone in estrogen-progestin regimens has the potential to ameliorate the stimulating effects of hormones on the breast. However, testosterone therapy alone cannot be recommended for estrogen deplete women because of the potential risk of enhanced aromatisation to estrogen in this setting.
Introduction
Testosterone exhibits important physiological effects in women, being both a precursor hormone for ovarian and extragonadal estrogen biosynthesis [1], and acting directly via androgen receptors (AR) throughout the body. Some postmenopausal women may develop symptomatic androgen insufficiency and may benefit from testosterone therapy in terms of improved libido, sexual satisfaction, quality of life and bone mineralization [2-9]. However, as the most recent large randomized placebo-controlled clinical trials have demonstrated a significantly increased risk of breast cancer in postmenopausal women taking oral combined hormone therapy [10,11] there is a need to establish whether adding testosterone to hormone therapy regimens will influence this risk.
Testosterone aromatization after menopause
Testosterone may exert biological effects by acting directly via the AR or indirectly through conversion to estrogen by the aromatase enzyme and dihydrotestosterone (DHT), which is a non-aromatizable androgen, by 5α-reductase type 1 and 2 [12,13]. Following menopause, the former conversion occurs primarily in adipose and target tissues such as breast, brain, bone, skin, vascular endothelium and vascular smooth muscle [13]. Increased aromatase activity and p450 aromatase gene expression has been demonstrated in adipose tissue with increasing age [14,15]. Plasma cytokines, known to be aromatase stimulating factors, such as interleukin-6, increase with age [16,17] and this may partially account for the increase in peripheral aromatase activity that is detected in older women.
Although this change is not influenced by menopause status [18], it is also possible that estrogen itself may inhibit its own bioformation in both human breast cancer cell lines [19] and animal models [20]. Consistent with these findings, intra-tumoral aro-matase activity has been shown to be higher in woman with lower circulating estrogen [21,22] and relative low activity of aromatase was found in women taking hormone therapy [19,23]. These data indirectly suggest that after menopause, when circulating estrogen levels are low, an increase in aromatase levels in the breast may maintain tissue concentrations of estrogen. Thus, aromatase may control the local production of estrogen through an autocrine loop. During the process of tranformation to malignancy, locally produced estrogen may stimulate the proliferation of tumor cells and vascular endothelial growth factor production. These effects are also likely to enhance tumor progression, development of angiogenesis, and, ultimately, metas-tasis of cancer. This has negative implications for the use of testosterone therapy in women who are post-menopausal and not receiving estrogen therapy.
With regard to 5α-reductase activity, a number of studies have substantiated 5α-reductase modulation by sex steroids [24-29]. Evidence for the relationship between 5α-reductase activity and menopausal status is lacking.
Effects of testosterone and DHT on breast: experimental studies
In experimental studies, estradiol has been consis-tently been shown to be a major mitogen in the breast [30-34] while testosterone and DHT have been shown to inhibit such estrogenic effects as follows.
Breast cancer cell lines
Anti-proliferative effects of androgens
Normal circulating levels in women are approxi-mately 0.2-0.8nmol/L for DHT and 0.5-2.3 nmol/L for testosterone. At physiological doses (1nM/L), testosterone and DHT inhibit proliferation of MCF-7 cells in both the absence of estradiol [35-37] or presence [35,38], although not all studies have found this association [39]. These inhibitory effects have also been reported in other breast cancer cell lines, ZR-75-1 [39] andT47-D [37,39]. Birrelletal. [39] reported no effect on proliferation of BT-20 cells, whereas Ortmann et al. [37] demonstrated a significant dose-dependent inhibition of cell growth by testosterone and DHT in this cell line. The contradictory result may be explained by the difference in cell proliferation assays used. Birrel et al. used a modification of a methylene blue whole cell binding assay while Ortman et al. usedMTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) method.
Apoptotic effects
Bcl-2 is considered an anti-apoptotic gene [40], and Bax is a pro-apoptotic gene [41]. DHT down-regulates Bcl-2 protooncogene levels via an AR-mediated mechanism in the ZR75-1 breast cancer cell line in either the presence or absence of 17p-estradiol [42]. In addition, in the AR positive human breast cancer cell lines, T47-D and ZR-75-1, DHT has been reported to be pro-apoptotic with maximal effects at 10 nmol/L concentration [43]. These findings are in-line with the growth inhibitory effects reported by Birrell et al. [44] in these two cell lines. This leads us to conclude that DHT may oppose the mitogenic action of estrogen in the breast by promoting apoptosis. Table 1. summarizes the results of breast cancer cell line studies.
Table 1. Effects of testosterone and DHT on breast cancer cell lines.
Animal studies
Normal model
In the Noble rat model, treatment with testosterone, either alone or in combination with estrogen, gave rise to high levels of Bax expression in ‘pre-malignant’ mammary glands [41]. This supports the hypothesis that testosterone may oppose the mitogenic action of estrogen in the breast by promoting apoptosis [45].
The addition of methyltestosterone to oral contra-ceptive therapy in rats causes a significant suppression of epithelial proliferation and a reduction in the number of progesterone receptor-labelled cells [46]. In rhesus monkeys either supra-or physiologic testosterone treatment reduced estradiol-induced epithelial proliferation [47,48], suggesting that testosterone can reduce breast cancer risk associated with estrogen treatment [48].
Tumor model
DHT therapy reduced the number of progressing tumors and increased the number of tumors that completely regressed in the DMBA-induced mammary carcinoma model in ovariectomized rats [49] and on ZR-75-1 breast cancer cells implanted into ovariec-tomized athymic mice in both the presence or absence of exogenous estradiol [50]. Table 2. summarizes the results from all animal studies.
Table 2.Effects of testosterone and DHT in animal studies.
Effects of testosterone on breast tissue: epidemiological stu
Endogenous testosterone and breast cancer
In premenopausal women, there are inconsistent results from two cross-sectional studies [51,52] and two prospective case-control studies have found no association between breast cancer risk and total testosterone levels [53,54].
Polycystic ovarian syndrome (PCOS) is a useful model of the effects of long-term exposure to hormone imbalance. Concentrations of testosterone, androstene-dione, and DHEAS, and the calculated free androgen index (total testosterone nmol/L divided by SHBG nmol/L x 100) are significantly higher in women with PCOS regardless of hirsutism [55]. An increased risk of endometrial cancer has been well documented in women with this condition [56] whereas the risk of breast cancer is not increased [56,57] despite hyper-androgenism and long term exposure to unopposed estrogen. Infact, Gammon and Thompson [58] have reported an aged-adjusted odds ratio for breast cancer in women with this syndrome of0.52 (95%CI 0.32-0.87). Although this risk reduction might be related to the hy-perandrogemia of this condition, cause and effect cannot be established.
In postmenopausal women, there are inconsistent results in both cross-sectional studies and prospective studies [53,59-68]. Table 3. shows the results from the prospective case-control studies. A reanalysis of prospective studies [69] reported that the relative risks associated with a doubling of total testosterone levels within the normal range were 1.37 (95%CI 1.15-1.65) for the three studies incorporating a purification step in their testosterone assay [66,67,70] and 1.44 (95%CI 1.21-1.72) for four studies that used a direct testosterone assay [62-65]. The considerable inaccuracy of direct total testosterone assays limits the interpretation of any data obtained by this methodology [71]. Furthermore, a subgroup analysis was not undertaken to determine the association between breast cancer and free testosterone. This is important as free testosterone, not total testosterone, is considered the best indicator of tissue exposure to testosterone.
Table 3. Endogenous testosterone and risk of breast cancer in postmenopausal women.
There are only two prospective studies available that have utilized measures of free testosterone [62,67]. Cauley et al. [67] measured free testosterone by equilibrium dialysis, considered to be a reasonably accurate methodology, in 97 cases and 244 controls and reported that breast cancer risk was three times greater in women with the highest concentration(≥13.17pmol/L), but that this was no longer significant after adjustment for estradiol. In contrast Berrino et al. [62] measured free testosterone by radioimmunoassay in 25 cases and 100 controls and reported a significant association between higher free testosterone levels(≥0.86pg/ml) and breast cancer after adjustment for estradiol. Again, as direct free testosterone assays are considered unreliable, the value of this data is questionable [71].
Exogenous testosterone and breast cancer
The specific evidence pertaining to the use of testosterone therapy and breast cancer risk has significant methodological limitations and provides inconclusive results. Breast cancer risk was not a primary endpoint for two of these studies [72,73]. Consequently, each had only a small sample size for this subgroup analysis [72,73]. A recent retrospective study had no control group [74]. Table 4 summarized study characteristics and their results.
Table 4. Exogenous testosterone and risk of breast cancer in postmenopausal women.
Clinical consideration
The term female androgen insufficiency (FAI) was proposed as a pattern of clinical symptoms in the presence of decreased bioavailable testosterone and normal estrogen status [75]. The addition of testosterone to hormone therapy has shown benefits for sexual dysfunction, mood, energy, psychological wellbeing, bone mineral density, muscle mass and strength and adipose tissue distribution [2-7,76]. However, there are some potential risks have been reported, for example acne, excess facial and body hair, permanent deepening of voice, weight gain, emotional changes (e.g. increased anger) and adverse effects on the lipid profile [77]. As androgens are converted to estrogens in vivo, estrogenic side-effects are also potential consequences of androgen therapy, including increased risk of breast and ovarian cancer [77].
Based on experimental studies covered in this re-view it is unlikely that physiological testosterone ther-apy in addition to estrogen therapy increases the risk of breast cancer. Alternatively, it appears that testosterone serves as a natural safe guard of the breast and limits mitogenic and cancer promoting effects of estrogen within the breast [35,37-39,42,43,45,49,50]. However, the existing evidence in women specific to exogenous testosterone therapy and the risk of breast cancer [72-74] has a significant limitations of research methodology to address this issue.
In case-control studies, high endogenous total testosterone levels have been associated with increased breast cancer risk in postmenopausal women [69], but not in premenopausal women [53,54]. It is almost impossible to be sure of a direct causal relationship between high levels of free testosterone and breast cancer although the positive association was found. Increased aromatase activity in the setting of estrogen depletion after menopause, and thus increased capacity to convert testosterone to estradiol in adipose tissue with age may be the major factors. That there is no longer an association between high levels of free testosterone and breast cancer risk after adjustment for estradiol levels would support this [67]. Furthermore, there has been no report of an increase in breast cancer risk in women with PCOS in which hyperandrogenism coexists with elevated estrogen.
As the epidemiological studies specific to exoge-nous testosterone therapy and breast cancer risk have major limitations, with the available data pertaining to the effects of testosterone on the breast, the inclusion of testosterone in hormonal regimens should be limited to women symptomatic of androgen insufficiency despite adequate estrogen replacement and involve regular measurements of circulating levels of free testosterone to avoid androgen excess.
Conclusion
Breast cancer is the most common carcinoma in women [78,79]. Evidence from the most recent large randomized, placebo-controlled clinical trials shows the risk significantly increases in postmenopausal women taking estrogen plus medroxyprogesterone ac-etate [10,11,80] but does not increase in those taking estrogen only [81] while the clinical studies specific to exogenous testosterone therapy and breast cancer risk yield inconclusive result due to significant limitations [72-74]. However, the preclinical studies suggest that testosterone may serve as a natural, endogenous protector of the breast and limit mitogenic and cancer promoting effects of estrogen on mammary epithelium [35,37-39,42,43,46,48-50]. In contrast, in the setting of estrogen depletion, ie postmenopause, aromatase activity is increased and higher free testosterone has been associated in one study with a small increase in breast cancer risk. Thus, we propose that whereas benefits may be achieved by adding testosterone to estrogen-progestin therapy, greater caution is required for the use of testosterone alone at this time. There is no doubt that further studies are required so that firm clinical advice can be given pertaining to the use of testosterone with or without estrogen therapy.
References
Simpson E, Rubin G, Clyne C, Robertson K, O’Donnell L, Jones M, et al. The role of local estrogen biosynthesis in males and females. Trends Endocrinol Metab 2000;11(5):184-8.
Burger HG, Hailes J, Menelaus M, Nelson J, Hudson B, Balazs N. The management of persistent menopausal symptoms with oestradiol-testosterone implants: clinical, lipid and hormonal results. Maturitas 1984;6(4):351-8.
Burger HG, Hailes J, Nelson J, Menelaus M. Effect of combined implants of estradiol and testosterone on libido in postmenopausal women. Br Med J 1987;294:936-7.
Davis SR, McCloud P, Strauss BJ, Burger H. Testosterone enhances estradiol’s effects on postmenopausal bone density and sexuality. Maturitas 1995;21(3):227-36.
Sherwin BB, Gelfand MM. Differential symptom response to parenteral estrogen and/or androgen administration in the surgical menopause. Am J Obstet Gynecol 1985;151(2):153-60.
Sherwin BB, Gelfand MM, Brender W. Androgen enhances sexual motivation in females: a prospective, crossover study of sex steroid administration in surgical menopause. Psychosom Med 1985;47(4):339-51.
Sherwin BB. Use of combined estrogen-androgen preparations in the postmenopause: evidence from clinical studies. Int J Fertil Womens Med 1998;43(2):98-103.
Sherwin BB. Randomized clinical trials of combined estrogen-androgen preparations: effects on sexual functioning. Fertil Steril 2002;77(Suppl. 4):49-54.
Shifren JL, Braunstein G, Simon J, Casson P, Buster JE, Red Burki RE, et al. Transdermal testosterone treatment in women with impaired sexual function after oophorectomy. N Eng J Med 2000;343(10):682-8.
Chlebowski RT, Hendrix SL, Langer RD, Stefanick ML, Gass M, Lane D, et al. Influence of estrogen plus progestin on breast cancer and mammography in healthy postmenopausal women: the Women’s Health Initiative Randomized Trial. J Am Med Assoc 2003;289(24):3243-53.
Rossouw JE, Anderson GL, Prentice RL, LaCroix AZ, Kooper-berg C, Stefanick ML, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results From the Women’s Health Initiative randomized controlled trial. J Am Med Assoc 2002;288(3):321-33.
Burger HG. Androgen production in women. Fertil Steril 2002;77(4):3-5.
Simpson ER. Aromatization of androgens in women: current concepts and findings. Fertil Steril 2002;77(4):6-10.
Cleland WH, Mendelson CR, Simpson ER. Effects of aging and obesity on aromatase activity of human adipose cells. J Clin Endocrinol Metab 1985;60(1):174-7.
Bulun SE, Simpson ER. Competitive reverse transcription-polymerase chain reaction analysis indicates that levels of aromatase cytochrome p450 transcripts in adipose tissue of buttocks, thighs, and abdomen of women increase with advancing age. J Clin Endocrinol Metab 1994;78(2):428-32.
Wei J, Xu H, Davies JL, Hemmings GP Increase of plasma IL-6 concentration with age in healthy subjects. Life Sci 1992;51(25):1953-6.
McKane WR, Khosla S, Peterson JM, Egan K, Riggs BL. Circulating levels of cytokines that modulate bone resorption: effects of age and menopause in women. J Bone Miner Res 1994;9(8):1313-8.
Longcope C, Baker S. Androgen and estrogen dynamics: relationships with age, weight, and menopausal status. J Clin Endocrinol Metab 1993;76(3):601-4.
Yue W, Berstein LM, Wang JP, Clark GM, Hamilton CJ, Demers LM, et al. The potential role of estrogen in aromatase regulation in the breast. J Steroid Biochem Mol Biol 2001;79(1-5):157-64.
Nakamura J, Lu Q, Aberdeen G, Albrecht E, Brodie A. The effect of estrogen on aromatase and vascular endothelial growth factor messenger ribonucleic acid in the normal nonhuman primate mammary gland. J Clin Endocrinol Metab 1999;84(4):1432-7.
Berstein LM, Larionov AA, Kyshtoobaeva AS, Pozharisski KM, Semiglazov VF, Ivanova OA. Aromatase in breast cancer tissue—localization and relationship with reproductive status of patients. J Cancer Res Clin Oncol 1996;122(8):495-8.
Bolufer P, Ricart E, Lluch A, Vazquez C, Rodriguez A, Ruiz A, et al. Aromatase activity and estradiol in human breast cancer: its relationship to estradiol and epidermal growth factor receptors and to tumor-node-metastasis staging. J Clin Oncol 1992;10(3):4386.
Pasqualini JR, Chetrite G, Blacker C, Feinstein MC, Delalonde L, Talbi M, et al. Concentrations of estrone, estradiol, and estrone sulfate and evaluation ofsulfatase and aromatase activities in pre-and postmenopausal breast cancer patients. J Clin Endocrinol Metab 1996;81(4):1460.
Cassidenti DL, Paulson RJ, Serafini P, Stanczyk FZ, Lobo RA. Effects of sex steroids on skin 5 alpha-reductase activity in vitro. Obstet Gynecol 1991;78(1):103-7.
Serafini PC, Catalino J, Lobo RA. The effect of spironolactone on genital skin 5 alpha-reductase activity. J Steroid Biochem 1985;23(2):191.
Serafini P, Lobo RA. Prolactin modulates peripheral androgen metabolism. Fertil Steril 1986;45(1):41-6.
Krieg M, Schlenker A, Voigt KD. Inhibition of androgen metabolism in stroma and epithelium of the human benign prostatic hyperplasia by progesterone, estrone, and estradiol. Prostate 1985;6(3):2330.
Kuttenn F, Rigaud C, Wright F, Mauvais-Jarvis P. Treatment of hirsutism by oral cyproterone acetate and percutaneous estradiol. J Clin Endocrinol Metab 1980;51(5):1107-11.
Dean HJ, Winter JS. The effect of five synthetic progestational compounds on 5 alpha-reductase activity in genital skin fibroblast monolayers. Steroids 1984;43(1):13-24.
Clarke RB, Laidlaw IJ, Jones LJ, Howell A, Anderson E. Effect oftamoxifen on Ki67 labelling index in human breast tumours and its relationship to oestrogen and progesterone receptor status. Br J Cancer 1993;67(3):606-11.
Clarke RB, Howell A, Anderson E. Estrogen sensitivity of normal human breasttissue in vivo and implanted into athymic nude mice: analysis of the relationship between estrogen-induced proliferation and progesterone receptor expression. Breast Cancer Res Treat 1997;45(2):121-33.
Laidlaw IJ, Clarke RB, Howell A, Owen AW, Potten CS, Anderson E. The proliferation of normal human breast tissue implanted into athymic nude mice is stimulated by estrogen but not progesterone. Endocrinology 1995;136(1):164-71.
Kyprianou N, English HF, Davidson NE, Isaacs JT. Programmed cell death during regression of the MCF-7 human breast cancer following estrogen ablation. Cancer Res 1991;51(1):162-6.
Perry RR, Kang Y, Greaves B. Effects of tamoxifen on growth and apoptosis of estrogen-dependent and -independent human breast cancer cells. Ann Surg Oncol 1995;2(3):238-45.
Ando S, De Amicis F, Rago V, Carpino A, Maggioloini M, Panno M, et al. Breast cancer: from estrogen to androgen receptor. Mol Cell Endocrinol 2002;193:121-5.
Boccuzzi G, Brignardello E, Di Monaco M, Gatto V, Leonardi L, Pizzini A, etal. 5-En-androstene-3 beta,17beta-diol inhibitsthe growth of MCF-7 breast cancer cells when oestrogen receptors are blockedby oestradiol. BrJCancer 1994;70(6):1035-9.
Ortmann J, Prifti S, Bohlmann MK, Rehberger-Schneider S, Strowitzki T, Rabe T. Testosterone and 5 alphadihydrotestosterone inhibit in vitro growth ofhuman breast cancer cell lines. Gynecol Endocrinol 2002;16(2):113-20.
MacIndoe JH, Etre LA. An antiestrogenic action of androgens in human breast cancer cells. J Clin Endocrinol Metab 1981;53(4):836-42.
Birrell SN, Bentel JM, Hickey TE, Ricciardelli C, Weger MA, Horsfall DJ, et al. Androgens induce divergent proliferative responses in human breast cancer cell lines. J Steroid Biochem Mol Biol 1995;52(5):459-67.
Herrmann J, Bruckheimer E, McDonnell T. Cell death signal transduction and bcl-2 function. Biochem Soc Trans 1996;24:1059-65.
Xie B, Tsao SW, Wong YC. Sex hormone-induced mammary carcinogenesis in female noble rats: the role of androgens. Carcinogenesis 1999;20(8):1597-606.
Lapointe J, Fournier A, Richard V, Labrie C. Androgens down-regulate bcl-2 protooncogene expression in ZR-75-1 human breast cancer cells. Endocrinology 1999;140(1):416-21.
Kandouz M, Lombet A, Perrot JY, Jacob D, Carvajal S, Kazem A, et al. Proapoptotic effects of antiestrogens, progestins and androgen in breast cancer cells. J Steroid Biochem Mol Biol 1999;69(1-6):463-71.
Birrell SN, Roder DM, Horsfall DJ, Bentel JM, Tilley WD. Medroxyprogesterone acetate therapy in advanced breast cancer: the predictive value of androgen receptor expression. J Clin Oncol 1995;13(7):1572-7.
Xie B, Tsao SW, Wong YC. Sex hormone-induced mammary carcinogenesis in the female Noble rats: expression ofbcl-2 and bax in hormonal mammary carcinogenesis. Breast Cancer Res Treat 2000;61(1):45-57.
Jayo MJ, Register TC, Hughes CL, Blas-Machado U, Sulisti-awati E, Borgerink H, et al. Effects of an oral contraceptive combination with or without androgen on mammary tissues: a study in rats. J Soc Gynecol Invest 2000;7(4):257-65.
Zhou J, Ng S, Adesanya-Famuiya O, Anderson K, Bondy CA. Testosterone inhibits estrogen-induced mammary epithelial proliferation and suppresses estrogen receptor expression. FASEB J2000;14(12):1725-30.
Dimitrakakis C, Zhou J, Wang J, Belanger A, Labrie F, Cheng C, et al. A physiologic role for testosterone in limiting estrogenic stimulation ofthe breast. Menopause 2003;10(4):292-8.
Dauvois S, Li SM, Martel C, Labrie F. Inhibitory effect of androgens on DMBA-induced mammary carcinoma in the rat. Breast Cancer Res Treat 1989;14(3):299-306.
Dauvois S, Geng CS, Levesque C, Merand Y, Labrie F. Additive inhibitory effects of an androgen and the antiestrogen EM-170 on estradiol-stimulated growth ofhumanZR-75-1 breast tumors in athymic mice. Cancer Res 1991;51(12):3131-5.
Secreto G, Toniolo P, Pisani P, et al. Androgens and breast cancer in premenopausal women. Cancer Res 1989;49:471-6.
Yu H, Shu XO, Shi R, Dai Q, Jin F, Gao YT, et al. Plasma sex steroid hormones and breast cancer risk in Chinese women. Int J Cancer 2003;105(1):92-7.
Wysowski DK, Comstock GW, Helsing KJ, Lau HL. Sex hormone levels in serum in relation to the development of breast cancer. Am J Epidemiol 1987;125(5):791-9.
Thomas HV, Key TJ, Allen DS, Moore JW, Dowsett M, Fentiman IS, et al. A prospective study of endogenous serum hormone concentrations and breast cancer risk in premenopausal women on the island of Guernsey. Br J Cancer 1997;75(7):1075-9.
Fox R, Corrigan E, Thomas PG, Hull MG. Oestrogen and androgen states in oligo-amenorrhoeic women with polycystic ovaries. Br J Obstet Gynaecol 1991;98(3):294-9.
Coulam CB, Annegers JF, Kranz JS. Chronic anovulation syndrome and associated neoplasia. Obstet Gynecol 1983;61(4):403-7.
Anderson KE, Sellers TA, Chen PL, Rich SS, Hong CP, Folsom AR. Association of Stein-Leventhal syndrome with the incidence of postmenopausal breast carcinoma in a large prospective study of women in Iowa. Cancer 1997;79(3):494-9.
Gammon MD, Thompson WD. Polycystic ovaries and the risk of breast cancer. Am J Epidemiol 1991;134(8):818-24.
Lipworth L, Adami HO, Trichopoulos D, Carlstrom K, Mant-zoros C. Serum steroid hormone levels, sex hormone-binding globulin, and body mass index in the etiology of postmenopausal breast cancer. Epidemiology 1996;7(1):96-100.
Secreto G, Toniolo P, Berrino E, et al. Serum and urinary androgens and risk of breast cancer in postmenopausal women. Cancer Res 1991;51:2572-6.
Garland CF, Friedlander NJ, Barrett-Connor E, Khaw KT. Sex hormones and postmenopausal breast cancer: a prospective study in an adult community. Am J Epidemiol 1992;135(11):1220-30.
Berrino F, Muti P, Michelli A, Bolelli G, Krogh V, Sciajno R, et al. Serum sex hormone levels after menopause and subsequent breast cancer. JNatl Cancer Inst 1996;88:291-291.
Dorgan JF, Longcope C, Stephenson Jr HE, Falk RT, Miller R, Franz C, et al. Relation of prediagnostic serum estrogen and androgen levels to breast cancer risk. Cancer Epidemiol Biomarkers Prev 1996;5(7):533-9.
Thomas HV, Key TJ, Allen DS, Moore JW, Dowsett M, Fentiman IS, et al. A prospective study of endogenous serum hormone concentrations and breast cancer risk in postmenopausal women on the island of Guernsey. Br J Cancer 1997;76(3):401-5.
Zeleniuch-Jacquotte A, Bruning PF, Bonfrer J, Koenig K, Shore R, Kim M, et al. Relation of serum levels of testosterone and dehydroepiandrosterone sulfate to risk ofbreast cancer in postmenopausal women. Am J Epidemiol 1997;145(11):1030-8.
Hankinson SE, Willett WC, Manson JE, Colditz GA, Hunter DJ, Spiegelman D, et al. Plasma sex steroid hormone levels and risk ofbreast cancer in postmenopausal women. J Natl Cancer Inst 1998;90:1292-9.
Cauley JA, Lucas FL, Kuller LH, Stone K, Browner W, Cummings SR. Elevated serum estradiol and testosterone concentrations are associated with a high risk for breast cancer. Study of Osteoporotic Fractures Research Group. Ann Intern Med 1999;130(4 pt 1):270-7.
Manjer J, Johansson R, Berglund G, Janzon L, Kaaks R, Agren A, et al. Postmenopausal breast cancer risk in relation to sex steroid hormones, prolactin and SHBG (Sweden). Cancer Causes Control 2003;14(7):599-607.
Endogenous sex hormones and breast cancer in postmenopausal women: reanalysis of nine prospective studies, J Natl Cancer Inst 2002;94(8):606-16.
Barrett-Connor E, Friedlander NJ, Khaw KT. Dehy-droepiandrosterone sulfate and breast cancer risk. Cancer Res 1990;50(20):6571-4.
Klee GG, Heser D. Techniques to measure testosterone in the elderly. Mayo Clin Proc 2000;75:S19-25.
Ewertz M. Influence of non-contraceptive exogenous and endogenous sex hormones on breast cancer risk in Demnark. Int J Cancer 1988;42:832-8.
Brinton LA, Hoover R, Fraumeni Jr JF. Menopausal oestrogens, and breast cancer risk: an expanded study case-control study. Br J Cancer 1986;54:825-32.
Dimitrakakis C, Jones R, Liu A, Bondy C. Breast cancer incidence in Australian women using testosterone in addition to estrogen replacement. In: Proceedings of the 85th Annual Meeting ofthe Endocrine Society. Philadelphia, MD: The Endocrine Society Press; 2003.
Bachmann GA, Bancroft J, Braunstein G, Burger H, Davis SR, Dennerstein L, et al. Female androgen insufficiency: the Princeton Consensus Statement on definition, classification and assessment. Fertil Steril 2002;77(660):665.
Sherwin BB. Affective changes with estrogen and androgen replacement therapy in surgically menopausal women. J Affect Disord 1988;14(2):177-87.
Bachmann G, Bancroft J, Braunstein G, Burger H, Davis S, Den-nerstein L, et al. Female androgen insufficiency: the Princeton consensus statement on definition, classification, and assessment. Fertil Steril 2002;77(4):660-5.
Forman D, Stockton D, Moller H, Quinn M, Babb P, De An-gelis R, et al. Cancer prevalence in the UK: results from the EUROPREVAL study. Ann Oncol 2003;14(4):648-54.
Edwards BK, Howe HL, Ries LA, Thun MJ, Rosenberg HM, Yancik R, et al. Annual report to the nation on the status of cancer, 1973-1999, featuring implications of age and aging on US cancer burden. Cancer 2002;94(10):2766-92.
Beral V. Breast cancer and hormone-replacement therapy in the Million Women Study. Lancet 2003;362(9382):419-27.
Hopkins TJ. Oestrogen only arm of women’s health initiative trial is stopped. BMJ 2004;328(7440):602.