17α-estradiol 17α-estradiol 17α-estradiol (also known as 17α-e2, 17-epiestradiol, alfatradiol, or estra-1,3,5(10)-triene-3,17α-diol) is a minor and weak endogenous steroidal estrogen that is related to 17β-estradiol (better known simply as estradiol). it is the c17 epimer of estradiol. it has approximately 100-fold lower estrogenic potency than 17β-estradiol. the compound shows preferential affinity for the erα over the erβ. although 17α-estradiol is far weaker than 17β-estradiol as an agonist of the nuclear estrogen receptors, it has been found to bind to and activate the brain-expressed er-x with a greater potency than that of 17β-estradiol, suggesting that it may be the predominant endogenous ligand for the receptor. affinities of estrogen receptor ligands for the erα and erβ ligandother namesrelative binding affinities (rba, %)aabsolute binding affinities (ki, nm)aaction erαerβerαerβ estradiole2; 17β-estradiol1001000.115 (0.04–0.24)0.15 (0.10–2.08)estrogen estronee1; 17-ketoestradiol16.39 (0.7–60)6.5 (1.36–52)0.445 (0.3–1.01)1.75 (0.35–9.24)estrogen estriole3; 16α-oh-17β-e212.65 (4.03–56)26 (14.0–44.6)0.45 (0.35–1.4)0.7 (0.63–0.7)estrogen estetrole4; 15α,16α-di-oh-17β-e24.03.04.919estrogen alfatradiol17α-estradiol20.5 (7–80.1)8.195 (2–42)0.2–0.520.43–1.2metabolite 16-epiestriol16β-hydroxy-17β-estradiol7.795 (4.94–63)50 ? ?metabolite 17-epiestriol16α-hydroxy-17α-estradiol55.45 (29–103)79–80 ? ?metabolite 16,17-epiestriol16β-hydroxy-17α-estradiol1.013 ? ?metabolite 2-hydroxyestradiol2-oh-e222 (7–81)11–352.51.3metabolite 2-methoxyestradiol2-meo-e20.0027–2.01.0 ? ?metabolite 4-hydroxyestradiol4-oh-e213 (8–70)7–561.01.9metabolite 4-methoxyestradiol4-meo-e22.01.0 ? ?metabolite 2-hydroxyestrone2-oh-e12.0–4.00.2–0.4 ? ?metabolite 2-methoxyestrone2-meo-e1<0.001–<1<1 ? ?metabolite 4-hydroxyestrone4-oh-e11.0–2.01.0 ? ?metabolite 4-methoxyestrone4-meo-e1<1<1 ? ?metabolite 16α-hydroxyestrone16α-oh-e1; 17-ketoestriol2.0–6.535 ? ?metabolite 2-hydroxyestriol2-oh-e32.01.0 ? ?metabolite 4-methoxyestriol4-meo-e31.01.0 ? ?metabolite estradiol sulfatee2s; estradiol 3-sulfate<1<1 ? ?metabolite estradiol disulfateestradiol 3,17β-disulfate0.0004 ? ? ?metabolite estradiol 3-glucuronidee2-3g0.0079 ? ? ?metabolite estradiol 17β-glucuronidee2-17g0.0015 ? ? ?metabolite estradiol 3-gluc. 17β-sulfatee2-3g-17s0.0001 ? ? ?metabolite estrone sulfatee1s; estrone 3-sulfate<1<1>10>10metabolite estradiol benzoateeb; estradiol 3-benzoate10 ? ? ?estrogen estradiol 17β-benzoatee2-17b11.332.6 ? ?estrogen estrone methyl etherestrone 3-methyl ether0.145 ? ? ?estrogen ent-estradiol1-estradiol1.31–12.349.44–80.07 ? ?estrogen equilin7-dehydroestrone13 (4.0–28.9)13.0–490.790.36estrogen equilenin6,8-didehydroestrone2.0–157.0–200.640.62estrogen 17β-dihydroequilin7-dehydro-17β-estradiol7.9–1137.9–1080.090.17estrogen 17α-dihydroequilin7-dehydro-17α-estradiol18.6 (18–41)14–320.240.57estrogen 17β-dihydroequilenin6,8-didehydro-17β-estradiol35–6890–1000.150.20estrogen 17α-dihydroequilenin6,8-didehydro-17α-estradiol20490.500.37estrogen δ8-estradiol8,9-dehydro-17β-estradiol68720.150.25estrogen δ8-estrone8,9-dehydroestrone19320.520.57estrogen ethinylestradiolee; 17α-ethynyl-17β-e2120.9 (68.8–480)44.4 (2.0–144)0.02–0.050.29–0.81estrogen mestranolee 3-methyl ether ?2.5 ? ?estrogen moxestrolru-2858; 11β-methoxy-ee35–435–200.52.6estrogen methylestradiol17α-methyl-17β-estradiol7044 ? ?estrogen diethylstilbestroldes; stilbestrol129.5 (89.1–468)219.63 (61.2–295)0.040.05estrogen hexestroldihydrodiethylstilbestrol153.6 (31–302)60–2340.060.06estrogen dienestroldehydrostilbestrol37 (20.4–223)56–4040.050.03estrogen benzestrol (b2)–114 ? ? ?estrogen chlorotrianisenetace1.74 ?15.30 ?estrogen triphenylethylenetpe0.074 ? ? ?estrogen triphenylbromoethylenetpbe2.69 ? ? ?estrogen tamoxifenici-46,4743 (0.1–47)3.33 (0.28–6)3.4–9.692.5serm afimoxifene4-hydroxytamoxifen; 4-oht100.1 (1.7–257)10 (0.98–339)2.3 (0.1–3.61)0.04–4.8serm toremifene4-chlorotamoxifen; 4-ct ? ?7.14–20.315.4serm clomifenemrl-4125 (19.2–37.2)120.91.2serm cyclofenilf-6066; sexovid151–152243 ? ?serm nafoxidineu-11,000a30.9–44160.30.8serm raloxifene–41.2 (7.8–69)5.34 (0.54–16)0.188–0.5220.2serm arzoxifenely-353,381 ? ?0.179 ?serm lasofoxifenecp-336,15610.2–16619.00.229 ?serm ormeloxifenecentchroman ? ?0.313 ?serm levormeloxifene6720-cdri; nnc-460,0201.551.88 ? ?serm ospemifenedeaminohydroxytoremifene0.82–2.630.59–1.22 ? ?serm bazedoxifene– ? ?0.053 ?serm etacstilgw-56384.3011.5 ? ?serm ici-164,384–63.5 (3.70–97.7)1660.20.08antiestrogen fulvestrantici-182,78043.5 (9.4–325)21.65 (2.05–40.5)0.421.3antiestrogen propylpyrazoletriolppt49 (10.0–89.1)0.120.4092.8erα agonist 16α-le216α-lactone-17β-estradiol14.6–570.0890.27131erα agonist 16α-iodo-e216α-iodo-17β-estradiol30.22.30 ? ?erα agonist methylpiperidinopyrazolempp110.05 ? ?erα antagonist diarylpropionitriledpn0.12–0.256.6–1832.41.7erβ agonist 8β-ve28β-vinyl-17β-estradiol0.3522.0–8312.90.50erβ agonist prinaberelerb-041; way-202,0410.2767–72 ? ?erβ agonist erb-196way-202,196 ?180 ? ?erβ agonist erteberelserba-1; ly-500,307 ? ?2.680.19erβ agonist serba-2– ? ?14.51.54erβ agonist coumestrol–9.225 (0.0117–94)64.125 (0.41–185)0.14–80.00.07–27.0xenoestrogen genistein–0.445 (0.0012–16)33.42 (0.86–87)2.6–1260.3–12.8xenoestrogen equol–0.2–0.2870.85 (0.10–2.85) ? ?xenoestrogen daidzein–0.07 (0.0018–9.3)0.7865 (0.04–17.1)2.085.3xenoestrogen biochanin a–0.04 (0.022–0.15)0.6225 (0.010–1.2)1748.9xenoestrogen kaempferol–0.07 (0.029–0.10)2.2 (0.002–3.00) ? ?xenoestrogen naringenin–0.0054 (<0.001–0.01)0.15 (0.11–0.33) ? ?xenoestrogen 8-prenylnaringenin8-pn4.4 ? ? ?xenoestrogen quercetin–<0.001–0.010.002–0.040 ? ?xenoestrogen ipriflavone–<0.01<0.01 ? ?xenoestrogen miroestrol–0.39 ? ? ?xenoestrogen deoxymiroestrol–2.0 ? ? ?xenoestrogen β-sitosterol–<0.001–0.0875<0.001–0.016 ? ?xenoestrogen resveratrol–<0.001–0.0032 ? ? ?xenoestrogen α-zearalenol–48 (13–52.5) ? ? ?xenoestrogen β-zearalenol–0.6 (0.032–13) ? ? ?xenoestrogen zeranolα-zearalanol48–111 ? ? ?xenoestrogen taleranolβ-zearalanol16 (13–17.8)140.80.9xenoestrogen zearalenonezen7.68 (2.04–28)9.45 (2.43–31.5) ? ?xenoestrogen zearalanonezan0.51 ? ? ?xenoestrogen bisphenol abpa0.0315 (0.008–1.0)0.135 (0.002–4.23)19535xenoestrogen endosulfaneds<0.001–<0.01<0.01 ? ?xenoestrogen keponechlordecone0.0069–0.2 ? ? ?xenoestrogen o,p'-ddt–0.0073–0.4 ? ? ?xenoestrogen p,p'-ddt–0.03 ? ? ?xenoestrogen methoxychlorp,p'-dimethoxy-ddt0.01 (<0.001–0.02)0.01–0.13 ? ?xenoestrogen hptehydroxychlor; p,p'-oh-ddt1.2–1.7 ? ? ?xenoestrogen testosteronet; 4-androstenolone<0.0001–<0.01<0.002–0.040>5000>5000androgen dihydrotestosteronedht; 5α-androstanolone0.01 (<0.001–0.05)0.0059–0.17221–>500073–1688androgen nandrolone19-nortestosterone; 19-nt0.010.2376553androgen dehydroepiandrosteronedhea; prasterone0.038 (<0.001–0.04)0.019–0.07245–1053163–515androgen 5-androstenediola5; androstenediol6173.60.9androgen 4-androstenediol–0.50.62319androgen 4-androstenedionea4; androstenedione<0.01<0.01>10000>10000androgen 3α-androstanediol3α-adiol0.070.326048androgen 3β-androstanediol3β-adiol3762androgen androstanedione5α-androstanedione<0.01<0.01>10000>10000androgen etiocholanedione5β-androstanedione<0.01<0.01>10000>10000androgen methyltestosterone17α-methyltestosterone<0.0001 ? ? ?androgen ethinyl-3α-androstanediol17α-ethynyl-3α-adiol4.0<0.07 ? ?estrogen ethinyl-3β-androstanediol17α-ethynyl-3β-adiol505.6 ? ?estrogen progesteronep4; 4-pregnenedione<0.001–0.6<0.001–0.010 ? ?progestogen norethisteronenet; 17α-ethynyl-19-nt0.085 (0.0015–<0.1)0.1 (0.01–0.3)1521084progestogen norethynodrel5(10)-norethisterone0.5 (0.3–0.7)<0.1–0.221453progestogen tibolone7α-methylnorethynodrel0.5 (0.45–2.0)0.2–0.076 ? ?progestogen δ4-tibolone7α-methylnorethisterone0.069–<0.10.027–<0.1 ? ?progestogen 3α-hydroxytibolone–2.5 (1.06–5.0)0.6–0.8 ? ?progestogen 3β-hydroxytibolone–1.6 (0.75–1.9)0.070–0.1 ? ?progestogen footnotes: a = (1) binding affinity values are of the format "median (range)" (# (#–#)), "range" (#–#), or "value" (#) depending on the values available. the full sets of values within the ranges can be found in the wiki code. (2) binding affinities were determined via displacement studies in a variety of in-vitro systems with labeled estradiol and human erα and erβ proteins (except the erβ values from kuiper et al. (1997), which are rat erβ). sources: see template page. selected biological properties of endogenous estrogens in rats estrogener rba (%)uterine weight (%)uterotrophylh levels (%)shbg rba (%) control–100–100– estradiol (e2)100506 ± 20+++12–19100 estrone (e1)11 ± 8490 ± 22+++ ?20 estriol (e3)10 ± 4468 ± 30+++8–183 estetrol (e4)0.5 ± 0.2 ?inactive ?1 17α-estradiol4.2 ± 0.8 ? ? ? ? 2-hydroxyestradiol24 ± 7285 ± 8+b31–6128 2-methoxyestradiol0.05 ± 0.04101inactive ?130 4-hydroxyestradiol45 ± 12 ? ? ? ? 4-methoxyestradiol1.3 ± 0.2260++ ?9 4-fluoroestradiola180 ± 43 ?+++ ? ? 2-hydroxyestrone1.9 ± 0.8130 ± 9inactive110–1428 2-methoxyestrone0.01 ± 0.00103 ± 7inactive95–100120 4-hydroxyestrone11 ± 4351++21–5035 4-methoxyestrone0.13 ± 0.04338++65–9212 16α-hydroxyestrone2.8 ± 1.0552 ± 42+++7–24<0.5 2-hydroxyestriol0.9 ± 0.3302+b ? ? 2-methoxyestriol0.01 ± 0.00 ?inactive ?4 notes: values are mean ± sd or range. er rba = relative binding affinity to estrogen receptors of rat uterine cytosol. uterine weight = percentage change in uterine wet weight of ovariectomized rats after 72 hours with continuous administration of 1 μg/hour via subcutaneously implanted osmotic pumps. lh levels = luteinizing hormone levels relative to baseline of ovariectomized rats after 24 to 72 hours of continuous administration via subcutaneous implant. footnotes: a = synthetic (i.e., not endogenous). b = atypical uterotrophic effect which plateaus within 48 hours (estradiol's uterotrophy continues linearly up to 72 hours). sources: see template. not to be confused with 17β-estradiol. this article is about 17α-estradiol as a hormone. for its use as a medication, see alfatradiol. 17α-estradiol names iupac name estra-1,3,5(10)-triene-3,17α-diol systematic iupac name (1r,3as,3br,9bs,11as)-11a-methyl-2,3,3a,3b,4,5,9b,10,11,11a-decahydro-1h-cyclopentaphenanthrene-1,7-diol other names 17α-e2; alpha-estradiol; alfatradiol; 17-epiestradiol; β-estradiol (obsolete, misleading) identifiers cas number 57-91-0 3d model (jsmol) interactive image chebi chebi:17160 chemspider 61840 echa infocard 100.000.322 pubchem cid 68570 unii 3vq38d63m7 comptox dashboard (epa) dtxsid8022377 inchi inchi=1s/c18h24o2/c1-18-9-8-14-13-5-3-12(19)10-11(13)2-4-15(14)16(18)6-7-17(18)20/h3,5,10,14-17,19-20h,2,4,6-9h2,1h3/t14-,15-,16+,17-,18+/m1/s1 smiles c12cc3(1cc2o)ccc4=c3c=cc(=c4)o properties chemical formula c18h24o2 molar mass 272.388 g·mol−1 except where otherwise noted, data are given for materials in their standard state (at 25 °c , 100 kpa). infobox references aging supplementation with 17α-estradiol increases the median lifespan of male mice by 19%, while not affecting female lifespan. this treatment does not lead to feminization of male mice. 17α-estradiol furthermore alleviates age-related metabolic and inflammatory dysfunction and improves glucose tolerance in male mice. the exact reason for this sex-specific increase in lifespan is unknown, however, the effect on male lifespan is gone in castrated mice, suggesting that the metabolic response to 17α-estradiol requires the presence of male gonadal hormones. whether these results are translatable to humans is currently unknown.