Testosterone in Women: How It Works, Why It Declines, and What Therapy Does

Where Testosterone in Women Actually Comes From

Women synthesize testosterone from three overlapping sources, not just the ovaries. The ovarian theca cells contribute roughly 25% of circulating testosterone, the adrenal glands another 25%, and peripheral tissues, primarily fat, skin, and muscle, account for the remaining approximately 50% through conversion of androstenedione and dehydroepiandrosterone (DHEA) [1]. This distributed production matters clinically: bilateral oophorectomy immediately drops serum testosterone by about 50% but does not eliminate it, because the adrenal and peripheral pathways persist [2].

The rate-limiting step in ovarian androgen production is LH-driven stimulation of theca cells via the CYP17A1 enzyme, which converts progesterone to androstenedione. Androstenedione then moves into granulosa cells, where aromatase (CYP19A1) converts part of it to estradiol. What is not aromatized leaves the ovary as androstenedione or is reduced to testosterone by 17β-hydroxysteroid dehydrogenase type 5 [3].

Sex hormone-binding globulin (SHBG) controls bioavailability. Only free testosterone (roughly 1 to 2% of total) and albumin-bound testosterone (roughly 30 to 40%) can enter cells. SHBG binds the rest tightly. Because the liver produces SHBG in response to estrogen, oral estrogen therapy raises SHBG concentrations by 2- to 4-fold, which suppresses free testosterone substantially, even when total testosterone looks unchanged on a lab report [4].

The Androgen Receptor Mechanism in Female Tissue

Testosterone's direct signaling runs through the androgen receptor (AR), a ligand-activated transcription factor encoded by the AR gene on the X chromosome. Binding testosterone causes the AR to dissociate from heat shock proteins, dimerize, translocate into the nucleus, and bind androgen response elements (AREs) in target gene promoters [5]. This classical genomic pathway changes gene expression over hours to days.

AR is expressed across a wide range of female tissues. These include the brain (hippocampus, hypothalamus, prefrontal cortex), skeletal muscle, bone, skin and hair follicles, clitoris, vaginal epithelium, and bladder trigone [6]. Each tissue exhibits different AR density and co-regulator profiles, which explains why testosterone influences mood, libido, muscle protein synthesis, bone mineral density, and urogenital atrophy through distinct downstream programs rather than a single shared effect.

A faster, non-genomic path also exists. Membrane-associated AR, or cross-talk with membrane estrogen receptors, activates kinase cascades (ERK1/2, PI3K/Akt) within minutes [7]. This rapid signaling partly underlies testosterone's acute effects on central arousal and attention.

Dihydrotestosterone (DHT) competes with testosterone for AR binding and has roughly 3- to 5-fold higher receptor affinity. The enzyme 5-alpha reductase converts testosterone to DHT predominantly in skin, genital tissue, and the prostate (in males). In women, 5-alpha reductase activity is lower than in men but clinically meaningful in the skin and clitoris, where DHT drives tissue sensitivity and engorgement responses [8].

Aromatization: How Testosterone Becomes Estradiol in Women

Aromatization is not a side effect. It is a designed conversion. The CYP19A1 enzyme catalyzes the irreversible conversion of testosterone to 17β-estradiol and of androstenedione to estrone. In premenopausal women, the ovarian granulosa cell is the principal aromatization site. After menopause, adipose tissue and muscle become the dominant sites, making peripheral aromatization the main source of circulating estradiol [9].

This has direct implications for testosterone therapy in postmenopausal women. Exogenous testosterone can raise estradiol concentrations, especially at supraphysiologic doses, because more substrate reaches peripheral aromatase. Monitoring both free testosterone and estradiol during therapy is therefore standard practice at HealthRX.

Aromatization also explains estrogen receptor signaling in contexts where clinicians measure testosterone. A woman with low testosterone often has low estradiol in tissues where local aromatization is the primary estrogen source, even if serum estradiol appears adequate. Skin, brain, and bone rely heavily on locally generated estradiol from testosterone precursors, so AR signaling and ERα/ERβ signaling in these compartments are linked [10].

Estrogen Receptor Mechanism: ERα and ERβ Cross-Talk with Androgens

Estrogen receptors alpha and beta (ERα, ERβ) are encoded by ESR1 and ESR2 respectively and belong to the same nuclear receptor superfamily as AR. Estradiol binds both with high affinity (Kd approximately 0.1 nM), while testosterone itself has only weak direct ER affinity, around 1,000-fold lower than estradiol [11]. The estrogenic effects of testosterone therefore depend almost entirely on prior aromatization.

ERα and ERβ have opposing effects in several tissues. ERα drives proliferative responses in the breast and endometrium, while ERβ generally acts as a brake on ERα-driven proliferation and may have protective roles in the breast [12]. In the brain, ERβ dominates in regions governing mood and cognition, which is one reason estradiol loss at menopause correlates with depressive symptoms and verbal memory decline independent of hot flashes.

Testosterone therapy, by generating local estradiol through aromatization, can activate both receptor subtypes in tissue-specific ratios. This is mechanistically distinct from administering estradiol directly, because the local substrate concentration and aromatase density shape which receptor subtype is preferentially activated. Clinically, this distinction partly explains why some women on combined estradiol plus testosterone report symptom relief that neither hormone provides alone at equivalent doses.

Progesterone Mechanism and Its Interaction with Testosterone Signaling

Progesterone acts through two nuclear progesterone receptors, PRA and PRB, and through membrane-bound progesterone receptors (mPR). PRB is the primary transcriptional activator; PRA modulates and often suppresses PRB activity depending on tissue context [13]. The progesterone receptor is itself estrogen-dependent: ERα upregulates PR expression, so without adequate estrogen signaling, progesterone receptor density falls and progesterone responsiveness diminishes.

Progesterone and testosterone share an indirect competitive relationship via SHBG. Progesterone does not bind SHBG significantly, but synthetic progestins, particularly those with androgenic structure like norethisterone, can displace testosterone from SHBG and raise free testosterone transiently. Micronized progesterone (oral Prometrium, vaginal Utrogestan) does not carry this effect and is generally preferred in women who are also using testosterone therapy to avoid unpredictable fluctuations in free androgen levels [14].

One underappreciated mechanism: progesterone inhibits 5-alpha reductase activity in skin, which may reduce DHT-mediated sebum production and hair follicle stimulation. Women using testosterone therapy alongside oral norethisterone-based progestins sometimes report acne or hair thinning partly because androgenic progestins block this protective 5-alpha reductase suppression [15].

Oral vs. Transdermal Delivery: The First-Pass Difference

Route of administration alters hormone bioavailability and the SHBG environment that governs free testosterone. Oral estrogen undergoes first-pass hepatic metabolism, during which the liver converts a large fraction of 17β-estradiol to the weaker estrone and estrone sulfate before systemic circulation [16]. More importantly for testosterone physiology, hepatic first-pass exposure drives a 2- to 4-fold increase in SHBG synthesis. This sharply reduces free testosterone availability even in women who are not taking exogenous testosterone.

Transdermal estradiol, delivered via patch, gel, or spray, bypasses first-pass metabolism. It maintains a steadier estradiol-to-estrone ratio closer to the premenopausal physiologic pattern and produces minimal SHBG elevation at standard doses [17]. A 2010 observational analysis published in BMJ (N=83,123) found that transdermal estradiol was not associated with elevated venous thromboembolism risk, while oral estradiol carried an odds ratio of 2.5 (95% CI 1.8 to 3.5) for VTE [18]. The SHBG difference means that a woman switching from oral to transdermal estrogen may notice increased energy and libido simply because her free testosterone concentration rises, without any change in testosterone prescription.

Testosterone itself is typically delivered transdermally in women (cream, gel, or low-dose patch) because oral testosterone undergoes near-total first-pass inactivation in the liver and produces toxic hepatic metabolites. Pellet implants offer sustained release over three to six months but make dose adjustment difficult once inserted. Injectable testosterone esters produce supraphysiologic peaks and troughs unsuitable for female dosing ranges [19].

HealthRX Clinical Framework: Matching Delivery Route to Hormonal Goal

| Goal | Preferred estrogen route | Preferred progesterone form | Testosterone add-on? | |---|---|---|---| | Symptom relief, lowest VTE risk | Transdermal | Micronized oral or vaginal | Yes, transdermal cream/gel | | Endometrial protection (intact uterus) | Either | Micronized oral (200 mg/night) or IUD | Yes, if indicated | | Maximizing free testosterone without Rx change | Switch oral to transdermal E2 first | Avoid androgenic progestins | Reassess free T before adding Rx | | Premature ovarian insufficiency (POI) | Transdermal (higher dose) | Micronized oral cyclically | Yes, often needed |

Estrogen Receptor Decline at Menopause and What It Means for Androgen Signaling

Menopause is defined as 12 consecutive months without menstruation, occurring at a median age of 51.3 years in US women [20]. Ovarian follicle depletion eliminates the granulosa cell aromatase pool, dropping serum estradiol from premenopausal levels of 100, 400 pg/mL to postmenopausal levels below 20 pg/mL in most women. Simultaneously, ovarian testosterone output falls roughly 50% because LH-driven theca cell stimulation loses its follicular support structure [21].

The loss of estradiol reduces ERα and ERβ expression in multiple tissues. Because PR expression depends on ERα, progesterone receptor density also falls, creating a cascade of receptor downregulation that goes beyond simple hormone deficiency. AR expression is similarly sensitive to the estrogenic environment: studies in rodent models show that castration-induced estrogen loss reduces AR protein levels in hippocampal tissue by approximately 30%, a finding with plausible human correlates given the cognitive changes reported at menopause [22].

This receptor-level decline compounds the substrate decline. A postmenopausal woman does not just have less testosterone circulating; she also has fewer receptors primed to respond to what remains. This is one mechanistic reason why hormone therapy often works best when initiated close to the menopausal transition, during what is now called the "critical window" or "timing hypothesis" formalized by Salpeter et al. [23]. Estrogen therapy restores receptor density, which may sensitize tissues to respond to both endogenous and exogenous testosterone more effectively.

The North American Menopause Society (NAMS) 2022 Hormone Therapy Position Statement notes: "For women who are bothered by symptoms attributable to androgen insufficiency, testosterone therapy may be considered as an adjunct to estrogen therapy after thorough evaluation and counseling regarding risks and benefits" [24].

Evidence Base for Testosterone Therapy in Women

The Global Consensus Position Statement on the Use of Testosterone Therapy for Women (2019), endorsed by NAMS, the British Menopause Society, and the International Menopause Society, reviewed 36 randomized controlled trials and concluded that testosterone therapy at physiologic doses significantly improves sexual function, specifically hypoactive sexual desire disorder (HSDD), in postmenopausal women [25].

The APHRODITE trial (N=814) demonstrated that transdermal testosterone 300 mcg/day increased satisfying sexual events by 74% vs. 41% placebo at 52 weeks (P<0.0001), with no significant increase in androgenic adverse events at that dose [26]. A Cochrane review (2019) of 35 trials (N=4,835) confirmed statistically significant improvements in sexual desire, arousal, and orgasm frequency with testosterone therapy, with acne and hair growth as the most common adverse effects, both dose-dependent [27].

Bone and muscle data are emerging but not yet sufficient for guideline-level recommendations. A 24-week RCT in postmenopausal women found that transdermal testosterone 10 mg/day (a higher-than-standard dose) increased lean mass by 1.0 kg and reduced fat mass by 0.9 kg vs. placebo (P<0.05) [28]. Whether standard female doses (0.5 to 2 mg/day transdermal) produce meaningful body composition effects without pharmacologic dosing remains under investigation.

The absence of an FDA-approved female testosterone product in the United States means clinicians use off-label compounded preparations or male products at fractional doses. The Global Consensus Statement recommends targeting a total testosterone level in the premenopausal physiologic range, roughly 15 to 70 ng/dL, using high-sensitivity liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays rather than standard immunoassays, which are unreliable at low female concentrations [25].

Monitoring Free Testosterone: Why Total T Alone Misses the Picture

Total testosterone measurements capture bound and unbound hormone together. Because SHBG varies substantially between individuals and with oral estrogen use, two women with identical total testosterone values may have free testosterone concentrations differing by 3-fold. The calculated free testosterone using the Vermeulen formula or direct equilibrium dialysis provides a more accurate clinical picture [29].

At HealthRX, baseline labs before any testosterone therapy include total testosterone by LC-MS/MS, SHBG, albumin (for Vermeulen calculation), estradiol, and a complete metabolic panel. Follow-up testing at 6 weeks after initiation and every 6 months during stable therapy checks free testosterone, hematocrit (testosterone stimulates erythropoiesis even at female doses in some women), and lipid panel.

The NAMS 2022 position statement specifies: "Measurement of serum testosterone using a validated, accurate assay, and clinical assessment of androgenic symptoms and signs, should guide therapy, with the goal of achieving premenopausal physiologic concentrations" [24].