Here is a hormone trivia question that stumps most people, including a surprising number of physicians: what is the most abundant biologically active sex hormone in women across most of their lifespan?
If you said estrogen, you are in good company. You are also wrong.
The answer is testosterone. And the story of how that fact got buried — and what it costs women when it stays buried — is one of the more consequential oversights in modern medicine.
The Biosynthetic Reality
Before getting into the clinical implications, it helps to understand the plumbing. Sex hormones are not manufactured independently. They are synthesized in a cascade, each one serving as a precursor for the next. The pathway runs roughly like this: cholesterol becomes pregnenolone, pregnenolone becomes progesterone and DHEA, those become androstenedione, androstenedione becomes testosterone, and testosterone becomes estradiol via the enzyme aromatase.
Read that pathway carefully. Testosterone does not appear downstream of estrogen. It is the precursor to estrogen. Estradiol cannot be made in the body without testosterone being made first.
In women, this synthesis happens in two primary locations: the ovaries and the adrenal glands. The ovaries produce androgens in the theca cells, which then diffuse to neighboring granulosa cells where aromatase converts them to estrogens. The adrenal glands contribute DHEA and androstenedione, which peripheral tissues also convert to testosterone. Some testosterone is also synthesized directly in peripheral tissues — fat, skin, muscle — from those circulating precursors.
The result is that women, at their reproductive peak, typically circulate testosterone at concentrations between 15 and 70 ng/dL (0.15–0.70 ng/mL), depending on assay method. The male reference range sits at 300–1,000 ng/dL. Women have roughly 10–20 times less testosterone than men, which is a meaningful difference — but it is emphatically not zero.
In fact, before menopause, testosterone concentrations in women are substantially higher than estradiol concentrations when you compare both in the same mass-based units (pg/mL). Testosterone operates at levels that are physiologically active in muscle, bone, brain, skin, and libido circuitry. It is not some trace residual from evolutionary accident. It is a primary hormone that has primary functions.
What Testosterone Does in the Female Body
Testosterone binds to androgen receptors distributed throughout the female body. Those receptors are not sparse. They are present in the brain (including areas governing mood, cognition, and sexual motivation), in skeletal muscle, in bone, in skin, in adipose tissue, in the cardiovascular system, and in the reproductive tract.
The clinical functions of testosterone in women include:
- Libido and sexual motivation. This is the most extensively studied domain. Androgen receptors in the hypothalamus and limbic system mediate sexual desire. Low testosterone in women is the most reliably documented cause of reduced libido unrelated to relationship factors or psychological distress.
- Energy and sense of wellbeing. Women with low testosterone frequently report fatigue that is qualitatively different from ordinary tiredness — a flat, motivational kind of exhaustion that does not fully resolve with rest.
- Muscle mass and physical performance. Testosterone has anabolic effects in women as well as men, though at much lower concentrations. Research by Huang, Basaria, and Bhasin (2014) at Boston University — a rigorous dose-response trial in surgically menopausal women — demonstrated that testosterone administration produced dose-dependent increases in lean body mass and muscle power.
- Cognitive function. There is accumulating, though not definitive, evidence for testosterone’s role in cognitive clarity, spatial processing, and verbal memory in women.
- Bone density. The skeleton is an androgen-responsive tissue. The androgens DHEA and testosterone contribute independently to bone mineral density maintenance, separate from estrogen’s contributions.
The Decline That Nobody Told Women About
Here is where the story gets less comfortable. Female testosterone levels do not decline sharply at menopause the way estrogen does. They decline earlier, and gradually, starting around age 20 and continuing through the fourth and fifth decades.
By the time a woman reaches her early 40s, her testosterone levels may be roughly half what they were in her 20s. By the time she reaches natural menopause — typically around age 51 in the United States — her testosterone levels have been declining for two decades. Surgical menopause, meaning removal of the ovaries (oophorectomy), produces a far more abrupt drop: studies have shown that bilateral oophorectomy can reduce testosterone levels by approximately 50% almost immediately, because the ovaries are responsible for a substantial share of total production even in the postmenopausal state.
This is why women who have undergone surgical oophorectomy tend to experience more severe sexual dysfunction than naturally menopausal women, even when given estrogen replacement — the estrogen addresses the estrogen deficiency but leaves the testosterone deficiency untreated.
Why Medicine Largely Ignored This
Several factors contributed to a decades-long institutional blind spot.
First, the hormone was associated with men. This was, from a historical perspective, a classification problem that calcified into an assumption. Testosterone was identified and named in the 1930s in the context of male physiology. Its presence in women was treated as background noise rather than signal.
Second, the clinical consequences of female testosterone deficiency are subtle relative to menopausal estrogen deficiency. The hot flashes, vaginal atrophy, and mood instability of estrogen withdrawal are hard to miss. Declining libido, low energy, and reduced muscle tone are easier to attribute to aging, stress, or “just getting older.”
Third, pharmaceutical infrastructure was never built around female testosterone. No major pharmaceutical company pursued an FDA-approved testosterone product specifically for women in the United States. The result was that clinicians had no clean regulatory pathway to follow, no approved dosing form, and limited clinical guidance — making it easier to simply not address the issue.
The irony is that the evidence for testosterone’s importance in female physiology is not particularly new. It was being published in peer-reviewed journals through the 1970s, 1980s, and 1990s. It simply did not accumulate into a clinical standard of care. Women experiencing fatigue, cognitive fog, reduced libido, and declining muscle mass were told these were expected consequences of aging. Some of them were. Some of them were not.
The Numbers That Matter
To put some order around the terminology: “low testosterone” in women is generally defined as total testosterone below 15–20 ng/dL, depending on the clinical context and the laboratory’s assay methodology. Because testosterone assays were designed and validated for male ranges, measuring female testosterone accurately has historically been difficult. Mass spectrometry-based assays (LC-MS/MS) are significantly more accurate at the low end of the female range than older immunoassay methods, a distinction that matters a great deal when the clinical cutpoints are in the 15–30 ng/dL range.
This assay accuracy problem is one reason female androgen deficiency has been underdiagnosed: the tools used to measure it were not calibrated for it.
Where This Series Goes
The story of testosterone in women’s health is ultimately a story about a clinical gap that is slowly being closed — by researchers, by evidence, and by patients who pushed back against being told their symptoms were not real. Over the next four posts, we will look at what low testosterone actually does to women’s health outcomes, why the regulatory environment around female testosterone treatment remains stuck, how dosing works when it differs from male protocols by a factor of ten, and where the research currently stands.
It turns out the hormone everyone associates with men is one of the most important hormones in women. Knowing that is step one.
Next up: Post 2 — What Low Testosterone Actually Does to Women: the clinical evidence for fatigue, low libido, reduced muscle, and why surgical menopause is its own category of hormonal disruption.
