Testosterone 1/2: The Hormone You Think You Know

This article is Part 1 of our two-part series on testosterone. Part 2 will look at what can actually be done to support appropriate levels, in both men and women.

You probably know testosterone for its association with masculinity, muscle, and libido. And most likely as a predominantly male hormone.

That is partly true, but this view is very incomplete.

Testosterone is an androgen, meaning a steroid hormone produced by both the male and female body. Its concentrations are significantly higher in men, but it is far from secondary in women: before menopause, women produce it daily in amounts at least comparable to, and probably greater than, estradiol.

In both sexes, testosterone contributes to sexual function, the maintenance of muscle and bone tissue, body composition, as well as certain metabolic and brain functions.

Its concentrations change throughout life. In men, they are generally highest in early adulthood, then tend to decline progressively. This evolution varies considerably and also depends on health status and lifestyle.

In women, the decline begins earlier than we might think. One large study found androgen levels to be approximately twice as low at ages 40 to 45 as at ages 20 to 25. A change that is still rarely discussed in women's hormonal health.

Sleep, metabolic health, body composition, certain medications, contraception, various medical conditions, and some environmental exposures can also influence testosterone production or availability.

Testosterone is an androgen. But what does that actually mean?

Androgens are a family of hormones that includes DHEA-S, androstenedione, testosterone, and dihydrotestosterone, or DHT.

They are often described as "male hormones" because of their role in male sexual development. But women produce them too.

In men, testosterone is mainly produced in the testes. In women, it comes from the ovaries, the adrenal glands, and the conversion of other androgens within tissues.

Its production is controlled by the hypothalamic-pituitary-gonadal axis: the brain releases signals, including GnRH and then LH, which stimulate the gonads. Sex hormones then send signals back to the brain to adjust this stimulation. This is known as negative feedback.

What testosterone actually does

In men, testosterone contributes to the development of the reproductive organs, sperm production, libido, and erectile function. It also helps maintain muscle mass and strength, bone density, red blood cell production, and body fat distribution.

In women, it contributes to sexual desire and response, ovarian function, the maintenance of muscle and bone tissue, and the physiology of genital tissues. It can also be converted locally into estradiol.

So testosterone is also a female hormone, although its interpretation differs from that in men.

A normal total testosterone level does not tell the whole story

In the bloodstream, testosterone circulates mainly bound to two proteins.

SHBG, produced primarily by the liver, binds testosterone strongly and limits its immediate availability. Albumin binds it more loosely, meaning this fraction can dissociate more easily. Finally, a very small amount circulates freely and can directly access androgen receptors.

Free testosterone and albumin-bound testosterone are considered bioavailable. Total testosterone includes all of these fractions.

A normal total testosterone level can therefore coexist with high SHBG and a lower free fraction. Total testosterone alone does not always reflect how much testosterone is actually available.

Recognizing low testosterone in men

In men, pathological testosterone deficiency is known as hypogonadism. Diagnosis is based on the combination of compatible symptoms and low testosterone concentrations confirmed by at least two morning measurements.

Sexual symptoms are generally the most suggestive:

• persistent decrease in sexual desire;

• fewer spontaneous or morning erections;

• difficulty achieving or maintaining an erection;

• impaired sperm production and fertility.

Other signs may also occur: reduced muscle mass and strength, increased body fat, reduced body hair, bone fragility, anemia, low energy, or depressed mood.

A persistent loss of morning erections can be a useful clue, but it is not enough to make a diagnosis: sleep, vascular health, certain medications, and other medical conditions can also affect them.

What about women?

In women, interpretation can be more complex.

Reduced desire, arousal, or pleasure, persistent fatigue, loss of strength, or more difficult recovery may be associated with lower androgen levels.

But unlike in men, there is no single biological threshold that can diagnose testosterone deficiency in women. No blood test can, on its own, explain low libido.

The strongest therapeutic evidence concerns certain postmenopausal women with persistent hypoactive sexual desire disorder, after other possible causes have been assessed. This does not support presenting testosterone as a general treatment for fatigue, brain fog, or muscle loss.

Can testosterone also be too high?

In women, excess androgens can manifest as persistent acne, increased body or facial hair, androgenic hair loss, irregular or absent periods, and ovulatory dysfunction.

Hyperandrogenism is one of the central features of Polyendocrine Metabolic Ovarian Syndrome (PMOS, formerly PCOS), the clinical condition most commonly associated with this profile. Certain ovarian or adrenal disorders can also cause markedly elevated androgen levels.

Symptoms that appear suddenly, progress rapidly, or are accompanied by virilization require medical evaluation. The change from PCOS to PMOS was announced in May 2026 to better reflect the endocrine and metabolic dimensions of the syndrome.

In men, spontaneous testosterone excess is much rarer. Very high concentrations are mainly seen with exogenous testosterone or anabolic steroid use. They can promote acne, increase hematocrit, affect mood or sleep, and suppress the body's natural production of testosterone and sperm.

External testosterone reduces the LH and FSH signals sent to the testes: intratesticular testosterone production slows down, and fertility can be significantly affected, sometimes even after treatment has been stopped.

Do men have less testosterone today than they used to?

Several studies have observed lower average testosterone levels in men of the same age from more recent generations. This does not necessarily apply to all men, but it suggests that aging is not the only factor involved.

A decline in testosterone rarely has a single cause. It may reflect changes in its production, the signals sent from the brain to the testes, its transport in the bloodstream, or overall health.

Sleep

In men, testosterone rises during sleep and generally reaches its highest concentrations in the morning.

In a small study of young men, one week of sleep restricted to five hours per night reduced daytime testosterone by around 10 to 15%. To put this into perspective, this is equivalent to the physiological decline normally seen over approximately 10 to 15 years of aging. Fragmented sleep and obstructive sleep apnea can also disrupt this rhythm.

Chronic stress and insufficient recovery

Prolonged stress can interfere with communication between the brain and the gonads. It also acts indirectly by worsening sleep, recovery, libido, eating habits, and metabolic health.

Metabolic health and body composition

Excess body fat, particularly abdominal fat, is frequently associated with lower testosterone concentrations in men.

Adipose tissue contains aromatase, an enzyme that converts some testosterone into estradiol, but insulin resistance, inflammation, and changes in hormonal signaling are also involved.

A vicious cycle can then develop: poor metabolic health contributes to lower testosterone, while a true testosterone deficiency can make it more difficult to maintain muscle mass.

Energy and nutrient intake

Testosterone is synthesized from cholesterol. Insufficient energy intake, highly restrictive diets, or diets excessively low in fat can interfere with its production.

Low zinc or vitamin D status may also be associated with less favorable hormonal function. High or regular alcohol consumption can also affect testicular function, the liver, sleep, and metabolic health.

Medications and exogenous hormones

Certain opioids, glucocorticoids, and treatments that affect the hormonal axis can reduce testosterone production or alter its availability.

In women, combined oral contraceptives lower both total and free testosterone on average while increasing SHBG. This biological effect is well established, but its consequences vary from one woman to another. It is also worth noting that elevated SHBG can persist for several months after stopping oral contraception.

Endocrine disruptors

Certain compounds found in plastics, cosmetics, packaging, pesticides, or indoor air can interfere with the hormonal system.

Bisphenols, phthalates, certain parabens, and microplastics do not all act in the same way. Depending on the substance, they may alter hormone production, transport, conversion, or action.

Human data remain complex, particularly because we are exposed to multiple substances at the same time. The concern therefore relates more to cumulative exposure than to the effect of a single isolated product.

What Lucis measures

Testosterone should never be interpreted in isolation.

Its value depends on how much is produced, how much is actually available, how well the hormonal axis is functioning, and the metabolic context. This is why Lucis assesses it alongside complementary markers: total and free testosterone, SHBG, DHEA-S, estradiol, LH, FSH, prolactin, thyroid function, glucose, HbA1c, insulin, vitamin D, and iron status.

The goal is not simply to determine whether a level falls within a reference range, but to understand what it means in context and which factors may be contributing to symptoms or observed variations.

The podcast episode

Bastien Intartaglia is a PhD in cognitive neuroscience, a men's health specialist and the founder of Code Primal. In this episode we look at what testosterone really is, what makes it fall, how to read a blood panel properly, and which natural levers actually work.

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The information in this article is provided for educational purposes only and does not constitute medical advice. It is not a substitute for consultation with a qualified healthcare professional. Lucis is not a medical device and does not provide diagnoses.

References

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British Menopause Society. Testosterone Replacement in Menopause. Tools for Clinicians. Updated May 2026.

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