Testosterone: Functions, Levels & Testing

Testosterone is the main androgen, or "male" sex hormone, although it is important in every body. It supports the development of reproductive tissues, muscle and bone, and contributes to mood, energy, and sex drive in people of all sexes.

What testosterone is

Testosterone is a steroid hormone built from cholesterol through a series of enzyme-driven steps shared with other steroid hormones such as cortisol and estradiol. It belongs to a family of hormones called androgens, which drive the development of typically masculine characteristics. Both men and women produce testosterone, though men make substantially more of it. Because it is fat-soluble rather than water-soluble, testosterone does not dissolve freely in blood plasma on its own; instead it travels attached to carrier proteins, and it acts inside cells by binding to the androgen receptor and influencing which genes are switched on.

Within tissues, testosterone is also a precursor for two other important hormones. The enzyme 5-alpha-reductase converts it into dihydrotestosterone (DHT), a more potent androgen that is especially active in the skin, hair follicles, and prostate. The enzyme aromatase converts a portion of testosterone into estradiol, an estrogen that matters for bone health, mood, and libido in people of all sexes. This means testosterone is not only a hormone in its own right but also a raw material the body adapts for several jobs.

Where it is produced

In men, the testes produce the majority of testosterone in specialized cells called Leydig cells. In women, the ovaries and the adrenal glands produce smaller amounts. In both sexes, the adrenal glands contribute precursor hormones such as DHEA and androstenedione that can be converted into testosterone in peripheral tissues like fat, skin, and liver. This peripheral conversion is one reason that body composition and overall metabolic health can influence circulating androgen activity.

The HPG axis: how production is controlled

Testosterone production is governed by a feedback loop known as the hypothalamic–pituitary–gonadal (HPG) axis. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses. These pulses prompt the pituitary gland to secrete two gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH signals the Leydig cells of the testes (or the ovaries) to make testosterone, while FSH supports sperm production in men and follicle development in women.

As testosterone rises, it feeds back to the hypothalamus and pituitary to slow GnRH and LH release — a self-correcting loop that keeps levels within a typical range. Part of this feedback works through estradiol, after testosterone is aromatized. Because the system is a loop, measuring LH and FSH alongside testosterone can help a clinician work out where in the axis a problem sits: low testosterone with high LH/FSH points toward the gonads themselves, while low testosterone with low or inappropriately normal LH/FSH points toward the pituitary or hypothalamus.

Testosterone in men also follows a daily rhythm, generally peaking in the early morning and declining through the day. Sleep is closely tied to this pattern, and disrupted or insufficient sleep can blunt the morning peak. These rhythms are a major reason that the timing of a blood draw matters when interpreting a result.

What it does across body systems

Testosterone has wide-ranging effects that extend well beyond reproduction:

• Reproductive development: During puberty it drives growth of the penis and testes, supports sperm production, and contributes to deepening of the voice and growth of facial and body hair.
• Muscle: It supports muscle protein synthesis and helps maintain lean mass and strength across the lifespan.
• Bone: Both testosterone and the estradiol made from it help maintain bone mineral density. Long-standing deficiency can be associated with weaker bones.
• Blood: It stimulates the bone marrow and the hormone erythropoietin to produce red blood cells, which is why androgen levels can affect the red cell count.
• Skin and hair: Through DHT, it influences oil (sebum) production and the pattern of body and scalp hair.
• Brain, mood, and libido: It contributes to sexual desire and may influence energy, motivation, and mood in people of all sexes, though these links are complex and individual.
• Metabolism: It interacts with fat distribution and insulin sensitivity, part of a two-way relationship in which body composition also affects hormone levels.

Free versus bound testosterone

Most testosterone in the bloodstream is not freely available. The majority travels tightly bound to sex hormone–binding globulin (SHBG), a smaller share is loosely bound to albumin, and only a small percentage circulates completely "free." The free fraction, together with the loosely albumin-bound portion, makes up what is sometimes called bioavailable testosterone — the part most readily available to tissues.

This matters because anything that changes SHBG changes the balance between bound and free hormone even when total testosterone looks unchanged. SHBG tends to be higher with aging, certain thyroid states, liver conditions, and some medications, and tends to be lower with obesity and insulin resistance. As a result, two people with the same total testosterone can have meaningfully different amounts of active hormone.

Illustrative concept only; the exact proportions vary by individual, age, and sex, and laboratories differ in how they measure or calculate each fraction.

What can raise or lower testosterone

Many factors nudge testosterone up or down, described here only in general terms. Levels can be lower in the context of aging, acute illness, poor or short sleep, significant physical or psychological stress, obesity, and certain chronic conditions. Some medications and treatments can also reduce it. Levels or measured values can appear higher with certain hormone-producing conditions or with use of androgen-containing products. Because so many of these influences are temporary, a single reading captures a moment in time rather than a fixed set point — which is why clinicians look at patterns rather than one number.

Age-related change

Testosterone rises sharply during puberty, tends to be highest in early adulthood, and in men generally declines gradually from midlife onward. This decline is slow and varies a great deal between individuals; many older men remain within typical ranges. In women, androgen levels also tend to decline with age, with shifts around the menopause transition. Whether an age-related change is clinically meaningful depends on symptoms, the overall picture, and repeat testing — not on age alone.

What high or low levels can be associated with

Lower-than-typical testosterone in men may be associated with reduced libido, fatigue, low mood, decreased muscle mass, and changes in bone strength. In women, elevated androgen activity can be associated with conditions such as polycystic ovary syndrome, sometimes with acne or excess hair growth; you can read more general background in the conditions section. These associations are qualitative; only a clinician can determine whether a level is meaningful for a given person.

How it is measured in blood

Testosterone is measured with a blood sample, often drawn in the morning when levels are highest. Laboratories may report total testosterone (bound plus free) and sometimes free testosterone measured directly or calculated from total testosterone and SHBG. Different methods — immunoassay versus mass spectrometry — can give somewhat different results, which is one reason values are interpreted against the specific lab's reference range. For a deeper walk-through of the test itself, see the testosterone blood test guide, and for general background on testing visit the blood tests overview. If a deficiency is confirmed, the testosterone replacement therapy overview explains, in neutral terms, how treatment is generally approached.