What is testosterone and how it works
Testosterone is the primary male sex hormone but it affects much more than reproductive function. The hormone influences muscle, bone, brain, energy, mood and many other body systems. Knowing what testosterone is and how it works provides foundation for understanding everything else about this important hormone. Here is the practical guide.
What testosterone is
Testosterone is a steroid hormone with specific chemical structure and biological functions. The basics matter for understanding everything else.
Chemical nature
Testosterone is a steroid hormone derived from cholesterol through several enzymatic steps. The molecular structure determines how it interacts with the body. Steroid hormones can pass through cell membranes directly which affects how they work compared to other hormone types.
Where it is made
About 95 percent of testosterone in men is produced in the testes by Leydig cells. Small amounts come from the adrenal glands. The location of production matters for how the body controls testosterone levels and for understanding what can go wrong.
How much is normal
Healthy adult men produce 4 to 7 milligrams of testosterone daily. Blood levels typically range 9 to 30 nmol/L (260 to 950 ng/dL). The production declines with age. Wide individual variation exists within healthy ranges.
Both sexes produce some
Women produce small amounts of testosterone from ovaries and adrenal glands. Female levels are much lower than male levels but the hormone still matters for female health. Testosterone is often called male but is biologically important in both sexes.
The mechanism
Testosterone works through specific mechanisms in the body. Knowing how helps understand its broad effects.
Receptor binding
Testosterone binds to androgen receptors in target tissues. The receptors are intracellular meaning they sit inside cells. Bound testosterone receptor complexes affect gene expression directly. The mechanism produces lasting cellular changes rather than brief effects.
Conversion to other hormones
Testosterone converts to other active hormones in specific tissues. DHT (dihydrotestosterone) via 5 alpha reductase in prostate, skin and hair follicles. Oestrogen via aromatase in fat tissue and other locations. The conversions extend testosterones effects.
Bound versus free
Most testosterone in blood is bound to proteins (SHBG and albumin). Only the small unbound fraction can directly affect tissues. The bound testosterone serves as a reservoir. The balance affects how much is biologically active despite total levels.
Tissue specific effects
Different tissues respond differently to testosterone. Muscle, bone, brain, skin and hair all have androgen receptors but produce different effects. The tissue specific responses explain why testosterone affects so many different body systems.
Body wide effects
Testosterone affects many body systems. The diverse effects explain why the hormone matters for overall health.
Reproductive function
Sperm production, libido, erectile function, masculine sexual characteristics. The primary functions that give testosterone its reputation as the male hormone. Reproductive effects are well known and well established.
Muscle and bone
Muscle protein synthesis supporting muscle mass. Bone formation and maintenance. Body composition (more muscle, less fat). The musculoskeletal effects matter for physical function throughout life.
Brain and mood
Mood regulation, motivation, cognitive function, energy levels. Brain receptor effects directly. Indirect effects through other body systems. The brain effects matter for quality of life and daily functioning.
Other systems
Red blood cell production. Skin oil production. Hair growth patterns. Voice deepening at puberty. Cardiovascular effects. The diverse effects extend through nearly all body systems. The hormone is genuinely body wide in its actions.
How testosterone changes
Testosterone varies dramatically across life stages. The variation produces different effects at different ages.
Prenatal and infancy
Brief testosterone surge during fetal development drives male sexual development. Brief postnatal surge in male infants for unclear reasons. Low levels through childhood. The early exposure shapes development for later life.
Puberty
Dramatic rise in testosterone from age 9 to 14 in boys. Drives all the changes of male puberty including voice changes, hair growth, muscle development, genital maturation, growth spurts. The peak development phase.
Young adulthood peak
Testosterone peaks between ages 18 and 30. The reference range for adult testosterone applies to this period. Peak levels support peak reproductive function, muscle mass and energy. The early adult years are testosterone optimised.
Gradual decline
Testosterone declines roughly 1 to 2 percent per year after age 30. By age 70, average levels are 30 to 50 percent below peak. Individual variation is significant. Some men maintain youthful levels into older age. Others see significant decline in middle age.
What testosterone is and how it works sits at the foundation of the Understanding Testosterone hub alongside articles on production, regulation and what testosterone does in specific body systems. For the complete library, see our Understanding Testosterone Hub.
More from the Understanding Testosterone hub
This guide sits inside the Understanding Testosterone hub covering everything from how the hormone works to lifestyle factors that affect levels, signs of deficiency and treatment options. Head back to the hub for the full library.
Keep reading
For production details, our How Testosterone Is Produced and Regulated covers the biological systems. Testosterone Levels Explained covers what numbers mean. And How Much Does Testosterone Decrease With Age covers age related changes.
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Testosterone Levels Explained