How testosterone is produced and regulated

Where testosterone comes from

Most testosterone is produced in the testes. Small amounts come from other tissues. The production process is regulated tightly by feedback systems.

Testicular production

About 95 percent of testosterone in men is produced in the testes. Specialised cells called Leydig cells produce the hormone from cholesterol through several enzymatic steps. The testes produce about 4 to 7 mg of testosterone per day in healthy young men.

Adrenal contribution

The adrenal glands produce small amounts of testosterone and testosterone precursors. The contribution is around 5 percent of total testosterone in men. In women the adrenal contribution is proportionally larger. Adrenal disorders can disrupt this small but real contribution.

Peripheral conversion

Some testosterone in the bloodstream comes from conversion of weaker androgens (like DHEA from the adrenals) into testosterone in peripheral tissues. The conversion happens in fat tissue, muscle and other locations. This source is small in healthy men.

Daily production rate

Healthy young men produce about 4 to 7 milligrams of testosterone per day. The amount declines with age. The continuous production replaces testosterone that is used or metabolised. The balance between production and clearance determines circulating levels.

How the body regulates production

The hypothalamic pituitary gonadal axis controls testosterone production. The three level system maintains levels within a target range.

Hypothalamus

The hypothalamus in the brain releases gonadotropin releasing hormone (GnRH) in pulses. GnRH signals the pituitary gland below. The pulsatile pattern of GnRH release drives the rest of the system. Disruption of GnRH affects everything downstream.

Pituitary gland

The pituitary gland responds to GnRH by releasing luteinising hormone (LH) and follicle stimulating hormone (FSH). LH stimulates the testes to produce testosterone. FSH supports sperm production. Both hormones travel through the bloodstream to the testes.

Testes

Leydig cells in the testes respond to LH by producing testosterone. The testosterone enters the bloodstream and reaches target tissues throughout the body. The testes also produce sperm in separate cells called Sertoli cells supported by FSH.

Feedback loops

Testosterone in the bloodstream signals back to the hypothalamus and pituitary, suppressing further GnRH and LH release when levels are adequate. The negative feedback maintains testosterone within a target range. The system is similar to a thermostat.

Where things go wrong

Disruption can occur at any level of the system. Knowing where helps understand specific testosterone problems.

Testicular problems (primary hypogonadism)

When the testes themselves cannot produce adequate testosterone despite normal signalling from above. Causes include testicular injury, infection, genetic conditions or chemotherapy. The pituitary increases LH trying to compensate, producing high LH alongside low testosterone.

Pituitary or hypothalamic problems (secondary hypogonadism)

When the signalling from the brain is disrupted. Causes include pituitary tumours, head injury, opioid use, chronic illness or genetic conditions. LH and FSH are low alongside low testosterone. The testes are normal but not receiving signals.

External testosterone

TRT or anabolic steroid use provides testosterone from outside the body. The system detects adequate levels and shuts down natural production. LH and FSH drop. Natural testosterone production stops. The effect is reversible after stopping in most cases.

Lifestyle factors

Sleep, weight, stress, alcohol and other factors affect the system at multiple levels. The effects are usually moderate but cumulative. Combined lifestyle factors can produce significant testosterone effects. Address these systematically for meaningful improvement.

How it works in the body

Once produced, testosterone affects many tissues throughout the body. Knowing what it does helps understand why it matters.

Direct effects

Testosterone binds to androgen receptors in target tissues, producing direct effects. Muscle, bone, skin, hair follicles, brain tissue and other organs respond directly to testosterone. The effects include muscle protein synthesis, bone mineralisation and various other functions.

Conversion to DHT

In some tissues, testosterone converts to dihydrotestosterone (DHT) which is more potent. DHT produces specific effects in prostate, skin and hair follicles. The conversion uses an enzyme called 5 alpha reductase. Some medications work by blocking this enzyme.

Conversion to oestrogen

Some testosterone converts to oestrogen (oestradiol) via the aromatase enzyme. Healthy men need small amounts of oestrogen for bone health and other functions. Excessive conversion can produce problems. Body fat increases this conversion which is one reason obesity affects men hormonally.

Free versus bound testosterone

Most testosterone in the 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 testosterone is biologically active despite total levels.

Keep reading

For a complete overview, our What Is Testosterone and How It Works covers the fundamentals. What Causes Low Testosterone covers when the system fails. And Total vs Free Testosterone covers the bound versus free distinction.