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Physiology · Reproduction, pregnancy and the fetus

Testosterone and its actions !

  • Mechanism
  • Exam priority: medium

Testosterone builds male body features, muscle and sperm; part of it is turned into a stronger form (DHT) and part into estrogen.

Updated 2026-11-01

Also called: Androgens, DHT

See it in 3D: Testes · Prostate · Seminal vesicles · Epididymis · Ductus deferens · Transition zone

What it is

Testosterone, made mainly by Leydig cells, acts directly and through two conversions: 5α-reductase turns it into dihydrotestosterone (DHT), a more potent androgen in the prostate, skin and external genitalia, and aromatase turns it into estradiol, which closes growth plates and gives feedback to the brain. Which product acts where explains many exam questions.

How it works, step by step

  1. LH stimulates Leydig cells to convert cholesterol to pregnenolone and then to testosterone.
  2. Most testosterone binds SHBG in plasma; the free fraction enters target cells.
  3. In prostate, skin and genital tissue, 5α-reductase type 2 converts testosterone to DHT, which binds the androgen receptor more tightly.
  4. In fat, bone and brain, aromatase converts testosterone to estradiol.
  5. The androgen receptor–hormone complex moves to the nucleus and changes gene transcription (anabolic, virilizing effects).

Hormone axes and feedback

Hormone axes and feedback: Hypothalamic–pituitary–testicular axis Kisspeptin · pulsed GnRH (continuous → suppresses)−−−++LH+FSH++HypothalamusGnRH (pulsatile)Anterior pituitary (gonadotrophs)LHFSHLeydig cellsTestosteroneSertoli cellsInhibin BTarget tissuesSpermatogenesis · male traits

Hypothalamic–pituitary–testicular axisLesion None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

  • Stimulates (+)
  • Inhibits / negative feedback (−)
  • Level ↑
  • Level ↓
Tap the Leydig cells to see the conversions of testosterone; choose a lesion to see feedback.

Reference values

ParameterValueNote
Total testosterone, adult man≈300–1000 ng/dLlaboratory ranges vary
Total testosterone, adult woman≈15–70 ng/dLlaboratory ranges vary
Fraction bound to SHBG and albumin≈98%free ≈2%

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Which hormone does what

HormoneFetal lifePuberty and adulthood
TestosteroneWolffian ducts → epididymis, vas deferens, seminal vesiclesSpermatogenesis, muscle, libido, voice, erythropoiesis
DHTPenis, scrotum, prostateProstate growth, facial and body hair, male-pattern baldness, acne
Estradiol (from aromatase)Brain organization (in some species)Epiphyseal closure, bone mass, feedback on LH

Clinical notes

  • 5α-reductase deficiency (46,XY): ambiguous or female-looking external genitalia at birth with normal Wolffian structures; virilization at puberty.
  • Complete androgen insensitivity (46,XY): testes and high testosterone, but a female phenotype with a blind vagina and no uterus (AMH still works).
  • Benign prostatic hyperplasia depends on DHT; 5α-reductase inhibitors shrink the gland, and anti-androgen therapy is used in prostate cancer.

Test yourself

Think of the answer, then tap the question to see it.

A 46,XY baby has female-looking external genitalia but normal epididymis and vas deferens. Which enzyme is missing?

5α-reductase type 2: testosterone builds the Wolffian ducts, but DHT is needed for the penis, scrotum and prostate.

Why do men with aromatase deficiency have low bone density?

Bone mass and epiphyseal closure depend on estradiol, which in men is made by aromatizing testosterone.

Why does SHBG matter when interpreting a total testosterone result?

Most testosterone is bound to SHBG; high SHBG (estrogen, hyperthyroidism, aging) can hide low free testosterone, and low SHBG (obesity, insulin resistance) can make total levels look low.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Biochemistry

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 27.1 Anatomy and Physiology of the Testicular Reproductive System (OpenStax (Rice University), 2022); 17.8 Gonadal and Placental Hormones (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 81). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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