[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) › Testosterone and its actions [Physiology](https://durgical.com/en/physiology/) · [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) # 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](https://durgical.com/en/anatomy/testis/) · [Prostate](https://durgical.com/en/anatomy/prostate/) · [Seminal vesicles](https://durgical.com/en/anatomy/seminal-gland/) · [Epididymis](https://durgical.com/en/anatomy/epididymis/) · [Ductus deferens](https://durgical.com/en/anatomy/ductus-deferens/) · [Transition zone](https://durgical.com/en/anatomy/transition-zone-of-prostate/) ## Key points - Testosterone itself: Wolffian duct development, spermatogenesis, muscle mass, libido, deepening of the voice, red cell production. - DHT (5α-reductase): external male genitalia and prostate in the fetus; prostate growth, scalp hair loss and acne in adults. - Estradiol from aromatization: closure of epiphyseal plates, bone density, part of the negative feedback. - About 98% circulates bound (SHBG and albumin); only free and albumin-bound fractions are bioavailable. - In women, androgens come from the ovaries and adrenals; excess causes hirsutism, acne and virilization. ## 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 - LH stimulates Leydig cells to convert cholesterol to pregnenolone and then to testosterone. - Most testosterone binds SHBG in plasma; the free fraction enters target cells. - In prostate, skin and genital tissue, 5α-reductase type 2 converts testosterone to DHT, which binds the androgen receptor more tightly. - In fat, bone and brain, aromatase converts testosterone to estradiol. - The androgen receptor–hormone complex moves to the nucleus and changes gene transcription (anabolic, virilizing effects). ### Hormone axes and feedback Hypothalamic–pituitary–testicular axis **Lesion** 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 | Parameter | Value | Note | | --- | --- | --- | | Total testosterone, adult man | ≈300–1000 ng/dL | laboratory ranges vary | | Total testosterone, adult woman | ≈15–70 ng/dL | laboratory 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 | Hormone | Fetal life | Puberty and adulthood | | --- | --- | --- | | [Testosterone](https://durgical.com/en/conditions/hypospadias/) | Wolffian ducts → epididymis, vas deferens, seminal vesicles | Spermatogenesis, muscle, libido, voice, erythropoiesis | | [DHT](https://durgical.com/en/conditions/benign-prostatic-hyperplasia/) | Penis, scrotum, prostate | Prostate 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. ## Memory aid Testosterone builds the Tubes and the muscles, DHT Designs the outside and the prostate, Estradiol Ends growth. ## 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. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/testosterone-and-its-actions/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Testes](https://durgical.com/en/anatomy/testis/) - [Prostate](https://durgical.com/en/anatomy/prostate/) - [Seminal vesicles](https://durgical.com/en/anatomy/seminal-gland/) - [Epididymis](https://durgical.com/en/anatomy/epididymis/) - [Ductus deferens](https://durgical.com/en/anatomy/ductus-deferens/) - [Transition zone](https://durgical.com/en/anatomy/transition-zone-of-prostate/) ## Related topics - [The male gonadal axis and spermatogenesis](https://durgical.com/en/physiology/the-male-gonadal-axis-and-spermatogenesis/) - [Sex determination and differentiation](https://durgical.com/en/physiology/sex-determination-and-differentiation/) - [Puberty](https://durgical.com/en/physiology/puberty/) - [Actions of estrogen and progesterone](https://durgical.com/en/physiology/actions-of-estrogen-and-progesterone/) ## Related conditions - [Benign prostatic hyperplasia](https://durgical.com/en/conditions/benign-prostatic-hyperplasia/) - [Prostate cancer](https://durgical.com/en/conditions/prostate-cancer/) - [Hypospadias](https://durgical.com/en/conditions/hypospadias/) - [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/) ## In Biochemistry - [Steroid hormone synthesis (overview)](https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/) - [Aromatase (CYP19A1)](https://durgical.com/en/biochemistry/enzymes/aromatase/) - [Nuclear receptors](https://durgical.com/en/biochemistry/topics/nuclear-receptors/) [← All of physiology](https://durgical.com/en/physiology/) 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](https://openstax.org/books/anatomy-and-physiology-2e/pages/27-1-anatomy-and-physiology-of-the-testicular-reproductive-system/) (OpenStax (Rice University), 2022); [17.8 Gonadal and Placental Hormones](https://openstax.org/books/anatomy-and-physiology-2e/pages/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). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/testosterone-and-its-actions/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/testosterone-and-its-actions/