Testosterone: A Comprehensive Physiology Review for Medical Students

 

 

Testosterone: A Comprehensive Physiology Review for Medical Students

1. Introduction

Testosterone is the principal androgenic steroid hormone in humans, playing a central role in male sexual differentiation, reproductive function, anabolic processes, and overall metabolic regulation. Although predominantly associated with males, it is also present and physiologically important in females.

It belongs to the class of C19 steroid hormones derived from cholesterol and is synthesized primarily in the testes, with smaller contributions from adrenal glands and ovaries.

2. Chemistry and Structure

Testosterone is a lipophilic steroid hormone synthesized from cholesterol via a series of enzymatic reactions:

• Molecular formula: C₁₉H₂₈O₂
• Structure: Four-ring steroid nucleus (cyclopentanoperhydrophenanthrene)
• Functional groups:
• Ketone group at C3
• Hydroxyl group at C17

Key Derivatives

• Dihydrotestosterone (DHT): More potent androgen (via 5α-reductase)
• Estradiol (E2): Formed via aromatization (important in bone and brain)

3. Sites of Synthesis

In Males

• Leydig cells of testes (major source, ~95%)

In Females

• Ovarian theca cells
• Adrenal cortex (zona reticularis)

4. Biosynthesis of Testosterone

Testosterone synthesis occurs via the steroidogenic pathway:

Cholesterol → Pregnenolone → Progesterone → 17α-hydroxyprogesterone → Androstenedione → Testosterone

Key Enzymes

• Cholesterol desmolase (CYP11A1)
• 17α-hydroxylase (CYP17A1)
• 17β-hydroxysteroid dehydrogenase

Regulation by LH

The process is stimulated by Luteinizing Hormone (LH) from the anterior pituitary.

5. Regulation: Hypothalamic–Pituitary–Gonadal (HPG) Axis

Testosterone secretion is tightly regulated by the HPG axis:

• Hypothalamus → releases GnRH
• Pituitary → secretes:
• LH → stimulates Leydig cells → testosterone production
• FSH → acts on Sertoli cells (spermatogenesis)

Negative Feedback

Testosterone inhibits:

• GnRH secretion
• LH secretion

6. Transport in Blood

Testosterone circulates in three forms:

• Bound to SHBG (Sex Hormone Binding Globulin) (~60%) → biologically inactive
• Bound to albumin (~38%) → weakly bound, bioavailable
• Free testosterone (~2%) → biologically active

7. Mechanism of Action

Testosterone acts via intracellular androgen receptors (ARs):

1. Diffuses through cell membrane
2. Binds to androgen receptor in cytoplasm
3. Hormone-receptor complex translocates to nucleus
4. Binds to DNA (androgen response elements)
5. Modulates gene transcription

DHT Action

• Higher affinity for androgen receptor
• Responsible for many androgenic effects (e.g., prostate growth, hair pattern)

8. Physiological Actions

A. Fetal Life

• Development of male internal genitalia (via testosterone)
• Development of external genitalia (via DHT)

B. Puberty

• Enlargement of testes and penis
• Development of secondary sexual characteristics:
• Facial, axillary, pubic hair
• Deepening of voice (laryngeal enlargement)
• Increased libido

C. Adult Male Functions

1. Reproductive System

• Maintenance of spermatogenesis (with FSH)
• Growth and function of accessory glands (prostate, seminal vesicles)

2. Anabolic Effects

• Increased protein synthesis
• Muscle mass and strength
• Bone growth and density

3. Hematopoietic Effects

• Stimulates erythropoiesis → ↑ hemoglobin

4. Metabolic Effects

• Mild increase in basal metabolic rate
• Fat distribution (central vs peripheral)

5. CNS Effects

• Libido
• Aggression and mood modulation

D. Functions in Females

• Contributes to libido
• Precursor for estrogen synthesis
• Role in bone health and muscle mass

9. Role of Dihydrotestosterone (DHT)

DHT is formed by 5α-reductase in target tissues:

Key Actions

• Development of prostate
• Male pattern baldness
• Sebaceous gland activity
• External genital differentiation

10. Circadian Rhythm

• Testosterone levels exhibit diurnal variation
• Peak: Early morning (6–8 AM)
• Decline with age (andropause)

11. Life Span Changes

Fetal Life

• High levels → sexual differentiation

Childhood

• Low levels

Puberty

• Sharp increase

Aging

• Gradual decline (~1% per year after 30)

12. Clinical Correlations

A. Hypogonadism

Causes:

• Primary (testicular failure)
• Secondary (pituitary/hypothalamic)

Features:

• Delayed puberty
• Infertility
• Decreased muscle mass
• Erectile dysfunction

B. Hyperandrogenism

• Excess testosterone (e.g., tumors, anabolic steroids)
• Features:
• Acne
• Aggression
• Infertility

C. Androgen Insensitivity Syndrome

• Defective androgen receptors
• XY individuals with female phenotype

D. Benign Prostatic Hyperplasia (BPH)

• Mediated by DHT
• Enlarged prostate → urinary symptoms

E. Male Pattern Baldness

• DHT-dependent hair follicle miniaturization

F. Polycystic Ovary Syndrome (PCOS)

• Elevated androgens in females
• Symptoms:
• Hirsutism
• Acne
• Menstrual irregularities

13. Pharmacology (Brief Overview)

Therapeutic Uses

• Testosterone replacement therapy
• Delayed puberty
• Gender-affirming therapy

Anti-androgens

• Finasteride (5α-reductase inhibitor)
• Flutamide (androgen receptor blocker)

14. Laboratory Assessment

• Total testosterone
• Free testosterone
• SHBG levels

Normal Range (Adult Male)

• ~300–1000 ng/dL

15. Summary (Exam-Oriented Points)

• Testosterone is the primary male androgen produced by Leydig cells.
• Regulated via HPG axis (GnRH → LH → Testosterone).
• Circulates mostly bound to SHBG.
• Acts via nuclear androgen receptors.
• Converted to:
• DHT (more potent androgen)
• Estradiol (important for bone)
• Responsible for:
• Sexual differentiation
• Secondary sexual characteristics
• Anabolic and metabolic effects
• Clinical relevance includes:
• Hypogonadism
• PCOS
• BPH
• Androgen insensitivity

16. Quick Revision Table

Feature	Testosterone
Type	Steroid hormone
Source	Leydig cells
Regulation	LH (HPG axis)
Transport	SHBG-bound
Active form	Free testosterone, DHT
Mechanism	Nuclear receptor
Functions	Reproductive, anabolic, metabolic

 

17. Conclusion

Testosterone is a multifunctional hormone essential for male physiology and significant in female health. Its precise regulation, conversion to active metabolites, and systemic effects make it a cornerstone topic in medical physiology and clinical medicine.

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