Chemistry, Secretion, Functions and Regulations of Gonadal hormones: Testosterone

Testosterone – Chemistry, Secretion, Functions, and Regulation

1. Chemistry

Testosterone is a steroid hormone.

It belongs to the group of androgens (male sex hormones).

Derived from cholesterol in the Leydig cells of testes.

It is lipid-soluble and easily passes through cell membranes.

2. Site of Secretion

Males: Mainly secreted by Leydig (interstitial) cells in testes.

Females: Secreted in very small amounts by the ovaries and adrenal cortex.

In blood, testosterone is mostly bound to sex hormone-binding globulin (SHBG) and albumin; only 2–3% is free and biologically active.


3. Normal Secretion

In Males:

Average adult production: 6–7 mg/day.

Blood levels: about 300–1000 ng/dL.

Secretion starts during fetal life (for male sex differentiation), decreases after birth, rises again at puberty, and gradually declines after 40–50 years (andropause).


In Females:

Much lower levels (20–70 ng/dL).

Contributes to libido, bone health, and muscle strength.


4. Functions of Testosterone

✅ Reproductive system

Development of male genital organs during fetal life.

Growth and maturation of testes, penis, prostate, and seminal vesicles.

Required for spermatogenesis (sperm production).


✅ Secondary sexual characteristics

Deepening of voice.

Growth of beard, mustache, and body hair.

Increased muscle mass and bone strength.


✅ Metabolic effects

Anabolic action – increases protein synthesis and muscle growth.

Promotes bone growth and mineralization.

Increases red blood cell production (stimulates erythropoietin).


5. Regulation of Testosterone Secretion

The Hypothalamic–Pituitary–Gonadal (HPG) Axis controls testosterone:

1. Hypothalamus releases GnRH (Gonadotropin Releasing Hormone).


2. Anterior Pituitary releases:

LH (Luteinizing Hormone) → acts on Leydig cells → ↑ Testosterone.

FSH (Follicle Stimulating Hormone) → acts on Sertoli cells → helps spermatogenesis (needs testosterone support).



3. Testosterone itself provides negative feedback to hypothalamus & pituitary → reduces excess LH & GnRH secretion.

Regulation Flowchart

Hypothalamus 
   ↓ (GnRH)
Anterior Pituitary 
   ↓ (LH & FSH)
Testes (Leydig cells) 
   ↓ 
Testosterone secretion
   ↓
Functions (reproductive, anabolic, behavioral)
   ↓
Negative feedback to Hypothalamus & Pituitary

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