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

Estrogen – Chemistry, Secretion, Functions, and Regulation

1. Chemistry

Estrogen is a steroid hormone, derived from cholesterol.

Belongs to the female sex hormone group.

Three main natural estrogens in humans:

Estradiol (E2) → most active, dominant in reproductive years.

Estrone (E1) → present after menopause.

Estriol (E3) → major estrogen in pregnancy.


Lipid-soluble and acts through intracellular receptors (nuclear receptors).


2. Site of Secretion

Females:

Mainly from ovarian follicles (granulosa cells).

Also secreted by corpus luteum and placenta (during pregnancy).


Males: Very small amounts secreted by testes and adrenal cortex.

In blood, estrogens are bound to sex hormone-binding globulin (SHBG) and albumin; only a small free fraction is active.


3. Normal Secretion

Adult Women:

Blood estradiol levels: 50–350 pg/mL (varies with menstrual cycle).

Peaks just before ovulation.


During Pregnancy: Very high levels (estriol predominates).

After Menopause: Levels fall drastically.



4. Functions of Estrogen

✅ Reproductive system

Growth and maturation of ovaries, uterus, fallopian tubes, and vagina.

Development of endometrium (inner lining of uterus).

Essential for ovulation and preparation of uterus for possible pregnancy.


✅ Secondary sexual characteristics

Development of breasts.

Female body fat distribution (hips, thighs, breasts).

Feminine voice and skin softness.


✅ Skeletal and metabolic effects

Stimulates bone growth and strength (prevents osteoporosis).

Promotes protein synthesis but less than testosterone.

Maintains cholesterol balance (increases HDL, decreases LDL).


✅ Cardiovascular and other effects

Protects heart and blood vessels (vasodilation).

Maintains healthy skin and hair.

Important for mood, cognition, and general wellbeing.



5. Regulation of Estrogen Secretion

Controlled by the Hypothalamic–Pituitary–Ovarian (HPO) Axis:

1. Hypothalamus → secretes GnRH.


2. Anterior Pituitary → releases:

FSH → stimulates granulosa cells → estrogen production.

LH → acts on theca cells → produce androgens (converted to estrogen in granulosa cells).



3. Estrogen levels exert feedback control:

Low/Moderate levels → negative feedback → suppress GnRH, FSH, LH.

High sustained levels (pre-ovulation) → positive feedback → sudden LH surge → ovulation.


6. Regulation Flowchart

Hypothalamus 
   ↓ (GnRH)
Anterior Pituitary 
   ↓ (FSH & LH)
Ovary (Granulosa + Theca cells) 
   ↓
Estrogen secretion
   ↓
Functions (reproductive, secondary sex, bone, metabolic, cardio-protective)
   ↓
Feedback:
   - Low/Moderate estrogen → Negative feedback on Hypothalamus & Pituitary
   - High sustained estrogen → Positive feedback → LH surge → Ovulation


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