Female Sex Hormones

Hypothalamic-Pituitary Control

The hierarchy of reproductive regulation begins in the hypothalamus, which secretes gonadotropin-releasing hormone (GnRH). This hormone is characterized by pulsatile secretion. The response of downstream structures depends on the current estrogen level: at low concentrations, GnRH stimulates the production of follicle-stimulating hormone (FSH), whereas at high concentrations, it switches to stimulating luteinizing hormone (LH).

The adenohypophysis synthesizes key ovarian regulators:

• FSH. Its targets are follicular cells, promoting their proliferation and follicular growth. A critical function of FSH is the activation of the enzyme aromatase, leading to enhanced estrogen secretion. In addition, FSH promotes the insertion of LH receptors into cell membranes and stimulates the release of inhibins. FSH is inhibited by estrogens and inhibins via negative feedback.
• LH. This hormone acts on theca interna cells, triggering androgen synthesis that is subsequently converted into estrogens within the stratum granulosum. The LH surge occurs 10–12 hours before follicular rupture and stimulates ovulation. Following oocyte release, LH induces progesterone secretion, causes luteinization of the remaining cells, and ensures the growth of the corpus luteum.
• A distinct role is played by prolactin, which is stimulated by prolactin-releasing factors and inhibited by dopamine. In combination with estrogens and growth hormone (GH), it ensures mammary gland growth. During lactation, prolactin triggers the synthesis of milk proteins, fats, and carbohydrates. It also maintains corpus luteum function and, together with progesterone, effectively suppresses the development of new follicles. Additionally, prolactin participates in water-electrolyte balance (increasing reabsorption of sodium, chloride, and water), enhancing the effects of vasopressin and aldosterone.

Ovarian Steroid Hormones

The primary peripheral hormones forming the female phenotype are estrogens and progesterone.

Estrogens (including estradiol, estrone, and estriol) are synthesized by granulosa cells. Their main physiological effects:

1. Control of puberty and the complete development of female secondary sexual characteristics.
2. Regulation of the ovarian-menstrual cycle with predominant control over its proliferative phase.
3. Maintenance of normal pregnancy progression and promotion of mammary gland development.
4. Participation in bone tissue development and stimulation of epiphyseal cartilage closure.
5. Metabolic effects: moderate anabolic action, nitrogen, salt, and water retention, and female-pattern adipose tissue distribution. Estrogens also have a pronounced effect on psychoemotional status.

During pregnancy, total estrogen levels increase approximately 30-fold, with about 90% of this amount represented by estriol produced by the placenta.

Outside of gestation, progesterone is produced predominantly by the corpus luteum, while starting from the second trimester of pregnancy, the placenta completely takes over this function. The hormone regulates the secretory phase of the cycle, carefully preparing the endometrium for potential blastocyst implantation. It is essential for maintaining pregnancy and induces specific remodeling in the uterus and mammary glands. Its metabolic effect is also well-known: progesterone increases basal metabolism and basal body temperature.

Accessory Ovarian and Adrenal Hormones

In addition to estrogens and progesterone, other important regulators are synthesized in the female body:

• Inhibins are peptide substances that inhibit GnRH production in the hypothalamus and FSH in the pituitary gland via negative feedback.
• Androgens are produced in females by the adrenal cortex and ovaries. Their main physiological tasks include stimulating anabolic processes and enhancing libido.

Endocrine Function of the Placenta

With the onset of pregnancy, the placenta becomes a powerful endocrine organ. It synthesizes and secretes a wide spectrum of hormones and biologically active substances that ensure fetal gestation and development:

• Human Chorionic Gonadotropin (hCG) — critical in early pregnancy because it stimulates continuous progesterone secretion by the corpus luteum of pregnancy.
• Relaxins — specific hormones that relax the myometrium throughout pregnancy. Immediately before labor, they increase the elasticity of the pubic symphysis tissues.
• The placenta also produces progesterone, prolactin, and estrogens.
• A unique property of the placenta is its ability to synthesize virtually all known hypothalamic releasing and inhibiting hormones, including corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), growth hormone-releasing hormone, somatostatin, and prolactin-releasing factors.