Endocrine System – Regulation of Endocrine Activities by the Pituitary Gland and Hypothalamus

Role of the Pituitary Gland

• The pituitary gland secretes multiple trophic hormones.
• These hormones regulate the endocrine activities of:
  • The adrenal cortex
  • The thyroid gland
  • The reproductive organs

The Three-Step Process of Hormone Regulation

1. Hypothalamus produces a regulatory hormone.
2. The regulatory hormone signals the pituitary gland to produce a second hormone.
3. The second hormone acts on specific endocrine tissues to produce hormones that circulate and perform their functions in the body.

• Example pathway:
  • Hypothalamus releases a releasing hormone → stimulates the anterior pituitary to produce hormone 1 → hormone 1 stimulates an endocrine gland (e.g., adrenal cortex) to produce hormone 2 (e.g., aldosterone).

• Actual Example: ACTH (Adrenocorticotropic Hormone)
  • Target tissue: Adrenal cortex (specifically, the cortex of the adrenal gland).
  • Process:
    1. The hypothalamus produces a regulatory hormone.
    2. It signals the pituitary gland to produce ACTH.
    3. ACTH travels to the adrenal cortex.
    4. The adrenal cortex produces specific hormones (like cortisol).
    5. These hormones circulate in the body and carry out their actions.

Homeostasis:  Hormone Regulation – Negative and Positive Feedback Loops, Neural Control, and Circadian Rhythm 

Nervous and Endocrine System Interaction

• The endocrine system is controlled by negative feedback loops and nervous system regulation.
• Both systems work together to regulate metabolic activities and maintain homeostasis.

Negative feedback loops are used to maintain hormone levels at appropriate levels.

• For example, when serum hormone levels become too high in the blood, hormone production from the hypothalamus and pituitary gland will be inhibited, bringing hormone levels back down.
• Conversely, when serum hormone levels become too low, the hypothalamus and pituitary gland will produce more hormone.
• Hormones have a half-life that ranges from seconds to days in a process that routinely involves degradation by the liver and kidney.

Positive Feedback Loops

• Are less common than negative feedback loop mechanisms
• Usually involved in short-term, specific processes such as childbirth.
• Example: Oxytocin during childbirth:
  • Baby’s head pushes against the cervix → nerve impulses send signals to hypothalamus → hypothalamus releases oxytocin → oxytocin stimulates uterine contractions → baby pushes harder, intensifying the process.
  • This loop continues, contractions become more frequent and stronger, until the baby is born, then the positive feedback loop stops.

Hormone Secretion and Circadian Rhythms

• Some hormones are secreted according to daily cycles:
  • Example: Growth hormone is primarily secreted at night, aligning with circadian rhythms.

Endocrine Disorders

Endocrine Disorders often stem from:

• Damage to endocrine glands
• Excess or insufficient hormone production
• Hormone degradation problems (due to liver or kidney failure)

Causes of Hormonal Imbalance

• Excess hormone production:
  • Tumors (e.g., adenomas producing too much hormone)
  • Congenital conditions
  • Autoimmune hypersensitivity
  • External factors (e.g., lung cancer producing excess ADH)

• Insufficient hormone production:
  • Tumors destroying tissue
  • Autoimmune attack
  • Genetic or congenital defects
  • Malnutrition
  • Gland atrophy, surgical removal, ischemia, or infection

Tropic Hormones

• The hormones produced by the pituitary gland are called tropic hormones.
• Etymology: Derived from Greek, meaning “turning” or “changing”.
• Function: Act as middlemen that stimulate specific endocrine tissues to produce hormones.
• Examples include:
  • ACTH: Targets adrenal cortex, stimulating the release of cortisol.
  • TSH (Thyroid Stimulating Hormone): Targets thyroid gland stimulating the release of thyroid hormones.
  • FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone): Target reproductive organs (testes and ovaries) and facilitate the maturation of sperm and oocytes.

Regulating Endocrine Glands

• Thyroid Stimulating Hormone (TSH): Stimulates thyroid gland to produce thyroid hormones (T₄ and T₃).
• Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH):
  • Travel to testes and ovaries.
  • Stimulate and/or regulate the secretion of:
    • Testosterone (by testes)
    • Estrogen and Progesterone (by ovaries)
    • Inhibin (by testes and ovaries)

Functions of FSH and LH in Reproductive Development

• During puberty and beyond the sex hormones (testosterone and estrogen):
  • Support development of primary and secondary sexual characteristics
  • Primary sexual characteristics develop during embryogenesis with the formation of the urogenital organs.
  • Secondary sexual characteristics develop during puberty in response to increased levels of testosterone and estrogen:
    • In males: widening of larynx, voice deepening, muscular and skeletal growth, increased body hair, and sweat gland activity.
    • In females: breast development, adipose tissue, skeletal development, and body hair.
• In males:
  • FSH is required for spermatogenesis
  • LH is required for testosterone production
• In females:
  • FSH, LH, estrogen, and progesterone regulate the:
    • ovarian cycle: the maturation of follicles and oocytes, and ovulation and the
    • uterine cycle: the monthly preparation of the uterus’s endometrial lining followed by menstruation (in absence of fertilization and implantation).

Reproductive Organ Secretions

• The testes and ovaries secrete hormones critical for sexual development and reproductive function:
  • Testosterone: Development of male secondary sexual characteristics.
  • Estrogen and Progesterone: Development of female secondary sexual characteristics and regulation of the menstrual cycle.  Progesterone is required for uterine lining thickening.  Both estrogen and progesterone are required in supporting pregnancy.
  • Inhibin: Regulates sperm production (in males) and follicular maturation (in females).

Role of Inhibin

• Inhibin is secreted by both testes and ovaries.
• It is essential in regulating the rate of sperm production in males.
• It also helps control follicular maturation and the ovarian cycle in females.

Key Points

• Tight regulation of hormones is crucial for maintaining homeostasis.
• Disruptions can lead to dysfunctions or diseases if feedback mechanisms fail.
• Understanding feedback mechanisms helps explain the dynamic balance of hormone systems in the body.