Endocrine feedback systems

Feedback Systems - The Body's Thermostat

Maintains homeostasis via feedback loops, adjusting hormone levels to a set point.

• Negative Feedback: The most common type. The final hormone inhibits its own production, creating stability. Think of a thermostat.

  • Sensor: Detects deviation from the set point (e.g., low thyroxine).
  • Controller: Compares input to the set point (e.g., hypothalamus/pituitary).
  • Effector: Produces the final response (e.g., thyroid gland releases thyroxine).

• Positive Feedback: Amplifies a stimulus, pushing the system further from its set point. It's a self-reinforcing cycle.

⭐ The classic USMLE example of positive feedback is the oxytocin-induced uterine contractions during childbirth, where stretching of the cervix triggers more oxytocin release.

HPT Axis - Thyroid's Tight Leash

• Initiation: Hypothalamus secretes Thyrotropin-Releasing Hormone (TRH).
• Stimulation: TRH prompts the Anterior Pituitary to release Thyroid-Stimulating Hormone (TSH).
• Synthesis & Release: TSH stimulates the thyroid gland to produce and release T4 (thyroxine) and T3 (triiodothyronine) from iodinated thyroglobulin.
• Negative Feedback: Rising T3/T4 levels inhibit both TRH and TSH secretion, ensuring stable hormone levels.

⭐ The Wolff-Chaikoff effect describes how high levels of iodide paradoxically inhibit thyroid hormone synthesis, a protective autoregulatory mechanism.

HPA Axis - Stress Control Central

• Hypothalamus releases Corticotropin-Releasing Hormone (CRH).
• Anterior Pituitary releases Adrenocorticotropic Hormone (ACTH).
• Adrenal Cortex responds. 📌 Mnemonic (GFR): Salt, Sugar, Sex.
  • Zona Glomerulosa (Salt): Aldosterone; regulated by RAAS.
  • Zona Fasciculata (Sugar): Cortisol; manages stress, ↑ blood glucose.
  • Zona Reticularis (Sex): DHEA (androgens).
• Cortisol provides negative feedback to the hypothalamus and pituitary.

⭐ The dexamethasone suppression test is a key diagnostic tool that leverages the HPA negative feedback loop to differentiate between causes of hypercortisolism (Cushing's syndrome).

Pancreatic Feedback - Sugar's Seesaw

• High Blood Glucose (↑):

  • Pancreatic β-cells release Insulin.
  • Insulin facilitates glucose uptake into muscle and adipose tissue via GLUT4.
  • Promotes glycogenesis (glucose storage as glycogen) and inhibits glycogenolysis/gluconeogenesis.
  • Result: ↓ blood glucose.

• Low Blood Glucose (↓):

  • Pancreatic α-cells release Glucagon.
  • Stimulates hepatic glycogenolysis and gluconeogenesis.
  • Result: ↑ blood glucose.

• Regulation:

  • Pancreatic δ-cells release Somatostatin, inhibiting both insulin and glucagon release.

⭐ Insulin administration can lead to life-threatening hypokalemia by driving potassium into cells via the Na+/K+-ATPase pump.

High‑Yield Points - ⚡ Biggest Takeaways

• Negative feedback is the dominant mechanism; final hormones like cortisol and thyroxine inhibit the hypothalamus and pituitary.
• In primary gland failure (e.g., Addison's disease), the target hormone is ↓ while the pituitary stimulating hormone (e.g., ACTH) is ↑.
• In secondary gland failure (a pituitary issue), both the stimulating and target hormones are ↓.
• Exogenous hormone use, like corticosteroids, suppresses the entire axis, causing adrenal atrophy.
• Positive feedback is rare but critical for the LH surge in ovulation and oxytocin during labor.
• The pancreas is a key exception; insulin and glucagon are regulated by blood glucose, not the pituitary.