Endocrine System

Endocrine System Overview - Hormone Harmony 101

• Hormone Types & Receptors:
  • Peptide (Insulin, GH): Water-soluble; act on cell surface receptors.
  • Steroid (Cortisol, Aldosterone): Lipid-soluble; act on intracellular receptors, alter gene expression.
  • Amine (Thyroxine, Epinephrine): Derived from amino acids; variable mechanisms.
• Key Signaling: Second messengers (cAMP, IP3, Ca2+) amplify signals from surface receptors.
• Control: Feedback loops maintain homeostasis.
  • Negative feedback: Predominant (e.g., thyroid axis).
  • Positive feedback: Less common (e.g., oxytocin in labor).

⭐ The hypothalamic-pituitary-adrenal (HPA) axis is a classic example of a negative feedback loop controlling stress response via cortisol.

Hypothalamus-Pituitary Axis - The Command Center

• Hypothalamus: CEO of endocrine system. Produces releasing hormones (RH) & inhibiting hormones (IH) that act on anterior pituitary. Examples: TRH, CRH, GnRH, GHRH, Somatostatin, Dopamine.
• Pituitary Gland:
  • Anterior Lobe (Adenohypophysis):
    • Connected by hypophyseal portal system.
    • Hormones: GH, PRL, ACTH, TSH, FSH, LH. (📌 FLAT PEG: FSH, LH, ACTH, TSH, PRL, GH).
  • Posterior Lobe (Neurohypophysis):
    • Neural extension of hypothalamus via infundibulum.
    • Stores & releases ADH (vasopressin) & Oxytocin (synthesized in hypothalamic supraoptic & paraventricular nuclei).

⭐ The posterior pituitary is not a true endocrine gland as it only stores and releases hormones (ADH, Oxytocin) produced by the hypothalamus.

Thyroid & Parathyroid Glands - Metabolic & Calcium Kings

• Thyroid Gland:
  • Follicular cells: T4 (thyroxine), T3 (triiodothyronine); T3 more potent. Regulate Basal Metabolic Rate (BMR), growth. Iodine essential.
  • Transport: Thyroxine-Binding Globulin (TBG).
  • Parafollicular (C cells): Calcitonin → ↓ serum Ca²⁺ (opposes PTH, minor role).
• Parathyroid Glands (Usually 4):
  • Chief cells: Parathyroid Hormone (PTH) → ↑ serum Ca²⁺, ↓ serum PO₄³⁻.
  • Kidney action: ↑ Ca²⁺ reabsorption, ↓ PO₄³⁻ reabsorption, stimulates 1-α-hydroxylase for Vit D activation.
  • 📌 PTH: "Phosphate Trashing Hormone".

⭐ MEN 1 (Wermer's) and MEN 2A (Sipple's) syndromes can involve parathyroid hyperplasia/adenomas leading to primary hyperparathyroidism.

Adrenal Glands - Stress & Steroid Sentinels

• Location: Retroperitoneal, superomedial to kidneys.
• Adrenal Cortex (Outer): Steroid hormones. 📌 GFR - Salt, Sugar, Sex (Outer to Inner layers & products).
  • Zona Glomerulosa: Mineralocorticoids (e.g., Aldosterone - regulates $Na^+$ reabsorption, $K^+$ secretion).
  • Zona Fasciculata: Glucocorticoids (e.g., Cortisol - ↑glucose, anti-inflammatory).
  • Zona Reticularis: Androgens (e.g., DHEA, androstenedione).
• Adrenal Medulla (Inner): Neuroectodermal origin. Chromaffin cells secrete Catecholamines (Epinephrine, Norepinephrine) for fight-or-flight response.

⭐ Pheochromocytoma, a tumor of adrenal medulla chromaffin cells, classically presents with the triad: Palpitations, Headache, Episodic sweating (PHE).

Endocrine Pancreas - Glucose Guardian

• Islets of Langerhans:
  • Alpha (α) cells: Glucagon (↑ blood glucose)
  • Beta (β) cells: Insulin (↓ blood glucose), Amylin
  • Delta (δ) cells: Somatostatin (inhibits insulin, glucagon)
• Insulin (Anabolic): ↑ Glucose uptake (muscle/adipose via GLUT4), ↑ glycogenesis, ↑ lipogenesis. Stimulus: ↑ Blood glucose.
• Glucagon (Catabolic): ↑ Glycogenolysis, ↑ gluconeogenesis. Stimulus: ↓ Blood glucose.

⭐ C-peptide is co-secreted with insulin in equimolar amounts; its measurement helps differentiate endogenous vs. exogenous insulin.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypothalamus controls the anterior pituitary (GH, PRL, ACTH, TSH, FSH, LH); posterior pituitary stores hypothalamic ADH & Oxytocin.
• Adrenal cortex zones (GFR): Zona Glomerulosa (Aldosterone), Zona Fasciculata (Cortisol), Zona Reticularis (Androgens). Adrenal medulla secretes catecholamines.
• Thyroid follicular cells produce T3/T4 (iodine is essential); parafollicular C-cells (neural crest origin) secrete calcitonin.
• Parathyroid hormone (PTH) from chief cells is the primary regulator of serum calcium and phosphate.
• Pancreatic islets: Beta cells (Insulin), Alpha cells (Glucagon), Delta cells (Somatostatin).