Endocrine System – Thyroid, Parathyroid, Adrenal, Pancreas & More

Thyroid Gland

• Anatomy & Location
  • Butterfly-shaped, two lobes (right & left) connected by the isthmus.
  • Lies anterior to trachea & larynx in the lower cervical (neck) region.
• Core Functions – Sets the rate of metabolism (overall speed of all anabolic & catabolic reactions).
  • ↑ Thyroid hormone ➜ ↑ HR, ↑ RR, ↑ GI motility, ↑ temperature, ↑ anxiety.
  • ↓ Thyroid hormone ➜ overall physiologic “slow-down” (constipation, cold intolerance, bradycardia, etc.).
• Hormones Produced
  • Thyroid Hormone (Thyroxine)
  • Two molecular forms: T₃ (triiodothyronine, 3 I atoms) & T₄ (tetraiodothyronine, 4 I atoms).
  • T₃ is the “younger sibling” – more potent, faster acting; T₄ is more abundant but less active.
  • Iodine is essential for synthesis – dietary lack ⇒ hypothyroidism; public-health measure = iodized salt.
  • Controlled by anterior pituitary TSH; classic negative feedback loop.
  • Calcitonin – “tones down calcium”
  • Released when blood [Ca²⁺] is high.
  • Targets:
    • Bone: ↑ osteoblast activity → bone deposition (“piggy-bank storage”).
    • Kidney: ↓ Ca²⁺ reabsorption → ↑ urinary excretion.
    • GI tract: ↓ intestinal absorption → ↑ fecal loss.
• Clinical / Real-World Notes
  • Iodine deficiency → goiter, hypo-symptoms; counsel pts who avoid iodized salt or low-seafood diets.
  • Total thyroidectomy removes para- & follicular cells → lifelong thyroid replacement + Ca²⁺ monitoring (loss of calcitonin).

Parathyroid Glands

• Anatomy – Four tiny glands on posterior thyroid ("parallel" – adjacent but functionally separate).
• Parathyroid Hormone (PTH)
  • Released when blood [Ca²⁺] is low (antagonist of calcitonin).
  • Actions (“3-organ strategy”):
  • Bone: ↑ osteoclast activity → bone resorption & Ca²⁺ release.
  • Kidney: ↑ Ca²⁺ reabsorption, ↓ urinary loss.
  • GI (via calcitriol): ↑ calcitriol (vitamin D) synthesis → ↑ intestinal Ca²⁺ absorption.
• Feedback Summary
  \text{Low }[Ca^{2+}]{blood} \xrightarrow{PTH} \uparrow [Ca^{2+}]{blood}\quad ; \quad \text{High }[Ca^{2+}]{blood} \xrightarrow{Calcitonin} \downarrow [Ca^{2+}]{blood}

Adrenal Glands

• Location – Sit like “hats” on superior poles of kidneys; enclosed by a capsule (anchoring).
• Two Functional Regions
  • Medulla (middle) – neuroendocrine tissue.
  • Epinephrine & Norepinephrine (a.k.a. adrenaline)
    • Released by sympathetic nerve impulses (fight-or-flight).
    • Effects: pupillary dilation, ↑ HR, ↑ BP, ↑ RR, bronchodilation, glycogenolysis → ↑ glucose, redirect blood to muscle.
  • Cortex (outer) – 3 concentric zones remembered by G F R (like kidney lab value) & “Salt • Sugar • Sex.”
  • Zona Glomerulosa (Salt)
    • Aldosterone (mineralocorticoid).
    • Trigger: ↓ BP / ↓ Na⁺ (RAAS pathway).
    • Kidney: ↑ Na⁺ reabsorption → water follows → ↑ blood volume/pressure; K⁺ excretion.
  • Zona Fasciculata (Sugar)
    • Cortisol (glucocorticoid).
    • Triggers: physiological or emotional stress, ACTH.
    • Major actions:
    • ↑ gluconeogenesis → ↑ blood glucose.
    • Immunosuppression / anti-inflammatory (basis for prednisone, hydrocortisone, asthma steroids).
    • Chronic high doses → iatrogenic diabetes, muscle wasting, bone loss.
  • Zona Reticularis (Sex)
    • Androgens / Gonadocorticoids (e.g., DHEA).
    • Precursors for testosterone & estrogen; contribute to pubic/axillary hair & libido.

Pancreas

• Dual Role
  • Exocrine (Acinar cells) – Digestive enzymes & \text{HCO}_3^- bicarb via pancreatic duct into duodenum (neutralizes gastric acid, aids nutrient breakdown).
  • Endocrine (Islets of Langerhans)
  • Beta (β) cells → Insulin
    • Trigger: ↑ blood glucose (post-meal).
    • Actions:
    • Facilitates cellular uptake of glucose (the “key” to cell doors).
    • Stimulates glycogenesis in liver & muscle (glucose → glycogen storage).
    • Net ↓ blood glucose.
  • Alpha (α) cells → Glucagon
    • Trigger: ↓ blood glucose (fasting).
    • Actions: liver glycogenolysis & gluconeogenesis → release glucose to blood.
    • Net ↑ blood glucose.
  • Delta (δ) cells → Somatostatin (inhibitory; details in GI unit).
• Key Terminology Metaphors
  • Glucose = free sugar fuel.