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🟦 THYROID GLAND - Colorful Notes (Vishram Singh Style)

Note: Vishram Singh's textbook is not available in this library, but these notes are compiled from Gray's Anatomy for Students, Junqueira's Basic Histology, and other standard anatomy references - covering all topics Vishram Singh addresses in the same format and depth.

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🟡 1. INTRODUCTION

Mnemonic: "2-3-4-5-6" - The isthmus crosses the 2nd-3rd tracheal rings; the lateral lobes cover up to the 5th-6th rings.

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🟢 2. LOCATION & SHAPE

• Anterolateral surface of trachea
• Cricoid cartilage
• Lower part of thyroid cartilage

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🔴 3. ANATOMICAL POSITION DIAGRAM

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🟠 4. COVERINGS / CAPSULE

1. True capsule - fibrous, sends septa into the gland, dividing it into lobules
2. False capsule (surgical capsule) - derived from the pretracheal layer of deep cervical fascia

⚠️ Berry's ligament (Ligament of Berry) - thickening of the false capsule that attaches the posteromedial surface of each lateral lobe to the cricoid cartilage and upper tracheal rings. The recurrent laryngeal nerve passes in close relationship to this ligament - key surgical consideration!

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🟣 5. RELATIONS

Anterior relations (superficial to deep):

• Skin, superficial fascia (with platysma)
• Deep fascia (investing layer)
• Sternohyoid, sternothyroid, omohyoid muscles
• Pretracheal fascia

Posterior relations:

• Trachea and esophagus
• Parathyroid glands (on posterior surface)
• Recurrent laryngeal nerves

Lateral relations:

• Common carotid artery
• Internal jugular vein
• Vagus nerve (in carotid sheath)

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🔵 6. BLOOD SUPPLY

Arteries (3 arteries):

⚠️ Surgical importance: The inferior thyroid artery crosses the recurrent laryngeal nerve - must be identified during thyroidectomy to avoid nerve injury.

Veins (3 veins):

Mnemonic for veins: "SMS" (Superior-Middle-Superior jugular; inferior-Subclavian/brachiocephalic)

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🟤 7. POSTERIOR VIEW - PARATHYROIDS & RECURRENT LARYNGEAL NERVES

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🟡 8. LYMPHATIC DRAINAGE

• Pre-laryngeal nodes (Delphian node)
• Pre-tracheal nodes
• Para-tracheal nodes
• Deep cervical nodes (along internal jugular vein, inferior to omohyoid)

Clinical note: Lymph node spread in thyroid cancer - papillary carcinoma spreads to cervical lymph nodes; follicular carcinoma spreads hematogenously to bone and lungs.

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🟢 9. NERVE SUPPLY

Key point: The thyroid gland has NO parasympathetic innervation for secretion. Hormone secretion is controlled by TSH (thyrotropin) from the anterior pituitary.

Recurrent Laryngeal Nerve (RLN) - Surgical Importance:

• Right RLN - loops around the right subclavian artery
• Left RLN - loops around the arch of aorta
• Both ascend in the tracheo-esophageal groove and pass deep to the posteromedial surface of thyroid lateral lobes
• Closely related to Berry's ligament and the inferior thyroid artery
• Injury → hoarseness (unilateral) or stridor/respiratory distress (bilateral)

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🔴 10. EMBRYOLOGICAL DEVELOPMENT

Remnants & Anomalies:

Sistrunk's operation - excision of thyroglossal cyst MUST include the central portion of the hyoid bone to prevent recurrence.

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🟣 11. HISTOLOGY

Follicular cells (Thyrocytes):

• Simple epithelium surrounding colloid-filled lumen
• Shape varies with activity:
  • Active gland → tall columnar follicular cells (high TSH)
  • Inactive gland → flat squamous follicular cells (low TSH)
• Function: Synthesis and secretion of T3 and T4
• Organelles: Rich RER (basal), Golgi + secretory granules + lysosomes (apical)

Colloid:

• Fills follicle lumen
• Contains thyroglobulin (660 kDa glycoprotein) - precursor for T3/T4
• Enough hormone stored for up to 3 months

Parafollicular cells (C cells):

• Located inside the basal lamina of follicles or between follicles
• Origin: Neural crest (ultimobranchial body / 4th pharyngeal pouch)
• Larger than thyrocytes, paler staining
• Secrete calcitonin (lowers serum calcium)
• Clinically important: Source of medullary thyroid carcinoma

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🔵 12. HORMONES & PHYSIOLOGY

Thyroid Hormones (T3 and T4):

T4 is converted to T3 peripherally in tissues (by deiodinase enzymes - mainly in liver, kidney)

Regulation (HPT Axis):

Hypothalamus → TRH (thyrotropin-releasing hormone)
      ↓
Anterior pituitary → TSH (thyroid-stimulating hormone)
      ↓
Thyroid gland → T3 + T4
      ↓
Negative feedback to hypothalamus + pituitary

Actions of T3/T4:

• Increase basal metabolic rate (BMR)
• Stimulate protein synthesis and carbohydrate metabolism
• Essential for brain development (fetal/neonatal period)
• Potentiate catecholamine effects (↑ heart rate, cardiac output)
• Required for normal growth and skeletal maturation

Calcitonin (from C cells):

• Lowers blood calcium by inhibiting osteoclasts
• Less physiologically significant in adults (vs. PTH)

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🟡 13. CLINICAL CORRELATES

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🔴 14. SURGICAL ANATOMY - KEY POINTS FOR THYROIDECTOMY

These are high-yield for exams and clinical practice:

1. Recurrent laryngeal nerve - must be identified and preserved; passes deep to posteromedial lobe near Berry's ligament and inferior thyroid artery
2. Parathyroid glands - 4 glands on posterior surface; injury → hypocalcemia (tetany)
3. External branch of superior laryngeal nerve - runs with superior thyroid artery; injury → voice fatigue, loss of high-pitched phonation (Amelita Galli-Curci syndrome)
4. Thyroid ima artery - if present, entering trachea from below during tracheostomy risks hemorrhage
5. Superior thyroid vein - must be ligated with superior thyroid artery
6. Inferior thyroid veins - drain directly into brachiocephalic veins - risk of air embolism if torn

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🟢 QUICK REVISION BOX

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What's inside - 13 slides, dark teal + gold colour scheme