Anatomy of the Thyroid Gland

CBME Curriculum — India MBBS Phase 1 (Anatomy)

CBME competency alignment (NMC): The student should be able to describe the gross anatomy, relations, blood supply, nerve supply, lymphatic drainage, and applied anatomy of the thyroid gland (Competency AN 47 / Surgery SU22.1).

1. Development (Embryology)

• Median anlage — endodermal thickening at the base of the tongue (foramen cecum), descends in the midline along the thyroglossal duct
• Lateral anlage — from the 4th–5th pharyngeal pouches; contributes C cells (parafollicular cells)

2. Gross Anatomy

Shape, Size, and Weight

Position

• Lies in the anterior triangle of neck, posterior to the sternohyoid and sternothyroid (strap) muscles
• Isthmus overlies tracheal rings 2–4, just caudal to the cricoid cartilage
• Each lobe extends from the mid-thyroid cartilage level down to the 5th–6th tracheal rings

Parts

1. Right lobe and Left lobe — the major components
2. Isthmus — connects the lower poles; lies over tracheal rings 2–4
3. Pyramidal lobe — a cranial extension from the isthmus (remnant of thyroglossal duct), present in ~50–75% of individuals
4. Tubercle of Zückerkandl — a small posterolateral projection of each lobe; important surgical landmark for the RLN

3. Relations

Capsule & Fascial Coverings

• The gland is invested by pretracheal fascia (forms the true capsule)
• This condenses as the suspensory ligament of Berry (posterior medial), which anchors the gland to the trachea — the RLN passes close to this ligament and may even traverse it in 25% of individuals
• Also forms the anterior suspensory ligament above the isthmus

4. Blood Supply

Arterial Supply

The inferior thyroid artery crosses the recurrent laryngeal nerve — this relationship is the most critical surgical landmark during thyroidectomy.

Venous Drainage

1. Superior thyroid veins — run with superior thyroid arteries → drain into internal jugular vein (IJV)
2. Middle thyroid veins — most variable; drain laterally → IJV
3. Inferior thyroid veins — form a plexus from the inferior poles → drain into brachiocephalic (innominate) veins

5. Lymphatic Drainage

• Primary drainage: Perithyroidal nodes in the central neck (Level VI) — bounded superiorly by the hyoid bone, inferiorly by the innominate artery, laterally by the carotid sheaths
• Secondary drainage: Level VI drains into lateral cervical nodes (Levels II, III, IV) along the jugular veins bilaterally
• Upper thyroid/pyramidal lobe → pretracheal and prelaryngeal (Delphian) nodes → Level VI
• Mediastinal nodes may also receive drainage inferiorly

6. Nerve Supply

RLN course (critical applied anatomy):

• Left RLN: arises from vagus at aortic arch, loops around ligamentum arteriosum, ascends in the tracheoesophageal groove
• Right RLN: arises at right subclavian artery, more oblique course, less predictable
• Non-recurrent RLN: right side in 0.5–1% (associated with aberrant right subclavian artery); left only with situs inversus — a catastrophic surgical surprise
• Both RLNs are most vulnerable near the ligament of Berry and at their crossing with the inferior thyroid artery

7. Histology (Microanatomy)

8. Applied Anatomy (Clinically Important)

9. CBME Phase 1 Competency Summary (NMC India)

• Describe the gross anatomy and position of the thyroid gland
• Describe its relations to surrounding structures
• Describe its blood supply, venous drainage, and lymphatic drainage
• Describe the nerve supply with emphasis on the RLN and external laryngeal nerve
• Explain the applied aspects and clinical implications (thyroiditis, goiter, thyroidectomy complications)
• Integrated teaching with Surgery (SU22.1 — applied anatomy and physiology of thyroid)

• Schwartz's Principles of Surgery, 11e — Ch. 38, Thyroid Anatomy
• Sabiston Textbook of Surgery — Ch. 73, Thyroid Embryology and Anatomy
• Mulholland & Greenfield's Surgery, 7e — Anatomy of the Thyroid
• Guyton & Hall Textbook of Medical Physiology — Ch. 77, Physiological Anatomy of the Thyroid Gland
• NMC CBME Curriculum Vol. I (Anatomy competencies; Surgery SU22.1)