Anatomy of the Thyroid Gland

1. Overview

• Weight: 12–20 g in adults (may be slightly larger in women and during pregnancy)
• Consistency: Soft, highly vascular
• Color: Brown-red (due to rich vascularity)

2. Location and Gross Structure

Components

• Two lateral lobes (right and left): Each lobe is pear-shaped, ~5 cm long, 3 cm wide, 2 cm thick
• Isthmus: Connects the lower thirds of the two lobes; lies over the 2nd–4th tracheal rings
• Pyramidal lobe: Present in ~50% of individuals; a finger-like projection upward from the isthmus (usually toward the left), representing a remnant of the thyroglossal duct

3. Capsule and Fascia

• The gland is enclosed by a thin fibrous capsule that sends septa (trabeculae) into the parenchyma, dividing it into lobules
• The capsule is surrounded by the pretracheal layer of deep cervical fascia, which tethers the gland to the larynx and trachea — this is why the thyroid moves with swallowing
• The false capsule (outer fascial layer) is surgically important as it separates the gland from surrounding structures

4. Anatomical Relations

Anterior

• Sternothyroid and sternohyoid muscles (strap muscles)
• Anterior jugular veins
• Skin and superficial fascia

Posterior

• Trachea and esophagus (esophagus lies slightly to the left)
• Cricothyroid muscles (medially)

Posterolateral

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

Posterior surface (critical surgical relations)

• Recurrent laryngeal nerves (RLN) — run in the tracheoesophageal groove; at risk during thyroidectomy
• Parathyroid glands (superior and inferior, 4 in total) — embedded in or attached to the posterior surface of each lobe

5. Blood Supply

Arterial

• The superior thyroid artery runs alongside the external branch of the superior laryngeal nerve (EBSLN) — injury causes voice changes (loss of high pitch)
• The inferior thyroid artery crosses the recurrent laryngeal nerve — the most important surgical relation in thyroid surgery

Venous Drainage

• The inferior thyroid veins form a plexus anterior to the trachea (thyroid impar plexus)

6. Lymphatic Drainage

Clinically important: Papillary thyroid carcinoma spreads early to regional lymph nodes via these channels.

7. Innervation

• The thyroid receives autonomic innervation only (no somatic sensory supply to the parenchyma)
• Sympathetic: Superior, middle, and inferior cervical ganglia (via cervical sympathetic chain) → vasomotor (vasoconstriction)
• Parasympathetic: Vagus nerve → vasodilatory effect
• Nerve fibers accompany blood vessels; they regulate blood flow but do not directly control hormone synthesis (TSH is the primary regulator)

8. Microanatomy (Histology)

Follicles (Functional Units)

• Spherical structures, 50–500 µm in diameter
• Lined by follicular cells (thyrocytes): simple cuboidal epithelium that becomes columnar when active and squamous when inactive
• Filled with colloid — a viscous material composed predominantly of thyroglobulin (the storage form of thyroid hormones T3 and T4)

Parafollicular Cells (C cells)

• Located between follicles (in the interfollicular space) and within the follicular basement membrane
• Produce calcitonin (involved in calcium homeostasis)
• Derived embryologically from neural crest cells (via ultimobranchial body), unlike follicular cells which arise from endoderm
• More concentrated in the middle third of each lobe

Stroma

• Delicate connective tissue septa carrying blood vessels, lymphatics, and nerves

9. Embryology (Brief)

• The thyroid descends from the foramen cecum along the thyroglossal duct to its final position by week 7 of gestation
• Failure of descent → lingual thyroid; incomplete obliteration of duct → thyroglossal duct cyst

10. Surgical Anatomy Highlights

Summary Diagram (Structural Overview)

         Hyoid bone
              |
     Superior thyroid artery
              |
      [  Left lobe  ]---[Isthmus]---[  Right lobe  ]
              |              |               |
       Parathyroids      2nd–4th         Parathyroids
       (posterior)     tracheal rings     (posterior)
              |                               |
         Inferior thyroid artery ← Thyrocervical trunk
              |
        Trachea / Esophagus (posterior)