32. THYROID AND TRACHEA

The thyroid is a butterfly-shaped endocrine gland in the anterior neck (vertebral levels C5-T1), comprising two lobes connected by an isthmus over the 2nd-4th tracheal rings. It is highly vascular, wrapped around the trachea and larynx, and enclosed in pretracheal fascia. The trachea is a cartilaginous tube extending from the larynx to the bronchi, located posteriorly to the thyroid isthmus. 

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Thyroid Gland Anatomy

• Structure: Two lateral lobes (left and right) connected by a central narrow band, the isthmus. A pyramidal lobe (remnant of embryonic development) may be present.
• Location: Located in the anterior neck, inferior to the thyroid cartilage (Adam's apple), straddling the trachea.
• Dimensions: Approximately 5 cm long, 3 cm wide, 2 cm thick, and roughly 25 grams in adults.
• Relations:
  • Anterior: Covered by strap muscles (sternothyroid, sternohyoid) and fascia.
  • Posterior: Adjacent to the trachea, larynx, esophagus, and pharynx.
  • Lateral: Carotid sheath, containing the common carotid artery and internal jugular vein.
  • Nerves: The recurrent laryngeal nerves run in the groove between the trachea and esophagus, behind the lateral lobes.
• Vascular Supply:
  • Arteries: Superior thyroid artery (from external carotid) and inferior thyroid artery (from thyrocervical trunk). A thyroid ima artery may exist in 10% of people.

  • Veins: Superior, middle, and inferior thyroid veins draining into the internal jugular and brachiocephalic veins. 

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Trachea Anatomy (Cervical Component)

• Position: Lies posterior to the thyroid gland, extending downwards from the cricoid cartilage (C6 level).
• Structure: Composed of 16-20 C-shaped hyaline cartilage rings that keep the airway open, with a posterior fibroelastic membrane.
• Relations:
  • Anterior: Thyroid isthmus (over 2nd-4th rings), inferior thyroid veins, strap muscles.

  • Posterior: Esophagus. 

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Clinical Significance

• Thyroid Enlargement (Goiter): Due to its fixation to the trachea via the suspensory ligament of Berry, the thyroid moves upward during swallowing.
• Thyroglossal Cyst: A remnant of embryological descent, presenting as a midline neck lump.
• Recurrent Laryngeal Nerve Damage: Risk during thyroid surgery, causing voice hoarseness