Thyroid Gland — Anatomy, Embryology, Blood Supply & Nerve Supply

Embryology

• The medial thyroid anlage (endodermal thickening of the pharyngeal floor) descends in the midline, anterior to the hyoid bone and larynx.
• During descent it remains connected to the foramen cecum via the thyroglossal duct (an epithelial-lined tube), which normally obliterates by the 8th week of gestation.
• The paired lateral anlages originate from the 4th branchial pouch (ultimobranchial bodies), which are neuroectodermal in origin. These fuse with the median anlage at approximately the 5th week and contribute the parafollicular C cells (calcitonin-secreting), which come to lie in the superoposterior region of each lobe.
• Thyroid follicles are apparent by 8 weeks; colloid formation begins by 11 weeks.

Developmental Anomalies

Treatment of thyroglossal duct cysts is the Sistrunk operation — en bloc cystectomy + central hyoid bone resection.

Gross Anatomy

• Weight: ~20 g (varies with body weight and iodine intake)
• Consists of two lobes connected by an isthmus just inferior to the cricoid cartilage
• A pyramidal lobe is present in ~50% of individuals, projecting superiorly from the isthmus (remnant of the thyroglossal duct)
• The lobes extend superiorly to the mid-thyroid cartilage and lie adjacent to the carotid sheaths and sternocleidomastoid muscles laterally
• Strap muscles (sternohyoid, sternothyroid, omohyoid) overlie the gland anteriorly
• Posterolaterally, the gland is in contact with the tracheoesophageal groove where the recurrent laryngeal nerve runs
• Each lobe has a posterior projection called the tubercle of Zuckerkandl at the level of the cricoid cartilage — a key surgical landmark for identifying the RLN

(Schwartz's Principles of Surgery, Fig. 38-2 — Thyroid anatomy anteriorly (A) and in cross-section (B))

Blood Supply

Arterial Supply

The inferior thyroid artery crosses the recurrent laryngeal nerve — the RLN must be identified before any arterial branches are ligated during surgery.

Venous Drainage

1. Superior thyroid veins — run with the superior thyroid arteries → drain into internal jugular veins
2. Middle thyroid veins — least consistent; drain into internal jugular veins
3. Inferior thyroid veins — often form a plexus → drain into brachiocephalic veins

Lymphatic Drainage

• Pretracheal, paratracheal, perithyroidal
• RLN chain nodes
• Superior mediastinal nodes
• Retropharyngeal and lateral cervical (deep jugular) nodes

Nerve Supply

1. Recurrent Laryngeal Nerves (RLN)

• Branch of the vagus nerve (CN X)
• Left RLN: arises where the vagus crosses the aortic arch, loops around the ligamentum arteriosum, ascends medially in the tracheoesophageal groove
• Right RLN: arises where the vagus crosses the right subclavian artery, loops posterior to it, then ascends more obliquely than the left
• Both RLNs may branch in the neck and can pass anterior, posterior, or interdigitate with branches of the inferior thyroid artery

• Unilateral injury → ipsilateral vocal cord paralysis (paramedian = weak voice; abducted = hoarse voice + ineffective cough)
• Bilateral injury → airway obstruction (may need emergency tracheostomy) or loss of voice

Non-recurrent RLN occurs in 0.5–1% on the right (associated with aberrant right subclavian artery). Extremely rare on the left (seen with situs inversus + right aortic arch). Important surgical pitfall.

The ligament of Berry is a critical zone: the last segment of the RLN courses below the tubercle of Zuckerkandl and is closely applied to this ligament. Nerve branches may traverse the ligament in 25% of individuals.

2. Superior Laryngeal Nerves (SLN)

• The external branch of the SLN (EBSLN) descends on the inferior pharyngeal constrictor alongside the superior thyroid vessels before innervating the cricothyroid
• Cernea classification describes its relationship to the superior thyroid artery:
  • Type 1: nerve crosses artery ≥1 cm above superior thyroid pole (safe zone)
  • Type 2a: nerve crosses artery <1 cm above the pole — occurs in ~20% of individuals (at risk)
  • Type 2b: nerve crosses below the thyroid pole (highest risk)
• Injury: inability to tense ipsilateral vocal cord → loss of high-pitched voice, voice fatigue, difficulty projecting voice (particularly devastating for singers)

3. Autonomic Innervation

• Sympathetic: from superior and middle cervical sympathetic ganglia → enter gland with blood vessels → vasomotor function
• Parasympathetic: from the vagus nerve via branches of the laryngeal nerves

Summary Diagram

(Bailey & Love's Short Practice of Surgery, Fig. 55.19)

Normal Respiratory Rate

Key Points

• Normal tidal volume is ~500 mL; at 12 breaths/min this gives a minute ventilation of ~6 L/min — Guyton & Hall Medical Physiology
• The acceptable range is 10–20 breaths/min in adults with normal respiratory physiology — Tintinalli's Emergency Medicine

• Normal is 30–60 breaths/min; count for a full minute with the infant resting or asleep
• Neonates increase minute ventilation almost entirely by increasing rate (not tidal volume)
• A resting rate >60 breaths/min during quiet breathing requires evaluation for tachypnea

• Tachypnea: >20 breaths/min in adults; >60/min in neonates
• Bradypnea: <12 breaths/min in adults
• Apnea: cessation of breathing; in neonates, pauses >20 seconds (or with bradycardia/cyanosis) are abnormal