Blood Supply of the Thyroid Gland and Clinical Anatomy

Arterial Supply

1. Superior Thyroid Artery (STA)

• Origin: First branch of the external carotid artery
• Course: Descends along the inferior constrictor muscle together with the superior thyroid vein
• Supply: Superior, anterior, and lateral parts of the thyroid lobe; gives off the superior laryngeal artery
• Relation: Lies posterolateral to the external branch of the superior laryngeal nerve (EBSLN) as the nerve courses through the fascia overlying the cricothyroid muscle

2. Inferior Thyroid Artery (ITA)

• Origin: Branch of the thyrocervical trunk (from the subclavian artery), which is itself a branch of the first part of the subclavian
• Course: Ascends along the anterior scalene muscle to the level of C6/C7, then turns medially and inferiorly behind the carotid sheath to enter the inferior pole
• Supply: Inferior, posterior, and medial parts of the thyroid; also the primary blood supply to both parathyroid glands
• Relation: Crosses either anterior or posterior to the recurrent laryngeal nerve (RLN) - anterior in ~70% of patients (variable, and often branching above this point)

3. Thyroidea Ima Artery (Arteria Thyroidea Ima)

• Origin: Inconstant (present in ~3-10%); may arise from the innominate (brachiocephalic) artery, common carotid, or aortic arch
• Course: Ascends in the midline anterior to the trachea to supply the isthmus and lower poles
• Clinical relevance: Can bleed unexpectedly during emergency tracheotomy or median sternotomy

Venous Drainage

Lymphatic Drainage

• Upper and middle thyroid: drain to deep cervical nodes along the IJV
• Lower thyroid: drain to pretracheal, paratracheal, and anterior mediastinal lymph nodes
• Clinically important in the spread of thyroid carcinoma (especially papillary carcinoma)

Clinical / Surgical Anatomy

1. External Branch of Superior Laryngeal Nerve (EBSLN) - "the forgotten nerve"

• Runs close to the superior thyroid vessels and is vulnerable during ligation of the superior thyroid pedicle
• Supplies the cricothyroid muscle (the only tensor of the vocal cord)
• Injury causes: reduced pitch, reduced voice range, loss of high notes - especially disabling in singers and professional speakers
• Safeguard: Ligate the STA branches individually at the thyroid capsule, not as a pedicle

2. Recurrent Laryngeal Nerve (RLN) and ITA Relationship

• The ITA crosses the RLN at the level of the inferior thyroid pole
• The RLN lies deep to the ITA in ~70% of cases; may lie anterior or pass between its branches
• The nerve is at greatest risk here during inferior pole dissection and during ligation of the ITA
• Right RLN: loops around the right subclavian artery, runs obliquely ~2 cm lateral to the trachea
• Left RLN: loops around the aortic arch (lateral to ligamentum arteriosum), runs more medially in the tracheoesophageal groove - slightly more protected
• Injury causes: unilateral - hoarseness; bilateral - aphonia, stridor, respiratory distress requiring tracheostomy

3. Non-Recurrent Laryngeal Nerve

• Rare (~1%), nearly always on the right side
• Enters the larynx directly from the vagus without looping under the subclavian
• Almost always associated with an aberrant retroesophageal right subclavian artery (arteria lusoria)
• High risk of unrecognized injury during thyroid surgery - must be anticipated

4. Parathyroid Gland Preservation

• The inferior thyroid artery is the dominant blood supply to both superior and inferior parathyroid glands (may also receive supply from STA or thyroidea ima)
• Routine mass ligation of the ITA risks devascularizing the parathyroids, causing postoperative hypoparathyroidism and hypocalcaemia
• Safeguard: Ligate the ITA branches individually close to the thyroid capsule (distal ligation), not the main trunk

5. Berry Ligament

• The posterior suspensory ligament of the thyroid; condenses the pretracheal fascia and connects the posterior lobe to the cricoid cartilage and first two tracheal rings
• The RLN passes immediately adjacent to or through the Berry ligament just before entering the larynx - the most common site of inadvertent nerve injury
• A small terminal branch of the ITA typically accompanies the nerve here

6. Middle Thyroid Vein

• Has no accompanying artery
• Its short, valveless course directly to the IJV means it tears easily during medial mobilization of the thyroid lobe - a common source of unexpected hemorrhage
• Must be identified and ligated early in thyroid surgery

7. Inferior Thyroid Veins and Tracheotomy

• The inferior thyroid veins descend in the midline anterior to the trachea
• Prone to injury during emergency tracheotomy, especially in children and in patients with a low-lying thyroid
• The thyroidea ima artery, if present, adds further hemorrhagic risk in the midline

Summary Table

• Cummings Otolaryngology Head and Neck Surgery - Surgical Anatomy and Embryology (thyroid)
• Color Atlas of Human Anatomy Vol. 2, Internal Organs - Gross Anatomy, pp. 592-593
• Pye's Surgical Handicraft, 22nd Ed. - Recurrent and Superior Laryngeal Nerve sections
• Mulholland & Greenfield's Surgery, 7th Ed. - Anatomy