Thyroid Gland and Its Arteries

Overview of the Thyroid Gland

Arterial Supply

1. Superior Thyroid Artery

• Origin: First branch of the external carotid artery
• Course: Descends along the lateral margin of the thyrohyoid muscle to reach the superior pole of the lateral lobe
• Branches at the gland:
  • Anterior glandular branch - runs along the superior border of the thyroid gland and anastomoses across the isthmus with its twin from the opposite side
  • Posterior glandular branch - passes to the posterior surface of the gland and may anastomose with the inferior thyroid artery
• Other branches: Gives off a superior laryngeal branch (which pierces the thyrohyoid membrane alongside the internal branch of the superior laryngeal nerve), and a cricothyroid branch

2. Inferior Thyroid Artery

• Origin: Branch of the thyrocervical trunk, which itself arises from the first part of the subclavian artery
• Course: Ascends along the medial edge of the anterior scalene (scalenus anterior) muscle, then arches medially and passes posteriorly to the carotid sheath to reach the posterior aspect of the thyroid gland
• Key surgical relationship: As it passes medially toward the gland, it crosses the recurrent laryngeal nerve (RLN) - the RLN may pass anterior, posterior, or between branches of the inferior thyroid artery. This is one of the most important surgical considerations during thyroidectomy
• Branches at the gland:
  • Inferior branch - supplies the lower part of the thyroid and anastomoses with the posterior branch of the superior thyroid artery
  • Ascending branch - supplies the parathyroid glands
• Parathyroid supply: The inferior thyroid artery is the primary blood supply to both superior and inferior parathyroid glands (the superior parathyroid may also receive branches from the superior thyroid artery, but the inferior thyroid is dominant for both)

3. Thyroid Ima Artery (Thyroidea Ima)

• An unpaired, inconstant vessel - present in approximately 1.5% to 12% of individuals (varies by source; about 2-10% cited across texts)
• Origin (variable): Can arise from:
  • Brachiocephalic (innominate) artery - most common
  • Arch of the aorta
  • Right common carotid artery
  • Internal thoracic artery
  • Subclavian artery
• Course: Follows a midline path ascending on the anterior surface of the trachea to enter the thyroid isthmus or inferior poles of the lobes
• Surgical significance: Must be identified before midline procedures such as tracheotomy or thyroidectomy to avoid unexpected hemorrhage

Summary Table

Venous Drainage (for completeness)

• Superior thyroid vein - travels with the superior thyroid artery; drains into the internal jugular vein
• Middle thyroid vein - travels without an arterial companion; drains into the internal jugular vein
• Inferior thyroid veins - unpaired vessels draining into the right and left brachiocephalic veins respectively

Clinical / Surgical Relevance

1. RLN and inferior thyroid artery relationship - The recurrent laryngeal nerve crosses the inferior thyroid artery in a variable pattern (nerve can be anterior, posterior, or between arterial branches). Surgeons must identify the RLN before ligating the inferior thyroid artery during thyroidectomy to avoid voice-damaging nerve injury.
2. Parathyroid preservation - Because the inferior thyroid artery is the dominant blood supply to all four parathyroid glands, inadvertent ligation of this artery close to its origin (rather than at the capsule) can devascularize the parathyroids and cause permanent hypoparathyroidism.
3. Thyroid ima artery - Absence or presence must be noted during tracheotomy. An unrecognized ima artery crossing the trachea can cause severe bleeding.
4. Highly vascular organ - The thyroid has a rich capsular arterial network connecting all named vessels, which is why thyroid surgery can be accompanied by significant blood loss if vessels are not carefully controlled.