Arterial Supply of the Thyroid Gland
The thyroid gland is one of the most vascular organs in the body, supplied by three pairs of arteries and occasionally a fifth unpaired artery.
Main Arteries
1. Superior Thyroid Artery
Origin: First branch of the external carotid artery
Course:
- Descends to the upper pole of the lateral lobe
- Accompanied by the external branch of the superior laryngeal nerve (runs medial to it)
- Divides into anterior and posterior branches at the upper pole
Branches at the gland:
- Anterior branch: runs down the anterior border of the lobe; anastomoses across the upper border of the isthmus with the opposite superior thyroid artery
- Posterior branch: runs down the posterior border; anastomoses with the inferior thyroid artery
Surgical note: The external branch of the superior laryngeal nerve (EBSLN) runs close to the superior thyroid artery before the artery enters the gland. It supplies cricothyroid muscle (tenses the vocal cords — affects high-pitched voice). Damage during ligation of the superior thyroid artery → loss of ability to produce high-pitched sounds. The artery must be ligated at the capsule of the upper pole to protect the EBSLN.
2. Inferior Thyroid Artery
Origin: Largest branch of the thyrocervical trunk (from the first part of the subclavian artery)
Course:
- Ascends behind the carotid sheath
- Loops medially at the level of C6 vertebra (Chassaignac’s tubercle)
- Passes behind the common carotid artery and sympathetic trunk
- Enters the posteromedial aspect of the gland
Branches at the gland:
- Inferior, medial, and posterior branches supply the lower pole and posterior surface
- Supplies parathyroid glands (critical)
Relation to RLN: The inferior thyroid artery crosses the recurrent laryngeal nerve at the posteromedial aspect of the lobe. The RLN may pass anterior, posterior, or between branches of the artery. The artery must be ligated laterally (away from the gland and nerve).
3. Thyroidea Ima Artery (Arteria Thyroidea Ima)
Incidence: Present in approximately 3–10% of individuals
Origin: Arises from the:
- Brachiocephalic trunk (most common)
- Arch of the aorta
- Right common carotid artery
- Internal thoracic artery
Course: Ascends in front of the trachea to supply the isthmus from below
Surgical significance: Must be identified and ligated before tracheostomy and during thyroidectomy — unexpected brisk haemorrhage if cut unrecognised.
Summary Table
| Artery | Origin | Supplies |
|---|---|---|
| Superior thyroid (paired) | External carotid artery | Upper pole + anterior surface |
| Inferior thyroid (paired) | Thyrocervical trunk (subclavian) | Lower pole + posterior surface + parathyroids |
| Thyroidea ima (unpaired, ~10%) | Brachiocephalic trunk / aortic arch | Isthmus (from below) |
Anastomoses
Rich anastomoses exist between:
- Right and left superior thyroid arteries (across the upper border of the isthmus)
- Superior and inferior thyroid arteries on each side (posterior surface)
- Right and left inferior thyroid arteries (across the isthmus inferiorly)
This rich anastomotic supply means ligation of one or two vessels rarely causes infarction — however, it also means unexpected haemorrhage after incomplete ligation.