Inferior thyroid artery

The inferior thyroid artery (ITA) is a paired artery of the anterolateral neck and the principal arterial supply to the thyroid gland in adults. It is one of the most variable arteries in the cervical region in terms of origin, course, and branching pattern.

Origin

The ITA most commonly arises as a branch of the thyrocervical trunk (approximately 68.5% of cases), which itself originates from the first part of the subclavian artery. In approximately 31.5% of cases, the ITA arises directly from the subclavian artery. Rare variant origins include the common carotid artery, the vertebral artery, the internal thoracic artery, and the brachiocephalic trunk. The distance from the origin of the subclavian artery to the ITA origin is shorter on the right side (29.1–53.1 mm) than on the left (43.7–68.1 mm), reflecting the asymmetric anatomy of the great vessels.

Course

From its origin, the ITA ascends along the anterior surface of the longus colli muscle, medial to the anterior scalene muscle. It then curves medially and inferiorly, passing posterior to the carotid sheath (common carotid artery, internal jugular vein, and vagus nerve) to reach the posteroinferior aspect of the thyroid gland. The artery typically divides into two or three terminal branches before entering the thyroid parenchyma at the level of the inferior pole of the corresponding lobe.

Branches and Territory of Supply

The ITA supplies the inferior pole of the thyroid gland and provides the dominant blood supply to the inferior parathyroid glands. In addition to glandular branches, the ITA gives off:

• Pharyngeal branches — to the inferior pharynx

• Esophageal branches — to the cervical esophagus

• Tracheal branches — to the cervical trachea

• Ascending cervical artery (when arising from the thyrocervical trunk, this may be a co-branch rather than a direct ITA branch)

The ITA also contributes to the anastomotic network with the superior thyroid artery along the posterior surface of the thyroid gland.

Relationship to the Recurrent Laryngeal Nerve

The relationship between the ITA and the recurrent laryngeal nerve (RLN) is highly variable and of paramount surgical importance. The most commonly used classification describes three patterns:

• RLN posterior to the ITA — most common on the left side (65–86%)

• RLN anterior to the ITA — most common on the right side (40.5–75%)

• RLN between the branches of the ITA — less frequent (1–5%)

Additional subtypes have been described, including the RLN running parallel to the artery and interlacing of nerve and arterial branches. This variability makes the ITA a useful but imperfect landmark for identifying the RLN during thyroid surgery.

Anatomical Variants

• Unilateral absence of the ITA (most often the left) occurs in 1–6% of cases; when absent, the contralateral ITA or a thyroid ima artery may compensate.

• A thyroid ima artery (arteria thyroidea ima), typically arising from the brachiocephalic trunk or aortic arch, may supplement or replace the ITA.

• Anthropological differences in ITA presence and origin have been documented between Caucasian and Asian populations.