## Thyroid Blood Supply — Arterial Anatomy **Key Point:** The thyroid gland receives a rich dual blood supply from the superior and inferior thyroid arteries; understanding their origins and course is essential for thyroid surgery and avoiding complications. ### Superior Thyroid Artery - First branch of the external carotid artery (above the level of the lingual artery) - Descends along the **lateral border** (not medial border) of the thyroid lobe - Supplies the upper pole of the thyroid and anastomoses with the inferior thyroid artery - The superior laryngeal nerve runs medial to this artery — at risk during ligation ### Inferior Thyroid Artery - Arises from the **thyrocervical trunk** of the first part of the subclavian artery - Ascends behind the carotid sheath - Supplies the lower pole and the parathyroid glands - The recurrent laryngeal nerve crosses this artery — critical surgical landmark ### Thyroidea Ima Artery (Arteria Thyroidea Ima) - **Accessory artery**, present in ~2–15% of individuals - Arises from the **brachiocephalic trunk** (or aortic arch, right common carotid, or right subclavian) - Ascends in the midline anterior to the trachea - Supplies the lower pole and isthmus - Important in thyroid surgery and central line placement | Artery | Origin | Course | Supply | | --- | --- | --- | --- | | Superior thyroid | External carotid (1st branch) | Lateral border of lobe | Upper pole, superior lobe | | Inferior thyroid | Thyrocervical trunk (subclavian) | Ascends behind carotid sheath | Lower pole, parathyroids | | Thyroidea ima | Brachiocephalic trunk (variable) | Midline anterior to trachea | Lower pole, isthmus (accessory) | **High-Yield:** The superior thyroid artery descends along the **lateral border** of the thyroid lobe, not the medial border. This is the key distinguishing feature and the trap in this question. **Clinical Pearl:** During thyroid surgery, the superior laryngeal nerve (which runs medial to the superior thyroid artery) can be injured if the artery is ligated too close to the thyroid capsule, resulting in loss of cricothyroid function and voice fatigue. **Mnemonic:** **"STAIN"** — **S**uperior thyroid from **E**xternal carotid; **I**nferior thyroid from **I**nconstant subclavian (thyrocervical trunk); **I**ma from **I**nconstant brachiocephalic. [cite:Gray's Anatomy 42e Ch 8]
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