Thyroid Gland
On this page
Direct answer
Largest purely endocrine gland in the body, the thyroid consists of two lateral lobes joined by an isthmus lying across the second to fourth tracheal rings. Wrapped in pretracheal fascia attached to the larynx, the gland rises on swallowing — a bedside sign separating thyroid swellings from other neck masses. Its surgery tests two nerves: the external laryngeal nerve with the superior thyroid artery, and the recurrent laryngeal nerve crossing the inferior thyroid artery's branches, with ligation rules protecting each.
What you must remember
- Parts and levels: each lobe from the thyroid cartilage to the fifth or sixth tracheal ring; isthmus over the second to fourth rings; pyramidal lobe in about half — a duct remnant.
- Fascia: the pretracheal sheath tethers the gland to larynx and trachea, so it moves on swallowing; the same fascia lets goitres descend behind the sternum into the superior mediastinum.
- Arteries: superior thyroid from the external carotid; inferior thyroid from the thyrocervical trunk; occasionally an arteria thyroidea ima from the aortic arch or brachiocephalic trunk.
- Ligation rules: superior thyroid artery tied close to the gland — the external laryngeal nerve diverges from it at the upper pole; inferior thyroid artery tied away from the gland, after identifying the recurrent laryngeal nerve, whose relation to the artery varies.
- Veins: superior and middle thyroid veins to the internal jugular; inferior thyroid veins to the brachiocephalic veins.
- Recurrent laryngeal nerve: ascends the tracheo-oesophageal groove, may pass in front of, behind or between the inferior thyroid artery's branches, and is tethered at Berry's ligament before entering the larynx behind the cricothyroid joint.
- Embryology: from the foramen caecum of the tongue, descending along the thyroglossal duct; parafollicular C cells from the ultimobranchial body.
- Clinical anchors: thyroglossal cyst — midline, moves on tongue protrusion; unilateral recurrent laryngeal injury — hoarseness; bilateral — airway obstruction; external laryngeal injury — monotone with loss of high pitch.
A thyroidectomy, narrated for the viva
A collar incision two finger-breadths above the sternal notch opens skin, platysma and investing fascia; the strap muscles are reflected and the pretracheal fascia opened to deliver the gland. At the upper pole the superior thyroid artery is ligated on the gland itself, because the external laryngeal nerve runs with the artery but peels away from it just before the pole — tying too high catches the nerve, and the patient loses the cricothyroid-tensed top of the vocal range. At the lower pole the inferior thyroid artery is tied away from the gland, but only after the recurrent laryngeal nerve is positively identified in the tracheo-oesophageal groove, since it crosses the arterial branches unpredictably and is tethered at Berry's ligament. The isthmus is divided between clamps, then drain and closure. Postoperative hoarseness from oedema is expected; stridor after bilateral surgery returns to theatre as an emergency.
The developmental checklist follows the same midline logic: a swelling anywhere from the foramen caecum to the sternal notch that climbs on tongue protrusion is thyroglossal, and its excision must include the body of the hyoid and the whole tract — the Sistrunk operation — because duct left behind recurs.
Where students slip
The ligation rule is reversed more often than any fact in anatomy: superior tied close to the gland, inferior tied away — the danger at the upper pole sits high (the nerve leaves the artery early), while the danger at the lower pole sits on the gland itself (the nerve loops up to it). Lateral neck cysts are mislabelled thyroglossal: thyroglossal cysts are midline and move with the tongue; branchial cysts lie at the junction of the upper and middle thirds of sternocleidomastoid and do not. And in the relations question, answer in order — lateral, the carotid sheath; posteromedial, the oesophagus and recurrent laryngeal nerve; on the posterior capsule, the parathyroids, whose blood supply rides in the inferior thyroid artery being ligated — the anatomical reason parathyroids are at risk in every thyroidectomy.
Frequently asked questions
At what level does the isthmus of the thyroid lie?
Across the second, third and fourth tracheal rings; tracheostomy is usually made below the isthmus, and a large or high-riding isthmus is a recognised intraoperative difficulty.
Why is the superior thyroid artery ligated close to the gland?
Because the external laryngeal nerve runs with the artery but diverges from it near the upper pole; ligating on the gland spares the nerve to cricothyroid, which tenses the cord for high notes.
Why is the inferior thyroid artery ligated away from the gland?
Because the recurrent laryngeal nerve crosses its branches close to the gland, in front of, behind or between them; the artery is tied only after the nerve is identified in the tracheo-oesophageal groove.
What is the developmental origin of the parafollicular C cells?
The ultimobranchial body associated with the fourth and fifth pharyngeal pouches, migrating into the thyroid — unlike the follicular cells, which descend from the foramen caecum with the thyroglossal duct.