Structure
The thyroid has two lobes joined in front of the trachea by a narrow isthmus. Each lobe is roughly pear-shaped and about 5 cm long. It reaches from the oblique line of the thyroid cartilage above to about the fifth or sixth tracheal ring below.
- Isthmus: crosses the second to fourth tracheal rings.
- Pyramidal lobe: often present, projecting up from the isthmus, usually to the left of the midline. It is a remnant of the thyroglossal duct.
- Capsules: a thin true fibrous capsule, and outside it a false capsule from the pretracheal fascia.
- Suspensory ligament (ligament of Berry): a thickening that binds the back of each lobe to the cricoid cartilage and upper tracheal rings. This is why the gland rises with the larynx on swallowing.
The gland develops from the floor of the pharynx at the foramen caecum of the tongue and descends in front of the hyoid and larynx along the thyroglossal duct.
Relations
The thyroid sits between the strap muscles in front and the airway, gullet and nerves behind.
- Anterior: sternothyroid, sternohyoid, the superior belly of omohyoid and, more laterally, sternocleidomastoid. Sternothyroid attaches to the oblique line and stops a goitre expanding upwards, so it grows down behind the sternum.
- Posterolateral: the carotid sheath with the common carotid artery, internal jugular vein and vagus.
- Medial: the larynx and trachea, the pharynx and oesophagus, cricothyroid, the external laryngeal nerve, and the recurrent laryngeal nerve in the tracheo-oesophageal groove.
- Posterior: the parathyroid glands, usually two on each side.
On this specimen the neighbouring airway structures are labelled too. The thyrohyoid membrane hangs the thyroid cartilage from the hyoid and is pierced by the internal laryngeal nerve and superior laryngeal vessels. Cricothyroid runs from the front of the cricoid up and back to the thyroid cartilage, tilting it forward to tense the vocal folds. The cricothyroid ligament fills the gap between the two cartilages in the midline. The trachea starts at the lower border of the cricoid at C6 and is held open by C-shaped cartilage rings. The brachiocephalic trunk, the first branch of the aortic arch, crosses in front of the trachea from left to right at the root of the neck and divides behind the right sternoclavicular joint.
Blood supply and innervation
The thyroid is one of the most vascular organs in the body, supplied by two paired arteries and sometimes a third, unpaired one.
| Artery | Origin | Course and key relation |
|---|---|---|
| Superior thyroid | External carotid, usually its first branch | Descends to the upper pole; the external laryngeal nerve runs close to it and parts from it just above the gland |
| Inferior thyroid | Thyrocervical trunk of the subclavian | Rises along scalenus anterior, then arches medially behind the carotid sheath to the back of the lower lobe; crosses the recurrent laryngeal nerve, in front, behind or between its branches |
| Thyroidea ima (inconstant) | Brachiocephalic trunk or aortic arch | Ascends on the front of the trachea to the isthmus |
Three pairs of veins drain the gland. The superior and middle thyroid veins drain to the internal jugular vein; the short middle vein is ligated first at surgery. The inferior thyroid veins form a plexus in front of the trachea and drain to the brachiocephalic veins. Sympathetic fibres from the superior, middle and inferior cervical ganglia reach the gland with its arteries and are vasomotor. Hormone release is controlled by thyroid-stimulating hormone from the pituitary, not by nerves.
Clinical relevance
Thyroid surgery is judged by the voice and the calcium afterwards.
- Recurrent laryngeal nerve injury: hoarseness from a paralysed vocal fold. Bilateral injury leaves both folds near the midline and causes stridor.
- External laryngeal nerve injury: a weak voice with loss of high pitch, prevented by ligating the superior thyroid vessels close to the upper pole.
- Parathyroid injury or removal: hypocalcaemia with tingling and tetany.
- Thyroglossal cyst: a midline neck lump that moves up on swallowing and on sticking out the tongue.
- Retrosternal goitre: extends into the superior mediastinum and can compress the trachea.
- Low tracheostomy: the brachiocephalic trunk crosses the lower trachea, and erosion by a low tube can cause a tracheo-innominate fistula with massive bleeding.
- Cricothyroidotomy: the emergency airway passes through the cricothyroid ligament, in its lower part, avoiding the small cricothyroid arteries that cross its upper part.
On the specimen
The specimen shows the right lobe of the thyroid with the airway in front and the great vessels at the root of the neck.
- Thyroid lobe or strap muscle: the lobe is lobulated, reddish-brown glandular tissue; the straps are flat parallel muscle fibres lying over it.
- Cricothyroid muscle or ligament: the muscle is a paired fan on each side of the front of the cricoid; the ligament is the midline membrane between the cartilages.
- Thyrohyoid membrane or cricothyroid ligament: the thyrohyoid membrane is above the thyroid cartilage, up to the hyoid; the cricothyroid ligament is below it, down to the cricoid.
- Trachea: identified by its horizontal cartilage rings below the cricoid, with the isthmus lying over the upper rings.
- Brachiocephalic trunk: the large artery rising to the right behind the manubrium, in front of the lower trachea.