Overview
A butterfly-shaped endocrine gland in the the visceral compartment of the anterior neck, spanning the C5 – T1 vertebrae and wrapping around cricoid cartilage and trachea. It produces thyroid hormone.
Structure
- Two lobes connecting centrally by an isthmus (covers the 2nd and 3rd tracheal cartilage)
- Pretracheal fascia splits around thyroid gland to form pseudocapsule
Arterial Supply
Superior thyroid artery
- Branch of external carotid artery
- Supplies superior poles and isthmus
- Anterior branch descends across isthmus, posterior branch descends posteriorly to anastomose with inferior thyroid artery
Inferior thyroid artery
- Branch of thyrocervical trunk of subclavian artery
- Supplies inferior poles and anastomoses with superior thyroid artery
Thyroid ima artery
- Normal anatomical variants – arises from the brachiocephalic trunk or aortic arch
Venous Drainage
- Superior and middle thyroid veins – Drains superior part of thyroid into internal jugular vein
- Inferior thyroid veins – Drains remaining gland into left brachiocephalic vein
Lymphatic Drainage
- Superior thyroid drains to superior deep cervical nodes
- Inferior thyroid drains to paratracheal nodes (anterior group), to inferior deep cervical nodes
Innervation
- Sympathetic – Superior, middle and inferior cervical ganglion of the sympathetic trunk.
- Parasympathetic – Superior laryngeal nerve and the recurrent laryngeal nerve – both branches of the vagus nerve.
- The superior laryngeal nerve arises from the inferior ganglion of the vagus nerve, a splits into internal laryngeal (sensory – laryngeal mucosa) and external laryngeal (motor – cricothyroid muscles).
Relations
- Superior: Cricoid cartilage, cricothyroid muscle, thyroid cartilage
- Anterior: strap muscles (sternohyoid, sternothyroid), superficial layer of deep cervical fascia, platysma
- Posterior: cricoid cartilage and trachea, parathyroid glands, buccopharyngeal fascia, retropharyngeal space, spinal nerve of accessory nerve
- Medial: recurrent laryngeal nerves in trachea-oesophageal groove (posteromedial), trachea
- Lateral: carotid sheath (common carotid artery, vagus nerve, internal jugular vein)
- Inferior: Suprasternal space (of Burns)
Development
- Derived from 1st and 2nd pharyngeal pouches at foramen caecum
- Arises as median outgrowth from base of tongue, foramen caecum is a remnant
Anatomical Variants
- Absent isthmus
- Ectopic thyroid tissue
- Pyramidal lobe extending to base of tongue
- Thyroglossal cyst or fistula
- Tubercle of Zuckerkandl
- Thyroid ima artery
- Recurrent laryngeal nerves may pass medial, lateral or through thyroid ligaments