Thyroid anatomy

• 2 main lobes connected via the isthmus
• Pyramidal lobe a variable conical 3rd lobe usually arising from the isthmus (remnant of thyroglossal duct)
• Weighs approximately 25-30g in adult
• Relations
  • Anterior to larynx and trachea
  • Inferior to the oblique line of the thyroid cartilage
  • Extends to the 6th tracheal ring and the isthmus is commonly found at the 2nd-4th
  • Anteriorly covered by the pretracheal fascia and posteriorly is fixed to the cricoid and tracheal cartilage by the ligament of Berry (posterior suspensory ligament)
    • Hence movement with swallow
• Lymphatic drainage to the deep cervical nodes to the thoracic duct
• Blood supply via the superior thyroid arteries (branch of the thyrocervical artery direct from the aorta- supplies via lateral, posterior and anterior branches) and inferior thyroid artery (branch of the common carotid)
  • (Occasionally the isthmus is also supplied via the aortic thyroidea ima artery)
• Venous drainage via the superior, middle (both via internal jugular) and inferior (via brachiocephalic) thyroid veins
• Important adjacent nerves
  • Superior laryngeal nerve can be damaged with ligation of the superior thyroid artery
  • Recurrent laryngeal nerve loops around the right subclavian artery and around the arch of aorta (on the left) before travelling with the inferior thyroid artery.  Risk of damage with ligation of this e.g. with thyroid surgery