Thyroid Anatomy Stephanie Johnson PGY 2 ENT Background What: brownish-red, highly vascular gland  Location: ant neck at C5-T1, overlays 2nd – 4th tracheal rings  Avg width: 12-15 mm (each lobe)  Avg height: 50-60 mm long  Avg weight: 25-30 g in adults (slightly more in women) **enlarges during menstruation and pregnancy**  Background Pyramidal lobe:  often ascends from the isthmus or the adjacent part of either lobe (usu L) up to the hyoid bone  may be attached by a fibrous/fibromuscular band  “levator” of the thyroid gland Transverse view: relationship to other NB structures in neck Structure     Under middle layer of deep cervical fascia (pretracheal)  thyroid inner true capsule  thin and closely adherent to the gland capsule extensions within the gland form septae, dividing it into lobes and lobules lobules are composed of follicles = structural units of the gland  layer epithelium enclosing a colloid-filled cavity colloid (pink on H&E stain) contains an iodinated glycoprotein, iodothyroglobulin (precursor of thyroid hormones). Structure   Follicles = variable size surrounded by dense plexuses of fenestrated capillaries, lymphatic vessels, and sympathetic nerves. Structure Epithelial cells = 2 types:  principal (ie: follicular) – formation of colloid (iodothyroglobulin)  parafollicular (ie: C cells -clear, light), lie adjacent to follicles w/in basal lamina  produce calcitonin Relation w/ Strap muscles Lateral - sternothyroid  Anterior - omohyoid muscle - sternohyoid  Inferior - SCM (lower portion) ** careful - motor nerve supply from the ansa cervicalis enters these muscles inferiorly.  Recurrent laryngeal nerve    Recall: innervates all larynx except cricothyroid Closely assoc with ITA (see next slides for details) NB: ‘non recurrent LN’ ~5/1000 pt’s on R side – When retroesophageal R SCA from dorsal aortic arch – NRLN - branches fr X at ~ cricoid cartilage – directly enters the larynx without looping around SC – L sided - only when R aortic arch and ligamentum arteriosum concurrent w/ L retroesophageal subclavian artery. Vascular Anatomy ARTERIAL:  superior and inferior thyroid arteries (occ thyroidea ima)  ++ collateral anastomoses (ipsi and contralaterally)  thyroid ima (when pres) originates from aortic arch or innominate artery, enters the thyroid at inferior border of isthmus. Vascular Anatomy Vascular Anatomy SUPERIOR THYROID ARTERY    first anterior branch ECA descends laterally to the larynx under the omohyoid and sternohyoid muscles runs superficially on the anterior border of the lateral lobe, sending a branch deep into the gland before curving toward the isthmus where it anastomoses with the contralateral artery Vascular anatomy SUPERIOR THYROID ARTERY: Relationship with SLN:  Cephalad to the superior pole, ext branch of SLN runs w/ STA before turning medially  supply cricothyroid muscle **careful when ligating artery** Vascular anatomy INFERIOR THYROID ARTERY SCA  thyrocervical trunk  ITA  ITA ascends vertically and then curves medially to enter the tracheoesophageal groove (posterior to carotid sheath)  Branches penetrate the posterior aspect of the lateral lobe Vascular anatomy Relationship with RLN:  RLN ascends in the TE groove and enters the larynx b/w the inferior cornu of the thyroid cartilage and the arch of the cricoid  RLN can be found after it emerges from the superior thoracic outlet: – Sup: thyroid lobe – Lat: common carotid artery – Medial: trachea Vascular anatomy **Careful - relationship between RLN and ITA highly variable (Redd, 1943 – described 28 variations) Examples:  Deep to ITA (40%)  superficial (20%)  b/w branches of the artery (35%) **also – only 17% of the time is the nerve/artery relationship the same on both sides **at level ITA – extralaryngeal branches RLN present 5% of the time Vascular anatomy VENOUS: 3 pairs of veins: 1) STV – asc along STA and becomes a tributary of the IJV 2) MTV – directly lateral  IJV 3) ITV (variable): – R – passes ant to innominate a  R BCV or ant trachea  L BCV – L – drainage  L BCV **occ – both inf veins form a common trunk “thyroid ima vein”  empties into L BCV Lymphatics    Extensive, multidirectional flow periglandular  prelaryngeal (Delphian)  pretracheal  paratracheal (along RLN)  brachiocephalic (sup mediastinum)  deep cervical  thoracic duct NB: regional mets of thyroid carcinoma are superior and lateral, along IJV ie: invasion of the pretracheal and paratracheal LNs and obstruction of normal lymph flow. Lymphatics Innervation Principally from ANS  Parasympathetic fibers – from vagus  Sympathetic fibers – from superior, middle, and inferior ganglia of the sympathetic trunk Enter the gland along with the blood vessels. References: 1) 2) 3) 4) 5) 6) Schwartz www.emedicine.com www.utdol.com Gray’s anatomy http://www.ncbi.nlm.nih.gov/books/b v.fcgi?rid=endocrin.box.330 Netter’s anatomy