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Thyroid Anatomy. Stephanie Johnson PGY 2 ENT. Background. What: brownish-red, highly vascular gland Location: ant neck at C5-T1, overlays 2 nd – 4 th 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)

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Thyroid Anatomy


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thyroid anatomy

Thyroid Anatomy

Stephanie Johnson

PGY 2 ENT

background
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**

background3
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
structure
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).
structure6
Structure
  • Follicles = variable size
  • surrounded by dense plexuses of fenestrated capillaries, lymphatic vessels, and sympathetic nerves.
structure7
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
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
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
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 anatomy12
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 anatomy13
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 anatomy14
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 anatomy15
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 anatomy16
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 anatomy17
Vascular anatomy

VENOUS:

3 pairs of veins:

  • STV – asc along STA and becomes a tributary of the IJV
  • MTV – directly lateral  IJV
  • 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
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.
innervation
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
References:
  • Schwartz
  • www.emedicine.com
  • www.utdol.com
  • Gray’s anatomy
  • http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=endocrin.box.330
  • Netter’s anatomy