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Thyroid Anatomy

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Thyroid
Anatomy
Stephanie Johnson
PGY 2
ENT
Background
What: brownish-red, highly vascular gland
пЃ® Location: ant neck at C5-T1, overlays 2nd
– 4th tracheal rings
пЃ® Avg width: 12-15 mm (each lobe)
пЃ® Avg height: 50-60 mm long
пЃ® Avg weight: 25-30 g in adults (slightly
more in women)
**enlarges during menstruation and
pregnancy**
пЃ®
Background
Pyramidal lobe:
пЃ® often ascends from the isthmus or the
adjacent part of either lobe (usu L) up
to the hyoid bone
пЃ® may be attached by a
fibrous/fibromuscular band пѓ вЂњlevator” of the thyroid gland
Transverse view:
relationship to other NB
structures in neck
Structure
пЃ®
пЃ®
пЃ®
пЃ®
Under middle layer of deep cervical fascia (pretracheal) пѓ thyroid inner true capsule пѓ 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 пѓ layer epithelium enclosing a colloid-filled cavity
colloid (pink on H&E stain) contains an iodinated
glycoprotein, iodothyroglobulin (precursor of thyroid
hormones).
Structure
пЃ®
пЃ®
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)
пЃ® parafollicular (ie: C cells -clear,
light), lie adjacent to follicles w/in
basal lamina пѓ produce calcitonin
Relation w/ Strap
muscles
Lateral - sternothyroid
пЃ® Anterior
- omohyoid muscle
- sternohyoid
пЃ® Inferior - SCM (lower portion)
** careful - motor nerve supply from the
ansa cervicalis enters these muscles
inferiorly.
пЃ®
Recurrent laryngeal nerve
пЃ®
пЃ®
пЃ®
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:
пЃ® superior and inferior thyroid arteries (occ
thyroidea ima)
пЃ® ++ collateral anastomoses (ipsi and
contralaterally)
пЃ® 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
пЃ®
пЃ®
пЃ®
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:
пЃ® Cephalad to the superior pole, ext
branch of SLN runs w/ STA before
turning medially пѓ supply cricothyroid
muscle
**careful when ligating artery**
Vascular anatomy
INFERIOR THYROID
ARTERY
SCA пѓ thyrocervical
trunk пѓ ITA
пЃ® ITA ascends vertically
and then curves
medially to enter the
tracheoesophageal
groove (posterior to
carotid sheath)
пЃ® Branches penetrate the
posterior aspect of the
lateral lobe
Vascular anatomy
Relationship with RLN:
пЃ® 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
пЃ® 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:
пЃ® Deep to ITA (40%)
пЃ® superficial (20%)
пЃ® 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 пѓ IJV
3) ITV (variable):
–
R – passes ant to
innominate a пѓ R BCV or
ant trachea пѓ L BCV
– L – drainage пѓ L BCV
**occ – both inf veins form a
common trunk “thyroid
ima vein” пѓ empties into
L BCV
Lymphatics
пЃ®
пЃ®
пЃ®
Extensive, multidirectional flow
periglandular пѓ prelaryngeal (Delphian) пѓ pretracheal пѓ paratracheal (along RLN) пѓ brachiocephalic (sup mediastinum) пѓ deep
cervical пѓ 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
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