The thyroid gland is a brownish-red, highly vascular gland located in the anterior neck at the level of C5-T1, overlaying the 2nd to 4th tracheal rings. It has an average width of 12-15 mm for each lobe and weighs around 25-30 grams in adults. The thyroid gland is composed of lobes and lobules which contain follicles - the structural units made up of a colloid-filled cavity surrounded by a layer of epithelial cells. The thyroid receives its blood supply from the superior and inferior thyroid arteries and drains into the internal jugular veins. The recurrent laryngeal nerve travels in close relationship with the inferior thyroid artery and supplies motor innervation to all of the larynx

1. Thyroid
Anatomy
Stephanie Johnson
PGY 2
ENT

2. 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**

3. 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

4. Transverse view:
relationship to other NB
structures in neck

5. 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).

6. Structure
 Follicles = variable size
 surrounded by dense plexuses of
fenestrated capillaries, lymphatic
vessels, and sympathetic nerves.

7. 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

8. 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.

9. 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.

10. 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.

11. Vascular Anatomy

12. 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

13. 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**

14. 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

15. 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

16. 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

17. 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

18. 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.

19. Lymphatics

20. 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.

21. References:
1) Schwartz
2) www.emedicine.com
3) www.utdol.com
4) Gray’s anatomy
5) http://www.ncbi.nlm.nih.gov/books/b
v.fcgi?rid=endocrin.box.330
6) Netter’s anatomy