Anatomy, vascular & nerve supply, applied in detail histology,development (brief)

2.  Largest & most vascular endocrine gland
 Earliest glandular tissue to develop & become functional
 Function- regulates BMR, stimulates psychosomatic growth,
important role in Ca metabolism
Features
 H- shaped & consists of right & left lateral lobes, joined by isthmus
 A third pyramidal lobe may project upwards from the isthmus
 Connected to body of hyoid bone by a fibrous or fibromuscular
band, levator glandulae thyroideae
 Accessory thyroid glands (small detached masses) are sometimes
present
 Weight- 25gm
 Each lobe measures 5×3×2cm
 Isthmus – 1.2×1.2cm

3.  Marked by 2 transverse parallel lines on the trachea-
upper 1.2 cm & lower 2.5 cm below the arch of cricoid
cartilage
 Upper pole is pointed and lower pole is broad & rounded
 Laterally overlapped by anterior border of
sternocleidomastoid
Location and Extent
 Vertebral level- C5, C6, C7 & T1
 Each lobe extends
-upwards up to oblique line of thyroid
cartilage &
-below up to 4th or 5th tracheal ring
 Isthmus present in front of 2nd, 3rd & 4th tracheal rings

4. 1. True capsule: peripheral
condensation of connective
tissue(fibrous stroma) of the gland
2. False capsule:
a) Derived from pretracheal layer of
deep cervical fascia
b) Thin along posterior border of lobe
c) Thick on medial surface of lobe,
thickens to form suspensory ligament
of Berry(explain why gland moves
with deglutition), connects the lobe to
the cricoid cartilage
d) Space between capsules contains
venous plexus, parathyroid gland
 APPLIED
1. To avoid haemorrhage during
operations, gland is removed
(thyroidectomy) along with the true
capsule.
2. Since a dense capillary plexus is
present deep to the true capsule.

5. Each lobe is conical & possesses:
 Apex: directed upwards & slightly laterally
extends up to oblique line of thyroid cartilage,
limited by the attachment of sternothyroid
 Base: extends upto 4th or 5th tracheal ring
Related to inferior thyroid artery & recurrent laryngeal nerve
 Lateral(superficial) surface: convex & covered by-
sternothyroid, sternohoid, anterior belly of omohyoid &
anterior border of sternocleidomastoid
 Medial surface: related to 2 tubes trachea &
oesophagus, 2 muscles inferior constrictor &
cricothyroid & 2 nerves external laryngeal & recurrent
laryngeal

6.  Posterolateral surface: related to carotid sheath & overlaps
common carotid artery
 Anterior border: thin & related to anterior branch of superior
thyroid artery
 Posterior border: thick & rounded, related to inferior thyroid
artery, anastomosis b/w superior & inferior thyroid arteries,
parathyroid glands & thoracic duct only on the left side
 ISTHMUS: connects lower parts of the 2 lobes.
2 surfaces- anterior & posterior
2 borders- superior & inferior
anterior surface- related to sternohyoid, sternothyroid anterior
jugular vein, skin & fascia
posterior surface- related to 2nd, 3rd & 4th tracheal ring
superior border- related to anastomosis b/w right & left
superior thyroid arteries
inferior border- inferior thyroid veins leave gland

8.  Superior thyroid artery:
 branch of external carotid artery
 Runs downwards & forwards related to external laryngeal nerve
 Pierces pretracheal fascia reach apex of lobe & Divides into anterior
& posterior branches
 Anterior branch- descends along anterior border of lobe along
upper border of isthmus, anastomose with its fellow of opposite
side
 Posterior br.- descends along posterior border, anastomose with
inferior thyroid artery
 Supplies upper 1/3rd of lobe & upper ½ of isthmus
 Inferior thyroid artery:
Branch of thyrocervical artery(from subclavian artery)
Runs upward, then medially finally downward to reach base
Give 4 or 5 branches, ascending branch anastomoses with posterior
branch of superior thyroid artery
Supplies lower 2/3rd of lobe & lower ½ of isthmus

9.  Lowest thyroid artery(thyroidea ima artery)
in 3% individuals
Arises from brachiocephalic or arch of aorta
Accessory thyroid arteries
Arises from tracheal & oesophageal artery
 Superior thyroid vein
 Emerges at apex
 Drains in IJV or common facial vein
 Middle thyroid vein
 Drain in IJV
 Inferior thyroid veins
 Emerge at lower border of isthmus
 After forming plexus drains into left
brachiocephalic vein
 Thyroid vein of Kocher
 Drain into IJV

10. Upper part- drains into upper deep cervical lymph
nodes directly or via prelaryngeal nodes
 Lower part- drains into lower deep cervical nodes
directly or via pretracheal & paratracheal nodes
 Parasympathetic - vagus & recurrent laryngeal nerve
 Sympathetic - mainly from middle cervical ganglion
 Also from superior & inferior ones

12.  Goitre- enlargement of gland, associated with hypofunction or
hyperfunction
 Enlarge downwards or backwards but not upwards due to
attachment of fascial sheath & sternothyroid to thyroid cartilage
Results in-
 Dyspnea(difficulty in breathing), due to pressure on trachea
 Dysphagia(difficulty in swallowing), due to pressure on
oesophagus
 Hoarseness of voice, due to pressure on recurrent laryngeal nerve
 Hypothyroidism- hyposecretion of T3 & T4
 causes myxoedema in adults & cretinism in children
 Hyperthyroidism(thyrotoxicosis)- hypersecretion of T3 & T4
 Clinically presents as: tachycardia, tremors, exophthalmos,
increased BMR

13. During operation to be take care of_
 Parathyroid gland
1. Superior thyroid artery is ligated near gland to
save external laryngeal nerve
2. Inferior thyroid artery is ligated away from
gland to save recurrent laryngeal nerve
 Retrosternal goitre- downward expansion
behind sternum leads to dangerous dyspnea

14.  Two types of secretory cells:
 Follicular cells-cuboidal epithelial cells forming
wall of spherical thyroid follicles.
 Secrete- T3 & T4
 Parafollicular cells or C-cells- lie between
basement membrane & follicular cells
 Also in spaces between follicles
 Secrete- thyrocalcitonin
 Function- important role in Ca metabolism
 Effects are opposite of parathormone

15.  Develop as endodermal thickening in midline of the
floor of pharynx, behind the tuberculum impar
 Thickening soon depressed to form thyroglossal duct
 Duct grows downwards in front of neck
 In front of hyoid binds around its lower border to
become retrohyoid
 Finally descends below hyoid with slight inclination to
one side(by the end of 7th week)
 Its tip bifurcates & proliferates to form bilateral
swellings, form thyroid gland
 Portion of duct near its tip form pyramidal lobe,
marked by foramen caecum
 Becomes functional in 3rd month