1. THYROID GLAND
Captain Rishi Pokhrel

2. Introduction
Unique endocrine gland
Located superficially
Uses raw material – supplied
externally ( Iodine )
Stores the product (2
months)
Rich blood supply
 5 ml/g/min
 5 l/hr.
0.4% of body weight - 2% of
total blood flow

3. Introduction – Historical background
 Eponymy – Gr. thyreos (Shield)
 Goiters were known long
before the thyroid gland itself.
 God Bes of ancient Egypt –
features of myxedema
 China 2700 B C
 Ayurveda 1400 BC –
“galaganda”

4.  Hippocrates (460-337 BC)
“...when glands of the neck
become diseased
themselves, they become
tubercular and produce
struma....” (struma – goiter)
 Hippocrates failed to
differentiate between the
thyroid and the cervical glands

5. Gallen (130-200 AD) described
operations on two boys by
ignorant physicians who
removed tubercular nodes
with their
fingernails, rendering one boy
mute and the other semi-
mute.
secretions of the thyroid
lubricated the larynx &
cartilage ; aphonia was
provoked by cutting the
laryngeal nerves

6.  Aetios 550 AD : goiter ->
aneurysm
Bronchocele, elephantias
is of the throat etc.

7. • Leonardo Da Vinci is
generally credited as
the first to draw the
thyroid gland as an
anatomical organ in
1508 AD

8. Andreas Vesalius (1514-
1564) correctly described
the anatomy of thyroid
gland in detail
B. Eustachius (1520-1547)
first used the term
isthmus
Thomas Warton (1614-
1673) gave the gland its
modern name of thyroid

9.  Robert James Graves, 1835 –
hyperthyroidism – Grave’s disease
 Partial thyroidectomy - P.S. Dessault (1744-
1795) in Paris.
 Guillance Dupuytren 1808 - total
thyroidectomy for tumor
 Ludwig Rehn, 1880, first successful
thyroidectomy for exophthalmic goiter.
 Thyroxine was identified only in the 19th
century
 In 1909, Theodor Kocher won Nobel Prize
in Medicine "for his work on the
physiology, pathology and surgery of the
thyroid gland”

10. Development
 Starts from 3rd week of
IUL -1st endocrine gland
to develop
 Proliferation of cells
from caudal end of
Thyroglossal duct -
endoderm
 PF or C cells –
Ultimibranchial body –
4th/5th pharyngeal
pouch – neural crest
cells

12. Week 3 (day 24)
 appears as midline
vesicular structure at
foramen cecum
 form a duct like
invagination of ventral
pharyngeal endoderm
 grows caudally to become
thyroglossal duct

13. Week 7
 finishes descent along
midline – forms median
isthmus & 2 lateral lobes
 2 lateral anlagen develop
from 4th-5th branchial
pouch, which contains
ultimobranchial body
 midline and lateral portions
of thyroid fuse
 Thyroglossal duct
disappears – remnants:
Pyramidal lobe (50%) and
levator muscles

14.  Week 9: cords and plates of follicular cells are formed
 Week 10:cords divide into small cellular groups, small follicular
lumina appear
 Week 11-12: colloid secretion appears, thyroid becomes functional
 Week 14: well developed follicles are lined by follicular cells and
contain thyroglobulin containing colloid in lumina
 Week 20: levels of TSH and T4 starts rising
 Week 35: TSH & T4 levels = adults
 Early growth and development is independent of TSH

15. Features
Features
 Fleshy mass in the neck, in front of
trachea, concealed by strap muscles of
neck
 2 symmetrical lobes united at isthmus.
 Lobes 5 x 3 X 2 cm; isthmus 1.25 x 1.25
cm
 25 – 30 gms in wt. – variable, larger in
females, varies with menstruation and
pregnancy

16. Features
 Lobes – Pear shaped, triangular in
cross section
apex: oblique line of thyroid
cartilage
base: 4-6 tracheal ring
 Isthmus flat and square: against
2-4 tracheal rings
 Pyramidal lobe (50%)
 Levator Glandulae thyroidae

17. Coverings
 Inner true capsule: condensation
of parenchyma
 Outer false capsule: formed by
splitting of pretracheal layer of
deep Cx fascia.
 Blood vessels ramify under true
capsule
 Ligament of berry – condensation
of PTF from false capsule to
cricoid cartilage- RLN runs in it ->
movement of thyroid gland with
larynx

18. Deep cervical fascia

19. Relations
 Lobes: Δr in cross section – 3 surfaces: ant-lat, med &
post
 Only posteromedial border is prominent.
 Med surface – 2 each; cartilage, muscle, tubes & nerves

21. Relations
• Upper pole tucked b/w 2 muscles
• Cannot extend sup.

22. Relations
Para thyroids lie in post.
Surface b/w 2 capsules
Capsule is thinner
posteriorly
Gland enlargement –
extends posteriorly &
inferiorly

23. Blood Supply

24. Blood Supply

26. Venous Drainage
Veins – wide lumen
No valves in lumen
Kocher’s vein - variable

27. Lymphatic Drainage

28. Microscopic Structure
 Stroma:
– Fibroelastic true
capsule -> septae ->
ill defined lobules ->
Pseudolobulated
– Septae: blood
vs, nerves lymphatics
– Intralobular loose CT

29. Parenchyma
 Follicles: arrangement of
cells in hollow spherical or
short cylindrical masses 0.2-
0.9 mm - Structural &
functional units
 Filled with gel like substance
- colloid- Thyroglobulin
 Simple Principal/Follicular
cells
 Parafollicular or ‘C’ cells

30. Resting Follicle Active Follicle

31. Principal/Follicular cells
Nuclei- Spherical, 1-2
nucleoli
Golgi, rER - prominent
Cytoplasm –
basophilic
Apical vacuoles
Microvilli

32.  Thyroglobulin - Stored follicle – iodine trapping and
iodination - reuptake (Scalloped margins) – lysosmes -
broken into T3 & T4 - secreted

33. (calcitonin)
– Lie beside follicle
– Enclosed in same BM
but not reaching lumen
– Larger, rounded & paler
– Nucleus round /oval,
eccentric
– Secretory granules –
Calcitonin (PTH
Antagonist)

34. Phylogeny
• Thyroid gland evolution -> adapt to the terrestrial
ecosystem with less supply of iodine.
• Jellyfish lack thyroid gland
• Endostyle of non-vertebrate chordates -> homologous
to thyroid (Endostyle: longitudinal ciliated groove on
ventral wall of the pharynx – produces mucus to gather
food particles)
• In lampreys, the larval endostyle transforms into adult
thyroid gland during metamorphosis
• Most primitive vertebrates - follicular thyroid gland but
non capsulated
• Thyroid is encapsulated in cartilaginous fish
• In the higher vertebrate forms, the thyroid is a one- or
two-lobed encapsulated structure.

35. Thyroid hormones
 Primary function of the thyroid -
production of T3, T4, and calcitonin
 T3 & T4 – essential for normal
growth, development & metabolism
 T4 -> T3 by peripheral organs like
liver, kidney, spleen
 T3 is 4 - 10 X more active than T4
 Hypothalamo – pitutary – thyroid
axis

36. Physiology
 Thyrocytes (follicular cells)
have four functions:
– collect and transport iodine
– they synthesize thyroglobulin
and secrete it into the colloid
– fix iodine to the thyroglobulin
to generate thyroid hormones
– remove the thyroid hormones
from thyroglobulin and secrete
them into the circulation.

37. Synthesis of thyroid hormones

38.  Thyroglobulin is synthesized in the rough endoplasmic reticulum and follows
the secretory pathway to enter the colloid in the lumen of the thyroid follicle
by exocytosis.
 Meanwhile, a sodium-iodide (Na/I) symporter pumps iodide (I-) actively into
the cell, which previously has crossed the endothelium by largely unknown
mechanisms.
 This iodide enters the follicular lumen from the cytoplasm by the
transporter pendrin, in a purportedly passive manner
 In the colloid, iodide (I-) is oxidized to iodine (I0) by an enzyme called thyroid
peroxidase.
 Iodine (I0) is very reactive and iodinates the thyroglobulin at tyrosyl residues
in its protein chain (in total containing approximately 120 tyrosyl residues).
 In conjugation, adjacent tyrosyl residues are paired together.
 The entire complex re-enters the follicular cell by endocytosis.
 Proteolysis by various proteases liberates thyroxine and triiodothyronine
molecules, which enters the blood by largely unknown mechanisms.

39. Calcitonin
• 32 - aa linear polypeptide - C cells
• Not under control of hypothalamus or pitutary
• Secretion -> Ca2+, gastrin and pentagastrin
• not essential for life – no replacement required
following thyroidectomy unlike parathyroids.
• antagonist to PTH - reduces Ca2+ level
• Inhibits: Ca2+ absorption by intestine, osteoclast
activity in bone & renal tubular cell reabsorption
of Ca2+
• Agonist to PTH -> Inhibits phosphate
reabsorption by the kidney
• Used clinically for Tt of hypercalcemia &
osteoporosis

40. Applied Anatomy
Congenital thyroid disorders
 Aberrant thyroid tissue
 Lingual thyroid
 Thyroglossal cyst
 50% close to or just
inferior to body of hyoid
bone
 Thyroglossal fistula –
secondary to rupture of
cyst

41. Hyperthyroidism Vs. thyrotoxicosis
 Graves’ disease—an autoimmune disease
involving autoantibody stimulation of TSH
receptors.
 Toxic multinodular goiter — nodular
enlargement of the thyroid in the elderly.
 Toxic nodule—autonomously functioning
thyroid nodule; most are adenomas
 Lymphocytic thyroiditis /Hashimoto’s
thyroiditis—inflammation causes release of
stored hormones (followed by hypothyroid
phase).
 Subacute (de Quervain’s) thyroiditis —
thyroiditis associated with a painful goiter.

42. Hypothyroidism
Myxedema Cretinism

43. Thyroid lumps
 Thyroid cysts.
 Nodule of multinodular
goiter.
 Follicular adenoma.
 Malignancy – 20%
• Papillary
• Follicular
• Medullary – C cells -> PNPS
• Malignant lymphoma
• Anaplastic

44. Applied anatomy
Thyroidectomy
 lobe, subtotal, total
 Transverse skin incision 2.5
cm above jugular notch
 Gap b/w ST & SH opened up
– trachea & isthmus exposed
 Muscles retracted laterally
or divided at upper ends –
preserve nerve supply from
ansa cervicalis

45.  Later lobes displayed
 Plane of cleavage: b/w 2 capsules
 Vessels ligated and divided – STA
right at the lower pole; ITA at
some distance from lower pole
 During removal of gland
 Ligament of berry released – RLN
injury
 Wedge shaped areas on post-medial
surface is left behind- PT

46. Complications
 ELN injury – CT paralysis,
hoarseness of voice,
temporary until the other
side takes over
 RLN injury – all intrinsic
muscles except CT
paralyzed, no recovery

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