Captain Rishi Pokhrel

THYROID GLAND

Introduction

Unique endocrine glandLocated 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

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”

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

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

Aetios 550 AD : goiter ->

aneurysm

Bronchocele,

elephantiasis of the

throat etc.

• Leonardo Da Vinci is

generally credited as

the first to draw the

thyroid gland as an

anatomical organ in

1508 AD

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

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”

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

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

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

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

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

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

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

Deep cervical fascia

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

Relations

• Upper pole tucked b/w 2 muscles

• Cannot extend sup.

Relations

Para thyroids lie in post.

Surface b/w 2 capsules

Capsule is thinner

posteriorly

Gland enlargement –

extends posteriorly &

inferiorly

Blood Supply

Blood Supply

Venous Drainage

Veins – wide lumen

No valves in lumen

Kocher’s vein - variable

Lymphatic Drainage

Microscopic Structure

Stroma:

– Fibroelastic true

capsule -> septae ->

ill defined lobules ->

Pseudolobulated

– Septae: blood vs,

nerves lymphatics

– Intralobular loose CT

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

Parenchyma

Resting Follicle Active Follicle

Principal/Follicular cells

Nuclei- Spherical, 1-2

nucleoli

Golgi, rER - prominent

Cytoplasm –

basophilic

Apical vacuoles

Microvilli

Thyroglobulin - Stored follicle – iodine trapping and

iodination - reuptake (Scalloped margins) – lysosmes -

broken into T3 & T4 - secreted

– Lie beside follicle

– Enclosed in same BM

but not reaching lumen

– Larger, rounded &

paler

– Nucleus round /oval,

eccentric

– Secretory granules –

Calcitonin (PTH

Antagonist)

Parafollicular or Clear cells or ‘C’ cells (calcitonin)

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.

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

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.

Synthesis of thyroid hormones

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.

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

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

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.

Hypothyroidism

Myxedema Cretinism

Thyroid lumps

Thyroid cysts.

Nodule of multinodular

goiter.

Follicular adenoma.

Malignancy – 20%

• Papillary

• Follicular

• Medullary – C cells -> PNPS

• Malignant lymphoma

• Anaplastic

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

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

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

What’s Your Message?POWERPOINT 2011

DISCUSSION

Nepal Army Institute of Health Sciences