Download Clinical Anatomy of Thyroid Gland

Clinical Anatomy and Embryology
of Thyroid Gland
Dr. A. Podcheko
2015
Intended Learning Outcomes
1.
2.
3.
4.
5.
6.
7.
Describe the anatomical locations of the thyroid gland of
adult human
Describe cells responsible for the hormone secretions of
the thyroid gland and functional outcomes of the
endocrine hormones from the thyroid gland
Describe the blood supply and innervation to the thyroid
gland
Describe the lymphatic drainage of the thyroid gland
Understand the embryological derivatives of the thyroid
gland
Medical imaging of thyroid
Clinical correlations
Why should I KNOW ?
• More than 12 percent of the population in US have
diseases of thyroid
• Thyroid cancer is the most common endocrine
cancer
• Most thyroid cancers respond to treatment, although
a small percentage can be very aggressive
• Clinically important aspects of thyroid anatomy are
commonly tested on USMLE step 1 and 2CK
Viscera of the Neck
Three layers of the cervical viscera
1. Endocrine layer: the thyroid and parathyroid glands
2. Respiratory layer: the larynx and trachea
3. Alimentary layer: the pharynx and esophagus
Thyroid gland as part of endocrine system
• The endocrine system consists of the endocrine
glands that release their secretions (hormones) into
the bloodstream to reach and act on target cells of
specific organs.
• The thyroid gland (largest endocrine gland) produces:
a. thyroid hormones (T3 - triiodothyronine (20%, highly
active) and T4 - thyroxine(80%, low activity), control
the rate of metabolism in almost all types of human
cells
b. Calcitonin, decreases calcium level in the blood by
direct inhibition of mediated bone resorption and by
enhancing calcium excretion by the kidney
Thyroid Hormones Actions
•
•
•
•
•
•
Increase basal metabolic rate
Normalize insulin levels
Improve insulin resistance
Increase glucose use for ATP production
Increase breakdown of fat into energy
Increase oxygen consumption and use of ATP
by cells
• Many others
Mechanism of Thyroid Hormone Action
Both T3 and T4:
1. Facilitate
normal growth
3. Facilitate
normal mental
development
(Brain
development!!!)
4. Increase the
local effects of
catecholamines
Any somatic cells (e.g. epithelium)
Histology of Thyroid Gland
•Thyroid follicles = follicular cells and large pools of
colloid
• Follicular cells produce
thyroid hormones and their
cell surfaces
possess thyroid-stimulating
hormone (TSH) receptors
• Thyroglobulin (aka
Colloid) is an inactive
storage for the T4 and T3
hormones
•Parafollicular cells (C cells) along the periphery of the thyroid
follicles secrete calcitonin
Thyroid Gland Location
•The thyroid gland lies deep to the sternothyroid and
sternohyoid muscles, located anteriorly in the neck at the
level of the C5 - T1 vertebrae
•Consists of right and left
lobes, anterolateral to the
larynx and trachea
•A relatively thin
isthmus unites the
lobes over the trachea,
usually anterior to the
second and third
tracheal rings, the
isthmus may be
incomplete
Thyroid Gland Location
•The thyroid gland lies deep to the sternothyroid and
sternohyoid muscles, located anteriorly in the neck at the
level of the C5 - T1 vertebrae
•Consists of right and left
lobes, anterolateral to the
larynx and trachea
•A relatively thin
isthmus unites the
lobes over the trachea,
usually anterior to the
second and third
tracheal rings, the
isthmus may be
incomplete
Sternohyoid m.
Sternothyroid m.
C5
C6
C7
T1
Thyroid Gland Location
•The thyroid gland lies deep to the sternothyroid and
sternohyoid muscles, located anteriorly in the neck at the
level of the C5 - T1 vertebrae
•Consists of right and left
lobes, anterolateral to the
larynx and trachea
•A relatively thin
isthmus unites the
lobes over the trachea,
usually anterior to the
second and third
tracheal rings, the
isthmus may be
incomplete
Pyramidal Lobe of the Thyroid Gland
•~50% of thyroid glands have a
pyramidal lobe
•Pyramidal lobe extends
superiorly from the isthmus of
the thyroid gland
•The fibrous tissue connecting
pyramidal lobe with hyoid
bone may contain levator
glandulae thyroidae muscle
(elevatior of thyroid gland)
•A band of connective tissue,
often containing accessory
thyroid tissue, may continue
from the apex of the pyramidal
lobe to the hyoid
Thyroid Imaging: Thyroid Scintigraphy
• Procedure producing one or more
planar images of the thyroid obtained within
15–30 min after intravenous injection of
Tc-99m pertechnetate or 3–24 hr
after the oral ingestion of radioactive
iodine (I-131)
•
Common Indications:
A. To relate the structure of the thyroid
gland to its function
B. To locate ectopic thyroid tissue
C. To evaluate a neck or substernal
mass
Gamma-camera or scanner
Thyroid Imaging: Thyroid Ultrasonography
• Should be performed with the
neck in hyperextension.
• Thyroid gland should be imaged
in the longitudinal and
transverse planes
• Common Indications:
A. Evaluation of the
presence, size, and
location of the thyroid
gland
B. Evaluation of the thyroid
gland for suspicious
nodules
C. Evaluation of the location
and characteristics of
palpable neck masses,
including an enlarged
thyroid
Thyroid Gland Capsule
•The thyroid gland is surrounded by a thin fibrous capsule,
which sends septa deeply into the gland
•Dense connective
tissue attaches the
capsule to the
cricoid cartilage and
superior tracheal
rings
•External to the
capsule is a loose
sheath formed by
the visceral portion
of the pretracheal
layer of deep
cervical fascia
Thyroid Gland Capsule
•The thyroid gland is surrounded by a thin fibrous capsule,
which sends septa deeply into the gland
•Dense connective
tissue attaches the
capsule to the
cricoid cartilage and
superior tracheal
rings
•External to the
capsule is a loose
sheath formed by
the visceral portion
of the pretracheal
layer of deep
cervical fascia
Platisma
Enlarging Thyroid is tend to grow downwards
and not upwards
•Upper pole of thyroid gland
can not expand above point of
attachment of sternothyroid
muscle
•The srernothyroid muscles
are attached onto the oblique
lines of the thyroid cartilage
•No limitation to the downward
expansion of the thyroid gland
•Pathologically enlarged
thyroid gland (goitre) extended
behind the sternum is termed
retrosternal goitre
Oblique line
of thyroid
cartilage
Sternothyroid M.
Retrosternal Goitre
Saggital CT section
Midsaggital CT section
Female 44y.o. Graves Disease
Arteries of the Thyroid Gland
•Thyroid gland is highly vascular
•Supplied by the superior and inferior
thyroid arteries
•Vessels lie between the fibrous
capsule and the loose fascial
sheath
•The first branch of the
external carotid artery, the
superior thyroid artery (STA),
descend to the superior poles
of the gland, pierce the
pretracheal layer of deep
cervical fascia, and divide into
anterior and posterior
branches supplying mainly the
anterosuperior aspect of the
gland
External
carotid
artery
Anatomical Relationships between Superior Thyroid
Artery and External Branch of Superior Laryngeal Nerve
cricothyroid muscle
Summary:
• Superior thyroid artery is
closely related to external
laryngeal nerve at its origin
• Nerve moves away from the
artery as artery approaches the
upper pole of the gland.
• In order to avoid injury of
external laryngeal nerve the
superior thyroid artery need to
be ligated during surgery just
near the superior pole of
thyroid gland
• A superior laryngeal nerve
injury will lead to changes in
the pitch of the voice and
causes an inability to make
explosive sounds due to
paralysis of the cricothyroid
muscle.
• A bilateral injury presents as a
tiring and hoarse voice.
Superior
Thyroid Artery
and Vein
External Branch
Superior Laryngeal
Nerve
Thyroid gland
- Do not ligate here
- Do ligate here
Arteries of the Thyroid Gland
•Inferior thyroid arteries
(ITA) are branches of the
thyrocervical trunks
arising from the
subclavian arteries, run
superomedially posterior
to the carotid sheaths to
reach the posterior aspect
of the gland
•Supply the posteroinferior aspect, including
the inferior poles of the
gland
•The right and left superior
and inferior thyroid
arteries form anastamoses
within the gland
STA
ITA
Left Subclavian artery
Thyrocervical trunk
Anatomical Relationships Between Recurrent
Laryngeal Nerve and Inferior Thyroid Artery
The terminal branches of inferior thyroid artery are
tightly related to recurrent laryngeal nerve
•Normally recurrent
laryngeal nerve passes
behind the ITA
•In some cases it passes
between branches of ITA
or in front of terminal
portion of inferior thyroid
artery
•It is easy to damage this
nerve during ligation of
inferior thyroid artery
•Always perform ligation
of inferior thyroid artery
as far away as possible
from thyroid gland!!!
Superior
Thyroid
Artery
Inferior
Thyroid
Artery
Recurrrent
Laryngeal
Nerve
Thyroid Ima Artery
•~10% of people have small unpaired thyroid ima artery
(branch of brachiocephalic trunk)
•Other possible sources of Ima artery: arch of the aorta, right
common carotid, subclavian, or internal thoracic arteries
•Ascends on the
anterior surface of the
trachea and continues
to the thyroid isthmus
•The presence of this
artery must be
considered before
tracheotomy (as a
potential source of
bleeding!)
Right common
carotid artery
Veins of the Thyroid Gland
•Three pairs of thyroid veins (superior, middle, inferior)
form thyroid plexus of veins on the anterior surface of the
thyroid gland and anterior to the trachea
•Superior thyroid veins
accompany the
superior thyroid
arteries
Middle thyroid veins do
not accompany but run
essentially parallel
courses with the
inferior thyroid arteries
Inferior thyroid veins
accompany the thyroid
ima artery (if artery is
present)
Thyroid Veins
•Superior and
middle thyroid
veins drain into
the internal
jugular veins
•Inferior thyroid
veins drain into
the brachiocephalic veins
posterior to the
manubrium of
sternum
Superior
Vena
Cava
Superior
Middle
Inferior
Thyroid
veins
Internal
jugular
veins
Brachiocephalic
veins
Inferior Thyroid Veins
Lymphatic Drainage of the Thyroid Gland
•Presence of metastases in lymphatic nodes of neck can be
first sign of thyroid carcinoma!!!
•Thyroid lymphatic vessels
communicates with
1st level: Prelaryngeal,
Pretracheal, and Paratracheal
lymph nodes
2nd level: Superior deep
cervical nodes (from the
prelaryngeal nodes) and
Inferior deep cervical nodes
(from the pretracheal and
paratracheal nodes).
•Some lymphatic vessels may
drain into the brachiocephalic
lymphatic nodes or the
thoracic duct
Brachiocephalic lymphatic nodes and thoracic duct
collect lymph form Thyroid
•Left Jugular V.
•Left Subclavian V.
•Left Brachiocephalic V.
Lymphatics of the Mediastinum
1, superior internal thoracic group;
2, superior phrenic or preaortic chain;
3, superior vagal or carotid subclavian
preaortic chain;
4, left brachiocephalic chain;
5, aortic arch chain.
6, inferior phrenic or prepericardial
chain;
Innervation of Thyroid Gland
•Derived from the superior, middle, and inferior cervical
sympathetic ganglia
•Nerves reach the
thyroid through:
• Cardiac periarterial
plexus
•Superior and inferior
thyroid plexus
•Only vasomotor fibers
causing constriction of
blood vessels
•Endocrine secretion from the thyroid gland is hormonally
regulated by the pituitary gland through TSH!
Thyroid Gland Development
• Thyroid glands arises as an endodermal diverticulum from the floor of
the pharynx
Week 3: Thickening in the floor between the first and second pharyngeal
pouches
Week 4: Endoderm evaginates ventrally into the mesoderm to form the thyroid
diverticulum
Week 5: a. Formation of thyroglossal duct b. Bifurcation on the tip of
Thyroglossal duct forms isthmus and the lateral lobes of the gland
Weeks 5-6: Growth of duct down to the neck, migration f thyroid gland down to
the neck
Week 7: Gland reaches the final position in relation to the larynx and the
trachea
Thyroid gland development: some important facts
•
•
•
•
•
As the thyroid glandular tissue migrates inferiorly, the duct portion begins to involute
Site of connection thyroglossal duct with pharynx makes a foramen caecum
The distal part of the thyroglossal duct may develop pyramidal lobe of the thyroid
gland
The thyroid gland may develop in any part at the midline of the thyroglossal duct and
results in lingual,suprahyoid, retrohyoid, or infrahyoid positions
The thyroid gland is relatively large in newborn babies
By the seventh week gland reaches
the final position
Thyroglossal Duct Cyst
• Thyroglossal duct normally
disappears but remnants of
epithelium may remain and form a
thyroglossal duct cyst
• The cyst is usually near or within the
body of the hyoid and forms a
swelling in the anterior part of the
neck always on the midline of the
neck!!!!
• Size 1 to 4 cm in diameter
• Lined by stratified squamous
epithelium, and cyst may harbor
lymphoid aggregates or remnants of
recognizable thyroid tissue –
important diagnostic sign!
A 16-year-old girl has noted a lump on her neck for the past
3 years. On physical examination there is a firm, non-tender,
2 cm discrete mass on her anterior neck at the level of the
hyoid bone. Which of the following is the most likely
diagnosis?
A: Thyroglossal duct cyst
Ectopic Thyroid Gland
•Definition: presence of thyroid tissue in locations other than the
normal anterior neck region between the second and fourth
tracheal cartilages
•Most frequent form of thyroid dysgenesis
•Up to 10% of adults may harbor ectopic tissue
Variants:
A. Lingual thyroid gland (90%)
B. Sublingual (suprahyoid, infrahyoid)
C. Others (trachea, submandibular,
lateral cervical regions, axilla, palatine
tonsils, carotid bifurcation, iris of the
eye, pituitary gland)
• Important to differentiate between an
ectopic thyroid gland and a thyroglossal duct cyst when excising a
cyst!!! Failure to do so may result in
total thyroidectomy!!!!
Accessory Thyroid Glandular Tissue
•Definition: additional thyroid
tissue in locations other than
the normal along with normally
positioned thyroid gland
•May appear anywhere along
the embryonic course of the
thyroglossal duct
•MC location of accessory
thyroid gland is on the
thyrohyoid muscle
•Caution: it is often of
insufficient size to maintain
normal function if the thyroid
gland is removed
Enlargement of the Thyroid Gland
•Definition: A non-neoplastic and
non-inflammatory enlargement of
the thyroid gland
•Most common causes of GOITRE:
A. Endemic Iodine deficiency
(N=150mcg/day), MC cause in the
world
B. Graves Disease (autoimmune)
+ Autoimmune Thyroiditis MC
causes in US
•Goitres can be associated with
increased or decreased production
of thyroid hormones
•May compress the trachea,
esophagus, and recurrent
laryngeal nerves
Goitre Classification
A. Toxic goitre Non-toxic goitre determined by levels of T3, T4, TSH and and clinical presentation
B. Diffuse goitre; Nodular goitre; Multinodular goitre determined by UltraSound examination and palpation of Thyroid
Multinodular goitre
Diffuse goitre
THYROID CARCINOMAS
• 1.5% of all cancers.
• A female predominance has been noted among patients who
develop thyroid carcinoma in the early and middle adult
years. In contrast, cases presenting in childhood and late
adult life are distributed equally among males and females.
• Most thyroid carcinomas (except medullary carcinomas) are
derived from the thyroid follicular epithelium
• There are 4 types of thyroid carcinomas:
• Papillary carcinoma (>85% of cases) - Most Common
• Follicular carcinoma (5% to 15% of cases) •
• Anaplastic (undifferentiated) carcinoma (<5% of cases)
• Medullary carcinoma (5% of cases)
Major type of treatment for thyroid carcinomas is
thyroidectomy!
Thyroidectomy
•Removal of the gland
•If possible, posterior part of each lobe of the thyroid
gland is usually preserved, a procedure called near-total
thyroidectomy, to protect the recurrent and superior
laryngeal nerves and to spare the parathyroid glands
•Technique: Collar incision
•Use of intraoperative RLN monitoring
•Complications:
•A. Postoperative hemorrhage after thyroid gland
surgery may compress
the trachea, making
breathing difficult
•B. Hypocalcemia–monitor
Ca level and clinical signs
and serum level of Ca2+!!!