thyroid gland- anatomy and physiology

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Thyroid Gland

Amador, Sheena G.Banaay, Anacleto Clent L.

Murga, Brenda Jim

Vicente Sotto Memorial Medical Center

Department of ORL NHS



Embryology

Begins its development at the foramen

cecum at the base of the tongue

Descends through the tissues of the neckand comes to rest overlying the trachea

Leaves a tract behind it± could be the

source of pathology in later life

Descent complete at 7th gestational week.



Embryology

If descent is completely arrested- lingualthyroid results.

If the inferiormost portion of thethyroglossal duct tract is maintained-pyramidal lobe is formed.

If the remnant of the thyroid tissue is left

along the thyroglossal duct tract- it cancystify and enlarge, forming athyroglossal duct cyst.



Thyroglossal

duct CystLingual

Thyroid

Pyramidal

Lobe



Anatomy

Composed of 2 lateral lobes

connected by an isthmus

Rests on the level of 2nd to

4th tracheal cartilages Each lobe measures

approximately 4cm high, 1.5

cm wide, 2cm deep. Ventral surface of the gland

is covered by the infrahyoid

strap muscle musculature.



Anatomy

Capsules:

True capsule

(thickening of the

connecting tissue)

False capsule

(pretracheal fascia)



AnatomyImportant relations:

Anterior:

Strap muscles

strenohyoid muscle sternothyroid

muscle

In surgery, retractingthese muscles willexpose the thyroidgland



Anatomy

Important relations:

Posterior:

trachea ± enlargement of thyroid

gland can compress

trachea and cause

difficulty in breathing

esophagus



Anatomy

Important relations:

Posterolateral:

Carotid artery

Internal jugular vein

Embedded inside the

gland:

2 pairs of parathyroidgland



Anatomy

Nerves:

Recurrent laryngeal nerve

Runs in the tracheoesophageal groove

Passes in between the branches of the

inferior thyroid artery

± During surgery, inferior thyroid artery will be

clamped. RLN can be accidentally damagedas well.

1 side damage: Husky voice

Both side damage: No voice & Stridor



AnatomyVenous Drainage:

Superior thyroid vein

± drains into internal jugular vein

Middle thyroid vein

± drains into internal jugular vein

Inferior thyroid vein

± drains into brachiocephalic vein



Anatomy

Lymphatic drainage

Pretracheal lymph node Paratracheal lymph node

Deep cervical lymph node



Microscopic Arrangement ± Arranged as follicles

± lined by cuboidal

epithelium

± cells secrete thyroxine

binds to globulin &stored as

thyroglobulin (colloid)

± in between follicles

parafollicular cells(secrete calcitonin)

medullary carcinoma

arise from here



Physiology

Thyroid Gland

produces 2

hormones: T3 andT4

Follicular lumen:

used for the storageof hormones and

their precursors.



Physiology

Iodide Transport

- The thyroid normally concentrates iodide

20-fold to 40-fold over the extracellular

space and against an electrical gradient of

approx. 40 mV.

- Sodium/iodide symporter- protein

which is the key to the trapping action of thyroid



Physiology

Iodide Transport

- Pendrin: an apical membrane protein

which aids in releasing iodide into the

follicular lumen

- Pendred¶s syndrome: associated with

mild hypothyroidism, goiter, and hearing

loss

: mutations in the gene and

coding protein of pendrin.



Physiology

Thyroglobulin

- a tissue-specific protein that serves as a

matrix for the synthesis of hormone and

as a vehicle for its storage.

- about 10% of its weight is carbohydrate,

and about 0.1% and 1% is iodine



Physiology

Iodination and Thyroperoxidase

- newly formed thyroglobulin and iodide meet atthe apical surface, where hormone synthesisoccurs.

- process includes:1. Oxidation of iodide.

2. Subsequent transfer to thyrosyl residueson thyroglobulin, producing MIT and DIT.

3. Coupling of 2 iodotyrosine molecules,either 1 each of MIT and DIT to form T3 or 2 of DIT to form T4.



Physiology

Storage and Release of Hormone

- Hormone release is initiated by retrieval

of thyroglobulin from follicular lumen.

- When released from thyroglobulin, the

thyroid hormones and their precursors

enter the cytosol; there MIT and DIT are

deiodinated by an iodotyrosine-deiodinase, and the released

iodotyrosine-enters the iodine pool.



Physiology

Circulating Thyroid Hormones

- < 1% of circulating thyroid hormones exist as

free iodo-amino acids.

- The remainder are bound in reversible,noncovalent linkage to 1 of several plasma

proteins.

- In humans, the most important is TBG,

accounting for approximately70% of circulating

hormones.

- Very high affinity to T4 and low affinity to T3.



Physiology

Metabolism of Thyroid Hormones

- Type 2 deiodinase is present primarily in

CNS, the pituitary, the placenta, and the

skin, and has more recently been found in

th thyroid.

- Major Role: major production of T3.

- May also contribute to circulating T3



Physiology

Control of Thyroid Function-Circulating levels of TSH

are controlled by the

opposing influences of

thyroid hormones, and TRHfrom the hypothalamus.

-As levels of thyroid

hormone increase in

response to TSHstimulation, T4 and T3

block the TRH-stimulated

release of TSH in the

thyrotrope.



Physiology

Control of Thyroid Function

Plasma T4 TSH

Plasma T4 TSH



Physiology

Antithyroid Agents

- can inhibit thyroid hormone synthesis

and metabolism.

Lithium- blocks thyroglobulin

endocytosis.

Iodinated radiocontrast agents-

potent inhibitors of thyroidhormone deiodination



Thank you!

thyroid gland- anatomy and physiology

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