Thyroid - Copy

8/7/2019 Thyroid - Copy

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Anatomy, physiology and

pathology of the thyroid gland

Dr Suchitra

I yr PG

Department of Prosthodontics

SRMC


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Anatomy


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Thyroid gland

Single, bilobed gland in the neck

Largest of all endocrine glands

Produces hormones thyroxine (T4) and tri-iodothyronine (T3) are

dependent on iodine and regulate basal metabolic rate

calcitonin which has a role in regulating blood calcium

levels


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Structure - macro

Brownish-red and soft during life

Usually weighs about 25-30g (larger in women)

Surrounded by a thin, fibrous capsule ofconnective tissue

External to this is a false capsule formed bypretracheal fascia

Right and left lobes United by a narrow isthmus, which extends across the

trachea anterior to second and third tracheal cartilages

In some people a third pyramidal lobe exists,

ascending from the isthmus towards hyoid bone


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Position and relations

Clasps anterior and lateral surface ofpharynx,larynx, oesophagus and trachea like a shield

Lies deep to sternothyroidand sternohyoidmuscle

Internal jugular vein and common carotidartery lie postero-lateral to thyroid


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Position and relations

Recurrent laryngeal nerve is an importantstructure lying between trachea and thyroid

may be injured during thyroid surgeryp ipsilateralVC paralysis, hoarse voice

Each lobe

pear-shaped and ~5cm long

extends inferiorly on each side of trachea (andoesophagus), often to level of 6th tracheal cartilage

Attached to arch of cricoid cartilage and tooblique line of thyroid cartilage

moves up and down with swallowing and oscillates

during speaking


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Anterior & Posterior view


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CTappearance


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Surface anatomy


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Muscular Landmarks

a. Sternocleidomastoid muscles lie laterally

b. Longus colli (prevertebral) muscles lie posteriorly

c. Strap muscles lie anteriorly


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Arterial supply

highly vascular

main supply from superior and inferior thyroid

arteries

lie between capsule and pretracheal fascia (false

capsule)


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Arterial supply

superior thyroid artery first branch of ICA

descends to superior pole of gland, pierces pretracheal fascia thendivides into 2-3 branches

inferior thyroid artery branch of thyro-cervical trunk

runs superomedially posterior to carotid sheath

reaches posterior aspect of gland divides into several branches which pierce pretracheal fascia to

supply inferior pole of thyroid gland

intimate relationship with recurrent laryngeal nerve


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Venous drainage

usually 3 pairs of veins drain venous plexus onanterior surface of thyroid

superior thyroid veins drain superior poles middle thyroid veins drain lateral parts

superior and middle thyroid veins empty into internal jugularveins

inferior thyroid veins drain inferior poles

empty into brachio-cephalic veins often unite to form a single vein that drains into one or other

brachio-cephalic vein


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Lymphatic drainage

communicate with a capsular network of lymph

vessels pass to prelaryngeal LNsp pretracheal andparatracheal LNs

lateral lymphatic vessels along superior thyroidveins pass to deep cervical LNs

some drainage directly into brachio-cephalicLNs or directly into thoracic duct


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Innervation

nerves derived from superior, middle and

inferior cervical sympathetic ganglia

reach thyroid through cardiac and laryngeal

branches of vagus nerve which accompany

arterial supply

postganglionic fibres and vasomotor indirect action on thyroid by regulating

blood vessels


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Structure - micro

Functional units are follicles responsible for synthesisand secretion of T3 and T4 Irregular spheroidal structures consisting of a single layer of

cuboidal epithelium + basement membrane Variable in size.

Occasional scattered clear cells/parafollicular cells/Ccells produce and secrete calcitonin

Gland enveloped by outer capsule of loose supportingconnective tissue and an inner fibro-elastic capsule

Fine collagenous septa extend into the gland dividing intolobules and conveying blood supply, lymphatics andnerves

Colloidis the secretory product of follicular cells Extra-cellular proteinaceous substance composed of thyroid

hormones linked together with protein (thyroglobulin)


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Structure - micro


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Embryology and development

first endocrine gland to appear in embryonic development

begins to develop ~24 days after fertilisation from median endodermalthickening of pharynx forms a downgrowth called the thyroid diverticulum

descends into the neck as the embryo and tongue grows passes anteriorly to

developing hyoid and laryngeal cartilages for a time is connected to tongue by thyroglossal duct

thyroid diverticulum is initially hollow but becomes solid and divides into Land R lobes connected by isthmus

assumes definitive shape and final location by 7 weeks gestation, and thethyroglossal duct disappears

initially consists of solid mass of endodermal cells, which are broken up intonetwork of epithelial cords by invasion of surrounding vascular mesenchyme lumen forms colloid forms by 11th week and thyroid follicles are formed,and synthesis of hormones commences


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Function


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Thyroid hormones function

Likely that all cells express thyroid hormone receptors

Metabolism

Increases basal metabolic rate

Increases carbohydrate and lipid metabolism

Normal growth

Normal development Especially CNS

Other systems CVS increases heart rate, cardiac output

CNS mental acuity

Reproduction fertility requires normal thyroid function


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Calcitonin function

Minor role in regulating (reducing) blood

calcium concentration

Suppresses osteoclastic bone resorption

Inhibits renal tubular reabsorption of calcium

and phosphorus


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Effects ofTSHon thyroid gland

Increased thyroglobulin proteolysisp increasedcirculating thyroid hormones

Increased activity of iodide pump - increasescellular iodine uptake

Increased iodination of tyrosine and coupling

Increased size and secretory activity of thyroid

cells Increased number of thyroid cells, plus change

from cuboidal to columnar epithelial structure


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Thyroid pathology

Normal thyroid function - euthyroidism

Disease states may result in hyper- orhypo-

thyroidism - relative excess or deficiency ofthyroid hormones

Any swelling of the thyroid may be termed agoitre

Toxic goitre: associated with increased thyroidhormone output

Non-toxic goitre: normal hormone levels

(Non-specific terms; dont relate to a particularpathology)


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Thyroid pathology

Thyroid enlargement may be diffuse or nodular

Irregular multinodular enlargement (goitre) of theentire gland is common, especially in the elderly

Focal nodular enlargement may be due to a tumour

Symmetrical slightly nodular (bosselated) firmenlargement of the whole gland is characteristic ofHashimotos disease

Symmetrical diffuse enlargement is usually associatedwith hyperthyroidism (eg. Graves disease)

Most thyroid enlargement (except Hashimotos)results from hyperplasia of thyroid follicles and

their cells


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Multinodular goitre

Common in the elderly

Often undetected

May present for cosmetic reasons (neck swelling)

or compression symptoms (eg. trachea)

Usually have normal thyroid function

Cause uncertain ? Uneven response of thyroid tissue to fluctuating

TSH levels over many years


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Graves disease

Most important cause of hyperthyroidism

Autoimmune thyroiditis

Diffuse thyroid enlargement and exophthalmos

Follicular cells stimulated by IgG antibody

(LATS) that causes constant thyroid hormone

production, independent of TSH

Large, fleshy thyroid gland with large follicles

lined by active cells


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Hashimotos disease

Destructive autoimmune thyroiditis

Common in middle age, women > men

Most common auto-antibodies are anti-

microsomal Ab and anti-thyroglobulin Ab

Diffusely enlarged thyroid, symmetrical

and firm


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Hypothyroidism

Cause Hormone concentrations Goitre

Primary failure ofthyroid gland q

T3 and T4, o TSH Yes

Secondary to

hypothalamic or

pituitary failure

qT3 and T4,q TSH and/orq TRH No

Dietary iodine

deficiencyqT3 and T4, o TSH Yes


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Thyroid malignancies

Follicular cell origin

Papillary carcinoma - 70%

Follicular carcinoma - 25%

Anaplastic carcinoma - rare

Parafollicular C cell origin

Medullary carcinoma - 5%


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Papillary carcinoma


Well-differentiated

Arises mostly in young adults Often multifocal

Metastasises via lymphatics to neck nodes

Slow-growing

Excellent prognosis

Treatment

Surgery - lobectomy/thyroidectomy

Iodine-131

Radiothera


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Follicular carcinoma


Most common in middle age

Metastasises via blood stream Characteristically spreads to bone, lung

Good prognosis

Treatment

Surgery

Iodine-131

Radiotherapy


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Anaplastic carcinoma


Occurs exclusively in the elderly

Poorly differentiated Rapidly progressive with direct invasion of

adjacent structures

Very poor prognosis

Treatment - poor response Surgery?

RT?

(Iodine-131?)


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Medullary carcinoma

Arises in parafollicular C cells

Small cells containing neuro-endocrine granules

Occurs in middle-aged and elderly

Slow-growing

Metastasises to lymph nodes

Secretes calcitonin (blood test)

Treatment Surgery

RT (but relatively radio-resistant)

Low uptake of iodine-131 - limited role


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Diagnostic tools

History and examination

Thyroid function tests

T3, T4, TSH Tumour markers

Thyroglobulin

Anti-TG antibodies

Iodine-123 or 131 scan

Ultrasound

Biopsy


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Summary - thyroid

Major endocrine gland

Located in the neck

Closely related to parathyroid glands, thyroidcartilage, trachea, important nerves (recurrentlaryngeal) and vessels

Important role in metabolic regulation via thyroid

hormones T3 and T4 Stored extracellularly in inactive form


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Summary

Hypo- and hyperthyroidism are common

conditions Benign and malignant pathology

Graves disease

Hashimotos disease

Papillary/follicular/anaplastic carcinoma Medullary carcinoma


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Thyroid - Copy