THYROID GLAND

THYROID GLAND

 

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ANATOMY OF TYHROID GLAND

 

– Thyroid gland is an endocrine gland present in lower part of front and sides of neck.

– It regulates basal metabolic rate and has a important role in the calcium metabolism.

– It is roughly H shaped and It has 2 lobes and a isthmus.

 

* Situation and extent

 

– Gland lies against the C5, C6, C7 and T1 vertebrae.

– Each love extends from middle lobe of thyroid cartilage to fourth or fifth tracheal rings and Isthmus extends from second to third tracheal rings .

 

* Dimensions and weight

 

– Each lobe is measures about 2 inches ( length ) , one inch ( breadth ) and one inch ( thickness).

– The isthmus is 1/2 inch  × 1/2 inch . And gland weighs about an average about 25g.

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* Capsules

 

– It has a true capsule and a false capsule.

 

* Blood supply

 

– It has superior and inferior thyroid arteries.

• Superior thyroid artery : It is a branch of external carotid artery and supplies upper 1/3 rd of lobe and upper half of isthmus.

• Inferior thyroid artery : It is a branch of thyroid cervical trunk from subclavian artery.
It supplies lower 2/3 rd of lobe and lower half of isthmus.

 

* Venous drainage

 

– It has 3 veins

(1) Superior thyroid vein

(2) Middle thyroid vein

(3) Inferior thyroid vein.

 

* Structure and function

 

It has 2 types of secretory cells : follicular and parafollicular cells

– The follicular cells secretes tri-iodo thyronine and tetra-iodo-thyronine.

– This stimulates basal metabolic rate and somatic and psyclic growth of individual.

– The parafollicular cells secretes thyrocalcitonine .

 

• Applied anatomy

 

– Goiter : any enlargement in thyroid gland

– Hyperparathyroidism

– Hypothyroidism.

• THYROID HORMONES

– There are 3 types of thyroid hormones

(1) Tetra-iodo-thyramin ( T4) / thyroxin

(2) Tri-iodo-thyramin ( T3)

(3) Calcitonin.

PHYSIOLOGY OF THYROID HORMONE

 

Synthesis of T3 and T4

 

– These are produced by  thryoid follicles.

– Thyroid follicles are made up of aceni which has a single layer of epithelial cells in centre.

– And it is filled with protein called as a colloid.

 

• Steps of thyroid hormone synthesis

 

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1) Iodine trapping

 

– Iodide is present in the blood which is consumed in diet as iodine.

– When it passes through the thyroid gland it is actively taken up and is converted into iodide in acinar cells of thyroid gland.

2) Oxidation

 

– Iodine inside the cell is gets oxidised again back to iodine.

3) Organification

 

Iodine binds with thyrosine and attached to thyroglobulin

– When the 2 molecules of iodine combined with thyrosine it forms diiodothyrosine.

– When only one molecules of iodine is combines with thyrosine it forms monoiodothyrosine.

 

4) Coupling

 

– After the formation of diiodothyrosine and monoiodothyrosine , the two molecules of diiodothyrosine is combined to form T4 and one molecule of monoiodothyrosine and one molecule of diiodothyrosine combines to form T3.

 

5) Release of T3 and T4

 

– After the formation of T3 and T4 in colloid hormones along with thyroglobulin is taken into acinar cells.

– Lysozymes in the cell releases T3 and T4 from thyroglobulin and then T3 and T4 is released into blood.

– Thyroglobulin is recycled for further hormonal production.

 

• Functions or actions of thyroid hormone

 

1) Calorigenic action

 

– Thyroid hormone increases activity of sodium – potassium pump and there by stimulates BMR.

2) Effects on metabolism

 

Carbohydrate metabolism

– Protein metabolism

– Lipid metabolism

– Vitamin metabolism

 

3) Effects on blood

 

– Stimulates erythropoiesis hence increase RBC and increase blood volume.

 

4) On Cardiovascular system

 

– Increase heart rate

– Increase force of contraction

– Increase cardiac output

– Increase SBP

– Decrease DBP

 

5) Effects of respiratory system

 

– Increase respiratory rate

– increase force  of respiration

– Increase demand of oxygen

 

6) On GIT

 

– Increase Appetite and hunger

– Increase GIT secretion

– Increase GIT motility

 

7) On CNS

 

– Thyroid hormone is essential for fetal growth.

– Lack of thyroid hormone during intra uterine life produces mental retardation.

– Thyroid hormone increases blood flow the brain  and maintains normal Function of brain .

 

8) Effects on skeletal muscles

 

– thyroid hormones are protein catabolic hormones

– They Decrease size and bulk of skeletal muscle

 

9) Effect on sleep

 

– Decreases sleep

 

10) Effect on Reproductive system

 

– Increases metabolic rate in reproductive system

– In females excess thyroid hormones produces oligomenoria

– In males , excess thyroid hormones produces impotence,

 

11) Effect on other endocrine glands

 

– Stimulates activity of endocrine gland.

 

12) On growth

 

– Stimulates bone growth and skeletal growth before puberty .

 

PATHOPHYSIOLOGY

 

• HYPERTHYROIDISM

 

– Increased secretion of thyroid hormone is called as Hyperthyroidism.

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* Causes 

 

1) Grave’s disease : primary

2) Thyroid adenoma : secondary.

• Grave’s disease

– It is an autoimmune disease.

 

* Pathology

 

√ B – lymphocytes produces autoimmune antibodies ( thyroid stimulating autoantibodies )

√ They start acting like TSH

√ Hypersecretion of thyroid hormone

√ Thyrotoxicosis

 

• Signs and symptoms

 

1) Eye

 

– Exoptalmus proptosis ( protrusion of eye balls due to edematous swelling of retro-orbital tissues leading to optic nerve damage )

– Patient is unable to close eye

– Can lead to dryness of cornea

– Infection , ulceration

 

2) CVS

 

– Tachchycardia

– Hypertension

– Increased BP

– Arrythmia

– Cardiac failure

 

3) GIT

 

Hypermobility of intestine which causes vomiting and diarrhea

 

4) Muscles

 

– Increase muscular weakness

– Edema over skin bones

– Whitish discolouration of skin

– Hair loss

 

5) CNS

 

– Nervousness

– Insomnia

– Mild tremor

– Anxiety

– Heat intolerance

6) Oligomenorrhea / Amenorrhea in females

 

* Investigation

 

1) Thyroid function test

– T3 , T4 is increased

– TSH is decreased

2) Detection of autoantibodies against thyroglobulin

3) Radioactive iodine uptake is increased

4) Fine needle aspiration

– Hyperplasia of thyroid cells

 

* Treatment

 

1) Anti-thyroid drugs

 

– Helps to suppress production of thyroid hormone

– Carbimazole

– Methimazole

2) Beta – blockers

– To relieve Tachycardia and heart rate

3) Radioactive iodine

– Destruction of functional thyroid cells

– Nitrates

– Thiocyanates

4) Surgery

– Thyroidectomy

* Secondary hyperthyroidism

 

– Thyroid adenoma

– Sometimes  , a localized tumor develops in the functional thyroid cells ( thyroid adenoma ) which secretes large quantity of thyroid hormone.

– Depresses the production of TSH.

 

•HYPOTHYROIDISM

 

Decreased secretion of thyroid hormone is called as hypothyroidism.

– Primary hypothyroidism : Decreased function of thyroid gland.

– Secondary hypothyroidism : Decreased secretion of thyroid hormone.

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• MYXEDEMA

 

– Hypothyroidism in adults.

– Life threatening complication of hypothyroidism.

– Patient goes in state of coma.

• Pathophysiology

– Deposition of myxedematous tissues of various organ.

– Responsible for organ dysfunction

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* Causes

 

1) Disease of thyroid gland

2) Iodine deficiency.

3) Deficiency of TSH

4) Auto immune

5) GI bleeding

6) Trauma or physical stress

7) Use of sedative diuretics.

 

*Signs and symptoms of MYXEDEMA

 

1) Edematous appearence throughout the body

2) Moon face

– Swelling of face

– Bagginess under eyes

– puffiness of face

– Non-pitting type of edema

3) Atherosclerosis

4) Arteriosclerosis

* General features

 

1) Tierdness

2) Weight gain

3) Frog like husky voice

5) Skin – Dry skin
Thick and rough

6) CVS – Bradycardia
Hypotension
Cadiac failure

7) CNS – Depression
Cold intolerance

8) Muscle – Stiffness

9) GIT – Constipation

10) Hematological

– Iron deficiency anemia.

– Pernicious anemia

– Decrease RBC synthesis

11) Reproductive – Menorrhagia
Infertility
Impotence.

 

* Investigation

 

√ Thyroid function test

– T3 and T4 decreases and TSH increases.

√ Radioactive iodine uptake decrease

√ Fine needle aspiration

– decreased thyroid cells.

 

* Management

 

– Replacement therapy

– Thyroxine therapy

– Supplementation of thyroid hormone.

• CRETINISM

– Hypothyroidism in children .

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* Causes

 

Congenital absence of thyroid gland

– Genetic disorder

– lack of iodine in diet.

 

* Features of Cretinism

 

1) At the time of birth , a newborn baby with hypothyroidism may appear normal because Thyroxine has been supplied from mother.

-But after weeks baby starts developing sluggish movements and croaking sound while crying .

– If not treated early , the baby will be mentally retarded permanently

2) Skeletal growth is affected more than the soft tissues.

– Bloated body.

– Tongue becomes big and there is a dripping of saliva.

– Obstruction in swallowing and breathing.

 

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