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Term Paper on Thyroid Gland
Term Paper Contents:
- Term Paper on Introduction to the Thyroid Gland
- Term Paper on Parts of the Thyroid Gland
- Term Paper on Capsule of the Thyroid Gland
- Term Paper on Blood Supply of the Thyroid Gland
- Term Paper on Nerve Supply of the Thyroid Gland
- Term Paper on the Parathyroid Glands
- Term Paper on the Thymus Glands
Term Paper # 1. Introduction to the Thyroid Gland:
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This is a large endocrine gland situated in front of the lower part of the neck and overlies 2 – 4th tracheal cartilages. Its functions are controlled by the thyroid- stimulating hormone from the anterior (adeno) hypophysis.
Development:
It develops from endodermal epithelial proliferation of median thyroid diverticulum.
The “C” cells of the gland are developed from the fourth branchial pouch.
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Situation:
Lower part of front of neck. The gland is situated in front of C5, C6, C7 and T1 vertebrae.
Term Paper # 2. Parts of the Thyroid Gland:
The gland has a pair of lobes united by an isthmus in the median plane.
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Size:
Each lateral lobe has approximate dimensions as follows:
Length- 5 to 6 cm
Width- 3 cm
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Thickness- 2.5 cm.
Isthmus:
Vertical length is 1.25 cm
Transversely – 1.25 cm.
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Weight:
About 25 to 30 gms.
Lateral Lobe of Thyroid:
(a) Apex or upper pole is pointed and extends superiorly up to the oblique line of thyroid cartilage.
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(b) Lower pole is blunt and rounded, extends inferiorly up to 6th tracheal ring. Lower pole on left side is related to thoracic duct.
Borders:
(a) Anterior border is sharp and related to anterior branch of superior thyroid artery.
(b) Posterior border is blunt and rounded and is related to posterior branch of superior thyroid artery and ascending branch of inferior thyroid artery which anastomose with each other.
It is also related to parathyroid glands.
Surfaces are antero lateral or superficial surface, postero lateral surface and medial surface.
Relations of Superficial Surface:
This is related to:
1. Skin.
2. Superficial fascia containing platysma and cutaneous nerves and blood vessels.
3. Investing layer of deep cervical fascia.
4. Sternohyoid muscle.
5. Sternothyroid muscle.
6. Omohyoid muscle.
7. Sternocleido mastoid muscle.
Superiorly the lobe is limited by “insertion of sternothyroid muscle”.
Medial Relations of Thyroid Lobe:
1. Two Viscera:
(a) Larynx and trachea
(b) Pharynx and oesophagus.
2. Two Muscles:
(a) Cricothyroid muscle
(b) Inferior constrictor of pharynx.
3. Two Nerve:
(a) External laryngeal nerve (X)
(b) Recurrent laryngeal nerve (X).
4. Two Cartilages:
(a) Thyroid cartilage
(b) Cricoid cartilage.
Postero Lateral Relations of Thyroid Lobe:
1. Carotid sheath and its contents:
i. Common carotid artery
ii. Internal jugular vein
iii. Vagus nerve (X).
2. Cervical sympathetic trunk with its middle ganglion.
Isthmus:
It crosses 2nd, 3rd and 4th tracheal rings.
Upper border shows pyramidal lobe and related to anastomosis between superior thyroid arteries.
Lower border is related to inferior thyroid vein.
Levator glandulae thyroidea connects pyramidal lobe to hyoid bone.
Term Paper # 3. Capsule of the Thyroid Gland:
It has two capsules:
1. False Capsule:
It is formed by pretracheal fascia and is weakest posteriorly.
Pretracheal fascia thickens posteriorly and is attached to cricoid cartilage. This thickened band is called suspensory ligament of Berry.
2. True Capsule:
It is formed by condensation of fascia around the gland. A plexus of veins lies deep to the true capsule. Therefore during thyroidectomy the gland is removed with its capsule to prevent bleeding.
Term Paper # 4. Blood Supply of the Thyroid Gland:
Thyroid gland is very vascular and supplied by:
1. Superior Thyroid Artery:
It is the first branch of external carotid artery. It descends downwards and forwards, reaches the upper pole of the gland – it divides into anterior and posterior branches to supply the gland near the apex. It is closely related to external laryngeal nerve. Hence during thyroid surgery artery is ligated close to the gland to save the nerve.
It supplies upper 1/3 of lateral lobe and upper 1/2 of isthmus of gland.
Anterior branch anastomoses with the branch of opposite side along the upper border of isthmus of gland.
2. Inferior Thyroid Artery:
It is a branch of thyrocervical trunk of subclavian artery. It passes deep to carotid sheath and middle cervical sympathetic ganglion, in front of vertebral vessels and reaches the posterior surface of the gland. It is accompanied by recurrent laryngeal nerve near the gland.
It divides into 4 or 5 branches and supplies lower 2/3 of the lateral lobe and lower half of isthmus of the gland. One branch ascends upwards along posterior border and anastomoses with the posterior branch of superior thyroid artery at the junction of upper 1/3 with lower 2/3 of the posterior border.
During surgery on thyroid this artery is ligated away from the gland to save recurrent – laryngeal nerve.
3. Thyroidea Ima Artery:
It is present only in 3% of individuals and is a branch from arch of aorta or brachiocephalic trunk. It runs upwards in the midline of neck to the isthmus of the gland.
4. Accessory Arteries:
Oesophageal and tracheal arteries also supply thyroid gland.
Venous Drainage:
Veins do not follow arteries.
1. Superior Thyroid Vein:
Drains upper part of thyroid lobe, crosses anterior to common carotid and drains into internal jugular vein.
2. Middle Thyroid Vein:
Also crosses anterior to common carotid artery to end into internal jugular vein. It drains the middle part of the thyroid lobe.
3. Inferior Thyroid Vein:
A pair of veins emerging from the lower border of isthmus, crosses anterior to trachea and terminates into either brachiocephalic veins or left brachiocephalic vein.
4. Vein of Kocker:
Sometimes a fourth thyroid vein emerges between middle and inferior thyroid veins and drains into internal jugular vein.
Lymphatic drainage into:
1. Pre-laryngeal lymph nodes
2. Pre-tracheal lymph nodes
3. Deep cervical lymph nodes
4. Recurrent chain of lymph nodes along recurrent laryngeal nerve (para tracheal group).
Term Paper # 5. Nerve Supply of the Thyroid Gland:
Sympathetic Supply:
Vasomotor in function.
Post ganglionic fibres from the superior, middle and inferior cervical sympathetic ganglia. These fibres form plexus around the thyroid arteries.
Parasympathetic Supply:
Vagus and recurrent laryngeal nerves.
The secretary functions of the gland is controlled by the T.S.H. of the anterior pituitary.
Accessory Thyroid (Ectopic Thyroid Tissue):
1. Lingual thyroid- Presence of thyroid tissue around the foramen caecum of the tongue.
2. Suprahyoid ectopic thyroid
3. Infrahyoid actopic thyroid
4. Mediastinal thyroid.
Ectopic thyroid tissue is found along the course taken by the thyroglossal duct.
Secretions of the Gland:
It secretes:
1. Thyroxin (T4) or Tetraiodothyronine.
2. Tri-iodo thyronine (T3).
Required for normal growth and development of body and maintain B.M.R. of body. (Both psychic and somatic growth.)
3. Thyrocalcitonin – secreted by the para follicular cells (“C” cells) – helps in deposition of calcium on bones – calcium metabolism.
Term Paper # 6. Parathyroid Gland:
Usually 2 to 6 in number.
Essential for life.
Two pairs – superior and inferior parathyroid glands.
It is a small endocrine gland.
Lies on the posterior border of the thyroid gland within the false capsule.
Development:
Superior parathyroid develops from endoderm of 4th pharyngeal pouch and is also called parathyroid IV.
Inferior parathyroid develops from endoderm of IIIrd pharyngeal pouch and is called parathyroid III.
Secretion:
It secretes hormones called para-thormone, which controls the calcium and phosphorus metabolism along with thyrocalcitonin.
It causes resorption of calcium from bones and makes them weak and thus liable to fracture.
Shape – Oval or lentiform (half pea shaped)
Size – Approximate size
Length – 6 mm
Width – 3-4 mm
Thickness – 2 mm
Weight – About 50 mgm
Position
Types of Parathyroid Gland:
Superior Parathyroid:
It is more constant in position and lies at the middle of the posterior border, here it is slightly prominent. This prominence is called as tubercle of Zuckerkandl. It is dorsal to recurrent laryngeal nerve.
Inferior Parathyroid:
More variable in position.
It may lie:
1. Within the thyroid capsule below the inferior thyroid artery and near the lower pole of thyroid lobe.
2. Behind and outside the thyroid capsule immediately above the inferior thyroid artery.
3. Within the substance of the thyroid lobe and ventral to recurrent laryngeal nerve.
Blood Supply:
Receives rich blood supply from inferior thyroid artery and from anastomosis between superior and inferior thyroid arteries.
Veins and lymphatic’s of the gland are associated with those of thyroid gland, i.e., inferior thyroid and middle thyroid vein.
Pre and paratracheal group of lymph nodes.
Nerve Supply:
Vasomotor nerves are derived from middle and superior cervical sympathetic ganglion.
Parathyroid Activity:
It is controlled by blood calcium levels – low levels – stimulate and high level inhibits the activity of the gland.
Applied Anatomy:
1. Tumours of para thyroid glands – lead to excessive secretion of parathormone – cause increased removal of calcium from bones making them weak and liable to fracture.
Hypercalcaemia and increased urinary excretion of calcium salts lead to formation of stones in urinary tract.
2. Hypoparathyroidism – may occur spontaneously or from accidental removal of glands during – thyroidectomy – results in hypocalcaemia leading to increased neuromuscular irritability causing muscular spasm and convulsions (Tetany).
Term Paper # 7. Thymus Gland:
It is an important lymphatic structure and essential for immunity of the body.
This is well developed at birth, continues to grow upto puberty, thereafter atrophy and replaced by fat.
Development:
It develops from the endoderm of the third pharyngeal pouch.
Situation:
Superior mediastinum and upper part of anterior mediastinum, extending above into the lower part of the neck.
Colour:
Pinkish grey.
Weight:
At the time of birth – 10 to 13 gms.
Greatest development during first two years of life.
At puberty- 35 to 40 gms.
Adult – It regresses after puberty and gets atrophied in the adult. It weighs only less than 10 gms.
Parts:
A pair of lobes united by connective tissue.
Cervical part of the thymus is usually rudimentary.
Upper end of each lobe extends upto the thyroid gland and lower end extends upto the 4th costal cartilage. It lies anterior to great vessels, pericardium and trachea.
Blood Supply:
Anterial Supply:
1. Internal thoracic artery.
2. Inferior thyroid artery.
Venous Drainage:
1. Left brachiocephalic vein.
2. Inferior thyroid vein.
3. Internal thoracic vein.
Lymphatic Drainage of Thymus:
1. Brachiocephalic lymph nodes.
2. Tracheo bronchial lymph nodes.
3. Parasternal group of lymph nodes.
Nerve Supply:
Sympathetic- Cervicothoracic ganglion.
Para sympathetic- Vagus.
Phrenic nerve- Supplies the capsule of the gland.
Functions:
1. Lymphoid organ – controls lymphopoiesis and maintains an effective part of circulating lymphocytes.
2. Organ of immunogenesis – immunologically competent lymphocytes is produced.
3. Abnormality of thymic functions may produce – autoimmune disorders like – myasthenia gravis, haemolytic anaemia, hashimotos thyroiditis.
4. Thymosin is a hormone secreted by gland – acts on lymph nodes to produce lymphocytes.
5. It produces T-lymphocytes.
6. It controls development of peripheral lymphoid tissues of the body during neonatal period. By puberty – main lymphoid tissues are fully developed.
Applied Anatomy:
1. Tumours of the gland – thymoma.
2. Thymic hyperplasia – causes myasthenia gravis.