Surgical anatomy of thyroid and parathyroid glands. hazem el folldocx

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Surgical Anatomy of Thyroid Gland

Embryological Development of Thyroid Gland

It is the first endocrine gland developing in the fetus.

The Gland derived from two distinct embryological structures, the endoderm of primitive pharynx, and the neural crest.

In the 4 th .Week I.U.Life ; median endodermal diverticulum arises in the floor of the primitive pharynx; at the point later indicated by a small depression; (the foramen caecum at the base of the tongue; at the junction of the posterior third with the anterior two thirds of the tongue.)

Endoderm cells in the floor of the pharyngeal anlage proliferate to form the median thyroid anlage.

The median thyroid anlage descends; along a line extending from the foramen caecum at the back of the tongue; in front of the primitive pharynx then; in-front of the developing hyoid bone; (and then loops behind it); and finally in-front of the larynx; into the neck; where it bifurcates; as bi-lobed solid organ; and developing into the isthmus and the majority of each lateral lobe. During its descent the anlage remains connected to foramen caecum via an epithelial-lined tube known as the thyroglossal duct.

The Endodermal Cells of the median thyroid anlage develop into the thyroid follicular cells.

In the 5 th .Week ; Paired lateral anlages originate from the Fourth branchial pouch ;( from the ultimobranchial bodies of the fourth branchial pouch); descend and fuse with the median anlage; develop into para- follicular calcitonin-producing ;( C) – cells. Ultimobranchial bodies have neuroectodermal origin as a result of the migration of cells from the neural crest. C-cells; come to lie in the posterio- superior region of the thyroid lobe and become dispersed among the thyroid follicles. . So; C-cells are neuro-ectodermal in origin.

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Normal Descend of Thyroid Gland

The lateral thyroid anlages; contribute less than 1% of the eventual thyroid mass.

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In the 6 th .Week; the foramen caecum ruptured and resorbs; (and it is not grossly visible.); and the Thyro-

glossal Duct becomes obliterated; leaving a regressive fibrous tract; (Thyro-glossal Duct tract); including the portion associated with the central hyoid bone).); which disappeared by the 8 th .Week.

In the 7 th . Week ; the Thyroid descends into its normal Cervical position.

In the 8 th. Week; the Thyro-glossal Duct tract disappeared.

The original proximal attachment of Thyro-glossal Duct in the pharynx; persists at foramen caecum; (at the junction of anterior 2/3 and posterior 1/3 of the tongue).

The very distal end of Thyro-glossal Duct; associated with the isthmus; may be retained and mature as a pyramidal lobe projecting from upper border of isthmus in the adult thyroid; (present in 30-50% of individules; usually from Lt side of the ismuth); the pyramidal lobe becomes connected to hyoid bone by fibro-muscular band; Levator Glandulae Thyroidae.

The caudal part of thyroglossal duct at the level of thyroid cartilage; deviates to one or the other side of the midline-usually Left side. So:-

Pyramidal Lobe often projects from Left side of isthmus.

Low Thyroglossal Duct Cyst- pre-laryngeal; pre-tracheal and supra-sternal (; (due to persistence of remananents of the thyroglossal duct); commonly presents as anterior neck mass within 2 cm off the midline commonly to Left side.

In the 8 th .Week; thyroid follicles develop; and in the 11 th .Week; colloid formation starts.

By the end of 3 rd .Month; thyroid follicular cells start to trap I2; and secrete thyroid hormones.

The para-follicular-C cells are the only component of the adult gland that is not of endodermal origin; but arise from neural crest; (neuro-ectodermal.)

C-Cells belong to a group of neural-crest derivatives known as APUD (amine precursor uptake and decarboxylation) cells.

 APUD system; comprise other Neuro-endocrine cells present in GIT; Pancreas; tracheal wall; and Adrenal Glands; tumors originate from these cells are collectively called "Apudomas."

Developmental Anatomical Anomalies of Thyroid Gland

(I)-- Thyroglossal Duct Cyst and Sinus:-

Thyroglossal duct cysts are the most commonly encountered congenital cervical anomalies; (60% of all Congenital Neck Masses.)

Thyroglossal duct cysts occur due persistence of the thyroglossal duct; in whole or in part; there are 2 Theories for the Etiology of Thyroglossal Duct Cyst:-

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(a)-- Cystic degeneration; recurrent throat infections could possibly stimulate the epithelial remnants of the tract causing it to undergo cystic degeneration.

(b)-- Retention Cyst; retained secretions block the thyroglossal duct remnant; (which is closed at both

ends); causing the cyst to develop; and continued secretions from mucinous and serous glands cause enlargement of the cyst.

Position; Thyroglossal duct cysts may occur anywhere along the descending path of the thyroid gland; (anywhere along the thyroglossal duct tract); from the foramen cecum at the base of the tongue to the level of the suprasternal notch; although; the most common site ;( 85%); are found in juxtaposition to the hyoid bone- (in the midline just above the thyroid cartilage).

The Thyroglossal duct Track; may get trapped in the substance of the body of hyoid bone, resulting in part of the tract running inside the body of the hyoid bone;/or becomes retro-hyoid .

No natural internal opening of thyroglossal duct has been demonstrated at the level of foramen cecum.

Commonest sites of Thyroglossal duct Cyst are Sub-Hyoid/Supra-Hyoid ;( in Floor of the mouth.)

Pathology :-

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Cyst contains thick jelly- like fluid which may contain cholesterol crystals.

Histologically, thyroglossal duct cysts are lined by secreting columnar or squamous epithelium,

Surrounded by a rim of lymphoid tissue; and Ectopic thyroid tissue present in 20-40% of cases. Rarely this could be the only functioning thyroid gland tissue.

Complications:

1. Recurrent infection.

2. Thyroglossal Duct Sinus; due to cyst infection with spontaneous or surgical drainage.

3. Malignancy (<1%): Usually (85%) papillary thyroid carcinoma; arises from the epithelial lining of the cyst which usually contains some ectopic thyroid cells.

4. In rare Lingual Thyro-glossal cyst causes swallowing and respiratory difficulties in the neonate. 

Thyroglossal duct sinuses result from infection of the cyst and secondary to spontaneous or surgical drainage of the cyst. The Sinuses are accompanied by minor inflammation of the surrounding skin.

Diagnosis :-

Most common congenital neck swelling; and 2nd most common benign neck mass after lymphadenopathy.

Age : -It can present at any age; but 90% of the cases present before age of 10 years /OR. It may remain asymptomatic until infected; (it can presents in Adult age.)

Presentation :-It presented as; 2- 4-cm midline neck mass ;( or within 2 cm off the midline commonly to

Left side; especially the Low Cyst- pre-laryngeal; pre-tracheal and supra-sternal); that moves upward and down with swallowing and protrusion of the tongue; (because of close anatomical relation to hyoid bone).

The Cyst is smooth, well-defined; firm; mobile horizontally but not vertically.

Examination of the Tongue; to look for any associated Lingual Thyroid; (due to failure of normal descend of thyroid anlage.)

Initial presentation; may be swollen, red, hot, and very tender neck mass; (Infected Cyst).

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Cervical Thyroglossal Cyst-Juxtahyoid Position

Lingual Thyro-glossal Cyst.

CT-Scan:-

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Thyroglossal duct sinuses result from infection of the cyst and secondary to spontaneous or surgical drainage of the cyst. The Sinuses are accompanied by minor inflammation of the surrounding skin.

Differential diagnoses : -- Other causes of midline neck swelling:-

1. Infected lymph node (Purulent secretion.)

2. Dermoid cyst (Cheesy secretion.)

3. Lipoma (Slip sign positive.)

4. Sebaceous cyst (Doughy feel.)

5. Cystic hygroma.

6. Hypertrophic pyramidal lobe of thyroid.

7. Thyroid nodule-(in the ismuth).

8. Branchial cyst. If thyroglossal cyst is low cervical and shifted off mid-line.

Treatment of Thyroglossal duct cysts :-

(I)—Pre-operative Diagnosis; aiming at to confirm the diagnosis; to define normal thyroid anatomy; and to detect if the cyst contains ectopic thyroid tissue, which may be the only functioning thyroid and excision would result in profound hypothyroidism.

1. Thyroid function test - (TFT).

2. Ultrasonography; confirm diagnosis; detect normal thyroid anatomy; and may also detect papillary carcinoma as a solid component or calcification.

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3. Radioisotope scan (123 I scan); to detect ectopic thyroid tissue; and to confirm the presence of normal thyroid.

4. Radiographic contrast examination ; may be used to confirm thyroglossal duct sinus.

(II)—Operative:-

“Sistrunk operation”, which consists of en bloc cystectomy; (Excision of the cyst and the core of tissue

encompassing duct tract from the cyst through the mylohyoid muscle up to the former foramen caecum at the base of the posterior tongue; and excision of the central hyoid bone to minimize recurrence.

The thyroglossal duct can descend either anterior or posterior; (loop behind) or even course through the hyoid bone. So; excision of body of hyoid helps in excision of any retro-hyoid; or intra-hyoid part of the tract.

In case of papillary carcinoma; found in the thyroglossal duct cyst; Total Thyroidectomy may be indicated however its indication is controversial.

US; and/or CT-Scan; will help in decision making to detect size of the tumor; extent; (invasion); and presence of L.Ns metastases:-

Excision of the cyst and the thyroglossal duct may be appropriate in small, limited carcinomas less than 1 cm.

Total Thyroidectomy is advised; in patients with large tumors, ≥ 1 cm; particularly if there are additional thyroid nodules and evidence of cyst wall invasion or lymph node metastases.

(II)— Anomalies of descent along the thyroid line:-

Thyroid gland may be found anywhere along the track from the foramen cecum to the normal Cervical site as a result of interruption of its normal descent. It may be-Lingual; Sub-lingual; Pre-laryngeal.

 Hyperdescent into the thorax; along thyro-thymic track may result in Intra-thoracic thyroid; (primarily retrosternal thyroid) 

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Lingual Thyroid:-

Lingual thyroid represents a complete failure of the median thyroid anlage to descend normally; along the

thyroid line; but develops in situ at the dorsum of the tongue; and in most of these cases, this may be

the only thyroid tissue present.

Lingual thyroid lies beneath the epithelium of the dorsum of the tongue, at the site of the foramen cecum.

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Complications may occur: - Enlargement of Lingual thyroid;

Obstructive symptoms; as choking, dysphagia, airway obstruction.

Hemorrhage.

Management :-

(1)-- Evaluation of normal thyroid tissue in the neck; US-Scan; and Radio-isotopic Scan- 123 I-Scan / Tc-Scan.

(2)—Medical:--

Exogenous thyroid hormone to suppress thyroid-stimulating hormone (TSH) and shrink its size; Or radioactive iodine (RAI) ablation followed by hormone replacement.

(3)—Surgical Excision; in presence of complications.

During Excision; careful dissection is required as the gland is well vascularized by the lingual arteries.

(III)—Accessory ;( Ectopic) Thyroid:-

(1)-- Normal thyroid tissue may be found; outside the pathway of normal thyroid descent:-

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Small ectopic thyroid tissue may be located under the lower poles of a normal thyroid gland; or in the thyro-thymic tract.

Anywhere in the central; (visceral) neck compartment, (including intra- esophageal, intra-tracheal).

Anterior mediastinum; (adjacent to the aortic arch; within the upper pericardium; or in the interventricular septum.)

(2)-- Thyroid tissue situated laterals to the carotid sheath and jugular vein, (previously termed Lateral aberrant thyroid,); almost always represents metastatic thyroid cancer in cervical lymph nodes ;(not remnants of the lateral anlage that had failed to fuse with the main thyroid). It may be the initial presentation of PTC; and the ipsilateral thyroid lobe contains a microscopic focus of papillary thyroid cancer (PTC), that may not be apparent with clinical examination and US-Scan.

However; very rare cases; in which ectopic thyroid tissue islands may be found as a nodule located lateral to the jugular vein; pulled away and attached by connective tissue to the mother gland.

Always consider the possibility of metastatic thyroid cancer of lateral aberrant thyroid nodules.

(3)--Struma Ovarii ;( ovarian thyroid); is Not a true congenital anomaly; but thyroid tissue may be present in Dermoid cysts and Teratoma of the ovary.

Struma ovarii may exist in approximately 1% of all ovarian tumors. Of these, 5-6% are bilateral and about 5% possess functioning thyroid tissue and causing Hyperthyroidism. Malignancy is a possible occurrence in up to 5% of all Struma ovarii; (metastatic Papillary Carcinoma.)

Surgical Anatomy of Thyroid Gland

The thyroid gland lies in the central compartment of the neck; bordered by the contents of the carotid sheath on each side.

The thyroid gland consists of two symmetrical; lateral lobes; clasping the upper part of trachea; and extends upwards for some distance on either sides of the larynx; and united by an isthmus of gland tissue just below cricoid cartilage; and overlying the upper tracheal rings; (second and third tracheal cartilages.)

Thyroid Lobe; is pear-shaped; and extends from the oblique line of thyroid cartilage-(above); to the 5th.-6th.Tracheal rings-(below).

The thyroid lobe encircles approximately 75% of the diameter of the upper trachea; and fills the space between the trachea and esophagus medially; overlapping the carotid sheath laterally.

Thyroid Lobe; measures approximately 5 cm in cranio-caudal; 2-3 cm in transverse; and 1-2 cm anteroposterior dimensions.

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Weight; normally developed adult thyroid weighs 15 - 20 g.

Thyroid Lobe has; Upper Pole; Narrow.

Lower Pole; Broader.

Anterio-Lateral ;( Superficial) Surface. Medial Surface. Posterior Surface.

Relations of Thyroid Gland:-

Upper Pole; rests on thyroid cartilage; and is tucked away beneath the upper end of sternothyroid muscle; which limits the upward extension of an enlarging lobe.

Lower Pole; at the level of 5th.-6th.Tracheal rings.

Anterio-Lateral Surface ;( Superficial):-It is covered by:-

Skin; Sub-cutaneous Fat containing the Platysma muscle. (Platysma lies deep to the superficial fat.)

Beneath Platysma; between it and the deep cervical fascia; there is a relatively avascular plane; in this plane; the superior and inferior flaps can be raised with minimal blood loss; during thyroidectomy.

Deep Cervical Fascia ;( The General Investing Layer.)

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The anterior border of the lower part of sternocleidomastoid overlaps these strap muscles.

Sterno-thyroid Muscle; closely applied on the anterio-lateral surface; (at deep plan); and Sterno-hyoid muscle at more superficial plan.

The Anterior layer of pre-tracheal fascia; encloses the Strap Muscles. These muscles are innervated by branches from the ansa cervicalis; enter the muscles through their inferio-lateral aspect.

The Upper Pole of thyroid lobe; lies beneath the upper end of sternothyroid muscle; (which is attached to oblique line of thyroid cartilage); so this will limit the upward extension of an enlarging lobe.

Medial Surface :-It moulded over the lateral side of Larynx-Above; and the anterior and lateral surfaces of upper Trachea-Below; with Lower Pole; extending along the side of the trachea as low as the fifth- sixth tracheal ring.

The Lower Pharynx and Upper Esophagus; lie immediately behind; i.e. at more posterior plan.

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The Medial surface of Upper 1/3 of thyroid lobe; is related to Two cartilages of Larynx; the Thyroid and Cricoid cartilages; with Two muscle (related to these cartilages); intervening; Crico-thyroid and Inferior Constrictor of Pharynx; more posteriorly.

Crico-thyroid Muscle; passes obliquely from side of Cricoid; and inserted into inferior horn of Thyroid Cartilage.

Inferior Constrictor of Pharynx attached to oblique line of Thyroid Cartilage and side of Cricoid Cartilage.

The Recurrent Laryngeal Nerve ascends; in-front of the groove between trachea and Esophagus as it approaches the medial surface of lower pole. The External Laryngeal Nerve descends immediately behind Superior thyroid Artery; as it approaches the medial surface of the upper pole; to supply the Crico-thyroid muscle.

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Posterior Surface :-

It overlaps the medial part of the carotid sheath; (Common Carotid Artery; in carotid sheath); lies over Longus Coli muscle. (If the Thyroid Lobe is enlarged; it may extend across the more laterally placed internal jugular vein.)

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Cervical sympathetic chain lies embedded in the loose areolar tissue behind the carotid sheath and anterior to prevertebral fascia.

Superior and Inferior Parathyroid Glands; usually lie in contact to posterior surface; between the true capsule of the thyroid and the Surgical Capsule ;( pre-tracheal fascial sheath).

Superior Parathyroids; normally lie between the true capsule of the thyroid and the pre-tracheal fascial capsule.

Inferior Parathyroids; may lie between the true capsule and the pre-tracheal fascial capsules, or within the thyroid parenchyma, (Intra-thyroidal); or lying on the outer surface of the pre-tracheal facial sheath. 

Inferior Thyroid A.; at the level of C6 (the carotid tubercle); it arches medially; behind the carotid sheath; (and in-front of Vertebral A.); to reach the posterior surface of thyroid lobe; at its midpoint.

Isthmus; it is bridge of thyroid tissue; approximately 1.25 cm in both transverse and vertical dimensions; connecting the 2-thyroid lobes.

Upper border typically crosses at or just below the level of the cricoid cartilage.

Ismuth lies over second; and third tracheal rings; connecting the two lateral lobes; towards their lower poles.

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At the upper border of the isthmus; there are anastomotic vessels between Rt. And Lt. Superior Thyroid Vs.

At the lower border of the isthmus; some veins emerge to Drain into the inferior thyroid veins.

The posterior surface of the isthmus is firmly adherent to the second and third rings of the trachea, (here; the pretracheal fascia; which investing the whole gland; is fixed between them.)

As; the Pre-tracheal fascia; investing the whole gland; forming a facial capsule to it; it is fixed on the posterior surface of isthmus; to tracheal rings; and it is blended upwards with the larynx; so the gland moving up and down with the larynx during swallowing.

Pyramidal Lobe:-It is finger of thyroid tissue projects from upper part of isthmus; and connected to the body of hyoid bone by fibro-muscular band; (levator glandulae thyroidae); which is the remnant of the distal part of thyro-glossal duct.

Pyramidal Lobe represents a development of glandular tissue from the caudal end of the thyroglossal duct.

Pyramidal Lobe; is usually present on the Left side; and it is Not always present; (present in 30-50% of individules.)-

The caudal part of thyroglossal duct at the level of thyroid cartilage; deviates to one or the other side of the midline-usually Left side; so Pyramidal Lobe often projects from Left side of isthmus.

Coverings of the Thyroid Gland; Outside; the Pre-tracheal facial Capsule; Inside; Fibrous Capsule. In-between there is a space containing plexus of anastomosing vessels.

Pre-tracheal facial Capsule ;( peri-thyroid sheath or surgical capsule); pre-tracheal fascia encloses the Visceral compartment of the Neck; Thyroid Gland; Trachea; and Esophagus-(and RLN.)

Upwards; it is blended with the Larynx; (so; thyroid gland moves up and down with swallowing.)

The Pre-tracheal facial Capsule; is denser and thicker on anterio-lateral surface of the gland; than the posterior surface; (so; in enlargement of the thyroid lobe; it tends to be pushed posteriorly around the sides of trachea and esophagus.)

The Pre-tracheal Thyroid facial Capsule is Not adherent to the gland; except between the isthmus and the 2nd. And 3rd. Tracheal rings.

Fibrous Capsule ;( True Capsule): it is thin fibrous connective tissue capsule; that is densely adherent layer to thyroid gland.

In Thyroidectomy; as the thyroid is mobilized away from surrounding structures; the plane is defined between the pretracheal facial capsule and the true thyroid capsule.

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In Goiters; as the upper pole of thyroid lobe is tucked beneath the upper end of sternothyroid muscle; which is inserted into thyroid cartilage; and the posterior layer of pre-tracheal facial capsule is thinner ; the enlarged thyroid lobe; may extend inferiorly into the mediastinum; and posteriorly; around the sides of trachea and esophagus.

The suspensory ligament; (ligament of Berry); it thickened portions of the visceral layer of pretracheal fascia from the upper medial surface of thyroid lobe; that is firmly attached to the thyroid and cricoid cartilages; so anchors the gland to the larynx.

Ligament of Berry is vascular; as it contains a small branch from inferior thyroid artery.

Ligament of Berry is closly related to RLN; which is most vulnerable to iatrogenic injury; in dividing of this ligament.

The anchor of the thyroid gland- the ligament of Berry.

Blood supply :-

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The Arteries supplying Thyroid Gland:-

The thyroid gland has the greatest blood supply per gram of tissue; (competes with the adrenal glands in this aspect.)

(1)-- Superior thyroid artery; from ECA.

(2)—Inferior thyroid artery; from Thyro-cervical Trunk; from first part of Subclavian A.

(3)—Thyroidea ima A.; small branch arises from Innominate A.; / right common carotid artery /or from arch of aorta ;( Not always present ;( only in 3% of people; and represents a persistent embryonic vessel that usually disappears.); ascends in front of trachea to reach the thyroid isthmus.

ITA; is larger than the STA; but is less constant, being absent or duplicated unilateral or bilateral in 10 % of individuals.

If the ITA is absent; on one side, the thyroidea ima Artery becomes larger; as it may be the principal source of blood supply to the lobe.

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(1)--Superior thyroid artery:-First Branch of ECA.

It arises from the anterior aspect of the external carotid; ;( just above CCA bifurcation); and descends downwards over the inferior pharyngeal constrictor muscle; deep to infra-hyoid muscles; accompanies the External laryngeal Nerve; (the Artery passes parallel to the Nerve.)

Superior Thyroid artery gives off:-

Branches:-

(a)-Superior laryngeal artery; it accompanies the internal laryngeal nerve; piercing the Thyro-hyoid membrane to reach inside the larynx.

(b)-Sternomastoid Branch; passes downwards to supply sternomastoid Muscle.

(c)-Infrahyoid Branch; passes over thyrohyoid muscle below hyoid bone to supply Infrahyoid muscles.

(d)—Crico-thyroid artery; to Crico-thyroid muscle.

(The Superior thyroid artery; accompanies the External laryngeal Nerve; while its branch; the Superior laryngeal artery accompanies the internal laryngeal nerve.)

It pierces the pretracheal fascia as a single vessel to reach the apex of the upper pole.

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As the vessel approaches the upper pole;(often 1 cm or more from the apex of upper pole); The External laryngeal Nerve crosses immediately behind the artery; passing medially to reach Crico-thyroid space.

On the surface of the gland; (upper pole); the Artery divides into an Anterior branch that runs down to the isthmus; (where the RT. And LT.As; anastomose along the upper border of isthmus) and a Posterior branch that runs down the posterior aspect of the lobe and anastomoses with an ascending branch of the inferior thyroid artery from the lower pole.

The Posterior branch; gives a small parathyroid artery passes to the superior parathyroid gland.

(When; the pyramidal lobe; presents; separate branch from Lt.Superior Thyroid A.; supplies it.)

(2)-- Inferior thyroid artery:-

It arises from Thyro-cervical Trunk; from first part of Subclavian A.; in the Root of the neck.

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Course; (inverted U-shape course;):- it ascends upwards and medially; along the medial border of Scalene Anterior muscle and behind the carotid sheath. Until the carotid tubercle ;( anterior tubercle of transverse process of C6.); where it arches medially; behind the carotid sheath; (and in-front of Vertebral A.); to reach the posterior surface of thyroid lobe at its midpoint.

It divides outside the pretracheal fascial capsule into four or five branches that pierce the fascia separately to supply the thyroid and parathyroid glands.

Parathyroid Branches : - small branches to the superior and inferior parathyroid glands.

The parathyroid small arterial branch crosses parallel and slightly anterior to the gland; and supplies the gland from the medial side.

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Two significant branches of the inferior thyroid artery (ITA) are routinely present; one crosses anterior and the other posterior to RLN. The posterior arterial branch passes through the Berry’s Ligament.

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Ascending branch from Inferior Thyroid Artery; ascends upwards over the posterior aspect of the thyroid lobe; supplying the posterior part of the lobe and small branch to superior Para-thyroid Glands; and anastomoses with the posterior branch of Superior Thyroid Artery.

Other Branches from Inferior thyroid artery :- Before inferior thyroid artery arches medially deep to carotid sheath at level of carotid tubercle;

arises the Ascending Cervical Artery. Before its terminal distribution into the thyroid gland; Internal Laryngeal Artery; Tracheal and

Esophageal branches.

(a)—Ascending Cervical Artery; it ascends in-front of transverse processes of cervical vertebrae; supplying pre-vertebral muscles; and gives off spinal branches enter the intervertebral foraminae; supplying the contents of spinal canal.

(b)—Internal Laryngeal Artery; It accompanies the recurrent laryngeal nerve; and both enter the larynx deep to inferior constrictor muscle of the pharynx; just posterior to Crico-thyroid muscle.

(c)—Tracheal and Esophageal branches.

The RT. And LT. Superior and Inferior Thyroid Arteries; anastomose along the surfaces of thyroid lobe and the isthmus; to ensure adequate blood supply; and providing Collateral Circulation between External Carotid and Subclavian Arteries.

Venous Drainage :-

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Veins of the thyroid gland form a plexus of vessels lying in the substance and on the surface of the gland. The plexus is drained by three pairs of veins, the superior, middle, and inferior thyroid veins.

(1)-- The superior thyroid vein from upper pole of thyroid lobe; drains into either the internal jugular or facial vein ;( equal proportion)

(2)—The middle thyroid vein; short and wide, (is usually present); passes from the middle of the lobe, crossing in front of CCA; and drain directly into the IJV.

(3)-- The inferior thyroid veins; form a venous plexus that lies with- in the pretracheal fascia of the lower pole and the isthmus, (in front of the trachea); drain into 2-3 inferior thyroid veins; passing in-front of trachea; drain into the brachiocephalic veins in superior mediastinum.

Pre-Laryngeal-(Delphian) and Pre-Tracheal L.Ns:-present along the inferior thyroid veins; and these are common sites of metastases in cancer thyroid.

Lymph drainage :-

Intra-glandular plexus of lymphatic vessels communicate between the 2- lobes through the isthmus.

The lymphatics follow the arteries:-

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From the upper pole; lymphatics follow superior thyroid A.; to anterio--superior group of deep cervical lymph nodes-(mid- jugular LNs). Some vessels may drain into one or more pre-laryngeal ("Delphian") lymph nodes just above the isthmus.

From the lower pole; lymphatics follow inferior thyroid A. ;( Back to its origin from the subclavian behind the carotid sheath); to pretracheal; trachea-esophageal; brachiocephalic nodes (superior mediastinal); and posterio-inferior group of deep cervical lymph nodes-(mid-and low jugular L.Ns).

Some lymphatics pass downwards from the isthmus; along thyroidae ima artery; to pretracheal nodes present along inferior thyroid veins.

Compartment and Levels of Cervical Lymph Nodes

Lymphatices from the Neck can pass into the Central and Lateral compartments of Cervical L.Ns.; the boundary between them is marked by the carotid sheath.

Classification scheme for regional lymph node basins in the neck. (Levels of Cervical Lymph Nodes)

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Central compartment L.Ns ;( level VI.)- bordered superiorly by the hyoid bone, inferiorly by the suprasternal notch, and laterally on each side by the carotid sheaths.

Central compartment L.Ns include; midline pre-laryngeal; pretracheal, para-tracheal, peri-thyroidal, RLN-(tracheo-oesphageal L.Ns), retropharyngeal; and Superior Mediastinal L.Ns; (Level VII)

Pre-Laryngeal and Pre-Tracheal L.Ns :-present along the inferior thyroid veins; both infra-and Supra-isthmic-(Pre-Laryngeal-Delphian L.Ns).

The tracheoesophageal lymph nodes ; lie adjacent and mostly anterior to the RLN.

These L.Ns are common sites of metastasis in papillary and medullary thyroid carcinoma.

Lateral compartment L.Ns; include:-

The (level I node)-Supra-hyoid compartment; (above the hyoid bone); bordered by anterior and posterior bellies of digastric muscle; and hyoid bone below.

It includes the submental and submandibular nodes.

The upper jugular (level II), mid-jugular (level III), and lower jugular (level IV) lymph nodes; along the jugular veins on each side; bordered laterally by the posterior border of the sternocleidomastoid muscle.

Level-IIExtending from base of skull to hyoid bone.

Level-III Extending from hyoid bone to cricoid cartilage.

Level-IV Extending from cricoid cartilage to clavicle.

Posterior triangle lymph nodes (level V.); bordered by posterior border of sternocleidomastoid muscle; anteriorly; trapezius muscle posteriorly; and clavicle inferiorly.

Regional lymph node metastases; from Thyroid cancers tend to involve level VI ;( Central) lymph node before involving levels II, III, IV, (Jugular) and ultimately level V. (Posterior triangle lymph nodes.)

There may be “skip” metastases directly to the lateral compartment with no (Central) level VI involvement; as in carcinoma of the upper 1/3 of thyroid lobe.

Level I (submental; submandibular L.Ns) and level VII (superior mediastinal L.Ns) involvement is less common.

Cancer Upper 1/3 of Thyroid Lobe; primarily involve pre-laryngeal ("Delphian") lymph nodes just above the isthmus; and may involve the mid-jugular nodes; both anterior and lateral to IJV.

Cancer in the Ismuth; can involve; pre-laryngeal ("Delphian") nodes above; and Pretracheal nodes present along inferior thyroid veins in-front of trachea below.

Cancers of Mid-and. Lower 1/3 of thyroid lobes involve initially the Pre-Tracheal and Tracheoesophageal nodes; then the mid- and lower jugular nodes and anterior mediastinal nodes.

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Nerve supply :-

Sympathetic fibres; from middle cervical ganglion; along inferior thyroid A. (Mainly); some Sympathetic fibres; from superior cervical ganglion; along superior thyroid artery.

Sympathetic fibres are vasoconstrictors.

Vagal Fibres; may pass to thyroid gland; but unknown function.

Structure of Thyroid Gland :- Capsule : Thin fibrous connective tissue capsule adherent to the parenchyma.

From the capsule, several septa extend within the thyroid parenchyma, which is subdivided into several lobules.

The Functional Units of the thyroid are Spherical thyroid follicles; - lined by single layer of epithelial (follicular) cells; and containing central store of colloid, (the iodine-containing thyroglobulin.) The colloid is secreted from the epithelial follicular cells under the influence of the pituitary hormone TSH; and containing the pre-formed Thyroid Hormones.

The amount of colloid in the follicular lumen depends on the level of thyroid activity. It becomes depleted in states of high or excess thyroid activity and accumulates during periods of thyroid inactivity.

Thyroid follicles are separated by thin connective stroma which is rich in both lymphatic and blood vessels.

The thyroid is thus unique in being the only endocrine gland to store its secretion outside the cells; in the colloid.

The colloid is iodinated when inside the follicle, and then reabsorbed by the cells. Inside the follicular cells; the thyroid hormones are released by lysosomal enzymes; before being discharged into blood capillaries.

The C- (para-follicular) cells ; represent less than 2% of the epithelial cells; and are scattered on the outer aspects of the follicles; in the inter-follicular stroma; (present as individual cells or clumped in small groups); and do not reach the Lumina of the follicles.

Para-follicular-C cells are concentrated mainly in the posterio-superior 1/3 of thyroid lobe. They secrete calcitonin; the calcium-lowering hormone.

The Nerves Related to the Thyroid Gland; and their significance during Thyroidectomy:--

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(1)--External Laryngeal Nerve; lies immediately behind the Superior thyroid artery at the upper pole of thyroid lobe; so in thyroidectomies the artery; (and the vein) should be ligated individually and right at the upper pole, not some distance from it, to avoid damage to the nerve.

Superior laryngeal nerve; it is branch from vagus Nerve in the Neck; it slopes downwards on the side wall of the pharynx deep to the internal carotid artery. At about the level of the hyoid bone ;( but often much higher); it divides into a large internal laryngeal nerve-(sensory to the mucosal lining of the supraglottic larynx) which pierces the thyrohyoid membrane; to reach the piriform recess; and a small external laryngeal nerve-(motor to the cricothyroid muscle, an important tensor of the vocal cords.)

External Laryngeal Nerve:-

External Laryngeal Nerve; together with the superior thyroid vein and artery, run downwards; over the inferior pharyngeal constrictor muscle; and then deep to the sternothyroid muscle.

At upper pole of thyroid lobe the Nerve lies immediately behind the Artery; (crossing behind the artery)

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Usually; 1 cm or more above superior thyroid pole; it crosses behind the superior-thyroid vessels passing medial to the upper pole into Crico-thyroid space; and down outside the larynx to supply cricothyroid muscle and possibly the cricopharyngeus part of the inferior constrictor.

In 20% of patients; the nerve crosses 1cm above superior pole; or below the apex of the superior thyroid pole; and in this place the nerve at a greater risk of injury.

The nerve crosses the artery ≥1 cm above the Summit of the superior pole of the thyroid lobe. (Usually.)

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The nerve crosses the artery <1 cm above the

Summit of Superior thyroid pole.

The nerve crosses the artery below the tip of the

Superior thyroid pole.

Type 2 variants are the most vulnerable to iatrogenic injury

So; in Thyroidectomy during moblisation of superior pole; the superior pole vessels should not be ligated en masse, but should be individually divided, low on the thyroid gland. This can be performed by applying caudal and lateral traction to superior thyroid pole to expose the cricothyroid space-(between the medial surface of the upper pole and the cricothyroid muscle); and distracts the Artery from the closely associated Nerve; and so facilitates the individual ligation of the superior-pole vessels close to the thyroid gland, staying lateral to the muscles of the pharynx and the larynx to avoid nerve injury.

Superior thyroid vessels should be doubly ligated; to avoid slipping of ligature and retraction of the vessels causing bleeding.

Injury to External Laryngeal Nerve ;) paralysis of Crico-thyroid muscle); may cause some temporary hoarseness of the voice which appears to recover (due to hypertrophy of the opposite cricothyroid)

Also will lead to inability to tense the ipsilateral vocal cord (to produce higher frequencies) and hence difficulty “hitting high notes, “As in singing; and shouting; difficulty projecting the voice, and voice fatigue during prolonged speech.

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(2)-- The Recurrent Laryngeal Nerve; is related to the medial surface of the thyroid lobe; but always lies

close behind the pretracheal fascia, and has a variable relationship to the Inferior Thyroid artery; (in-front/or behind/or through branches of inferior thyroid A.)

Vagus Nerve; descends downwards in the neck within the posterior part of carotid sheath; in-between the carotid Artery and IJV.

In the Root of the Neck; it crosses in-front of subclavian artery; and passes into the mediastinum.

The left vagus nerve; as it crosses arch of aorta; it gives- off the Left recurrent laryngeal branch; which hooks around ;( “Recurs” around) the ligamentum arteriosum and behind the aortic arch, and ascends medially in the neck in a typical vertical course; in the groove between trachea and esophagus.

The right vagus nerve; as it crosses the subclavian artery in the root of the neck; it gives- off the Right recurrent laryngeal branch; which hooks around ;( “Recurs” around) the right subclavian artery; passes posterior to the artery and then crosses behind the right CCA; before ascending in the neck), passing from lateral to medial; (so its course being more oblique than the left); and may be 1 - 2 cm lateral to the trachea; (in paratracheal position.)

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The recurrent laryngeal nerve ;( RLN) arises from vagus nerve; The RT.Nerve hooks around the subclavian artery; The LT.Nerve hooks around the ligamentum arteriosum and the arch of the aorta.

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The Rt.RLN; has more oblique course; than the Lt.N; which has a typical vertical course in the neck.

The RT.R.L.N. Hooks around the subclavian artery; passing behind it and then behind the C.C.A.; and ascends in neck passing from lateral to medial and may be 1-2 cm from the trachea-in para-tracheal position.

The Lt. R.L.N. ascends in the neck in a more vertically in in the groove between trachea and esophagus.

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Course of RLN:-

The Nerve ascends; in-front of the groove between trachea and Esophagus as it approaches the medial surface of lower pole of the thyroid lobe; always; close behind the pretracheal facial capsule of the thyroid gland; usually in front of trachea-esophageal groove;/Or more anteriorly in a para-tracheal location-(in 90% of patients.)

Anatomical Variations of the course of RLN; between trachea and esophagus:-

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The RLN; mostly-(in 90% of individules); lies in the tracheoesophageal groove; or more anterior to the groove (paratracheal); few-(10%); lies more posterior (para-esophageal); or the nerve lies within the gland; (intra-thyroid.)

During thyroid surgery, when the gland is dislocated forward and medially, the nerve usually hugs the side of the trachea; so can be located not in the tracheo-oesphageal groove but on the posterolateral aspect of the trachea.

Then; (in middle 1/3 of thyroid lobe); it passes in-front/ or behind the inferior thyroid artery/ or in-between its branches. RLN; always passes close behind the pretracheal facial capsule; and may even appear to be buried within the parenchyma.

Relation of Recurrent Laryngeal N. to Inferior Thyroid A.:-

The recurrent laryngeal nerve crosses the inferior thyroid artery at the middle third of the thyroid lobe.

Recurrent Laryngeal N.may pass in-front/or behind/or through branches of inferior thyroid A.

In most individules; (approximately 80% of RLN in both sides); the nerve crosses either posterior to or in - between the branches of the inferior thyroid artery.

Rt.RLN; most frequently crosses in-between arterial branches; the Lt.RLN; usually crosses behind the artery. 

Variations of the Relations of Right RLN ; to Inferior Thyroid A.

Right RLN.; Lies in-between branches of Inferior Thyroid A.; in 50% of individules.

Lies either In-front/or Behind Inferior Thyroid A.; in equal proportion of individules; (25%)

Variations of the Relations of Left RLN ; to Inferior Thyroid A.

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Left RLN.; lies behind Inferior Thyroid A.; in 50% of patients.

Lies in-between branches of Inferior Thyroid A.; in 35% of patients.

Lies in-front of Inferior Thyroid A.; in 15% of patients.

At the level of cricoid cartilage ; RLN; usually; passes posterior; (Deep to); / or; sometimes; through the Suspensory Ligament of Berry.

Variation in relationship of recurrent laryngeal n. to suspensory ligament of Berry.

The Nerve is always behind the pretracheal fascia (including the thickening of it attached to the cricoid cartilage ;( the suspensory ligament of Berry).

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Sometimes; (In about 25% of patients); RLN; may pass through the Suspensory Ligament of Berry; (contained within the ligament as it enters the larynx.)

Or splits into large anterior motor and small sensory branches; at the ligament outside the larynx.

At the level of the upper border of the isthmus ;( just below the inferior border of the cricoid cartilage); the nerve often; (in 40% - 80% of patients); divides into two. If so, the Anterior (larger) branch is the motor branch to laryngeal muscles, and the Posterior branch is sensory only.

Finally ; RLN; passes under the lower border of the inferior constrictor (cricopharyngeus part); just posterior to cricothyroid joint; to enter the larynx (the piriform recess); and then penetrates the laryngeal wall.

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The recurrent laryngeal nerve runs in the tracheoesophageal groove, passes through/ or Deep to the Berry ligament, and then dives under the inferior pharyngeal constrictor muscle before innervating the intrinsic muscles of the larynx.

Recurrent Laryngeal Nerve Supplies; Tracheal; Esophageal branches; and possibly the cricopharyngeus part of the inferior constrictor ;( before entering the pharynx and larynx).

Then; after entering the larynx; it supplies the laryngeal muscles (except cricothyroid) and the laryngeal mucosa below the vocal folds. (The cricothyroid muscles are innervated by the external laryngeal nerves.)

Exposure of RLN :-

Simon's triangle-RLN Triangle; aids in RLN Identification; bordered; medially by RLN; laterally; carotid A.; and above-(base) by ITA.

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RLN- Triangle-(Simon Triangle): RLN forms the medial border of a triangle bounded superiorly by the inferior thyroid artery and laterally by the common carotid artery. 

Upward and medial traction of the thyroid lobe; will make the recurrent laryngeal nerve taut; and may be palpated caudal to the inferior pole of the gland; as a tight strand over the tracheal surface; this will help identification, and exposure of the nerve. (Avoid excessive traction; compression, or stripping the surrounding connective tissue.) 

At the level of Cricoid cartilage; the Nerve is tethered to the Berry’s Ligament and may be stretched by excessive traction and so liable to injury. Small Arterial branch from inferior thyroid artery may enter the Berry’s Ligament and causes bleeding in this area which further put the nerve at risk of injury due to inadequate visualization and may be the use of diathermy/or ligature for hemostasis.

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Superiorly; RLN; can be identified as it enters the larynx posterior to cricothyroid articulation; deep to lower border of inferior pharyngeal constrictor.

RLN; is safest and least visible when it lies in the tracheoesophageal groove.

It is most vulnerable; at the inferior thyroid pole; during ligating inferior thyroid veins as the RLN may be anterior than usual; in the lower 2/3 of its course; where it may traverse the thyroid parenchyma; (intra-glandular); and at the level of cricoid cartilage; it is tethered to/ or may pass through the Suspensory Ligament of Berry; (contained within the ligament as it enters the larynx.) It must be identified and protected before the ligament is divided.

Common Causes of Injury to Recurrent Laryngeal N.:-

Recurrent Laryngeal Nerve Injury occurs in:-

Mass ligation of the vessels of the lower pole of the thyroid; (inferior thyroid veins). Such ligation may include RLN; if it is more anterior than usual.Tunneling through thyroid tissue; (intra-glandular course.)

Distal part of RLN course; ;( at the level of cricoid cartilage) "Just below that point where the nerve passes under the lower fibers of the inferior constrictor muscle to become intra-laryngeal. At this level the nerve is tethered to the suspensory ligament of Berry; passing either deep; superficial; or within ;( contained) the ligament.

In Thyroidectomy;

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Once the RLN has been exposed; the branches of inferior thyroid artery are ligated close to the thyroid parenchyma, (Capsular Dissection); so preserving the blood supply to the parathyroid glands. Truncal ligation of the inferior thyroid artery should be avoided.

In dissection of the medial surface of thyroid lobe to expose the RLN; the ligament of Berry is divided sharply from its attachment to cricoid cartilage; / or small remnant of thyroid tissue can be left in place where the RLN contacts the thyroid parenchyma ;( penetrate the parenchyma); at the ligament of Berry, to preserve the integrity of the RLN.

The parathyroid glands should be preserved in situ by dissecting them downward, away from the capsule of the thyroid gland. When a parathyroid gland cannot be preserved in situ, it is removed and a small portion submitted for frozen-section examination to confirm the presence of parathyroid tissue. The remainder of the gland is minced and autotransplanted into a pocket of the sternocleidomastoid muscle. 90% of freshly autotransplanted parathyroid glands retain function.

The occasional Non-Recurrent Right inferior laryngeal nerve:-

It is more common on the Rt.Side; than the Lt.

In approximately 1% of the population, the right subclavian arises from left side of aortic arch and passes posterior to the esophagus, and the right recurrent laryngeal nerve arises directly from the vagus N. in the neck and enters the larynx directly without forming a loop; (non-recurrent); and with no relation to inferior thyroid artery.

The perpendicular course of the Non-Recurrent Right inferior laryngeal nerve mimics the usual course of the inferior thyroid artery and must be distinguished from it.

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Rt. Non-Recurrent Laryngeal N.; passes directly from vagus N.; into the larynx.

The arteria lusoria vascular anomaly; non-Recurrent Rt.Inferior Laryngeal N.; in which the innominate artery is absent and the right common carotid and right subclavian arteries originate directly from the

aortic arch. The RT subclavian artery; arises from left side of aortic arch and takes a retro-esophageal course.

Left Non-Recurrent Laryngeal N.; may occur in situs inversus viscerum; (the aortic arch must be right sided; the origin of the left subclavian artery and the ligamentum arteriosum must be displaced to the right.)

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Surgical Anatomy of Para-Thyroid Glands

Development of Parathyroid Glands

The superior gland; develops from the Fourth pharyngeal pouch; in conjunction with the lateral thyroid anlages; and descend with the Thyroid Gland.

(Lateral Thyroid Anlage; descends with the Median Thyroid Anlage; fuses with it; and develops into Para-Follicular C-Cells; that localised in the posterio-superior region of thyroid lobe; and dispersed in-between thyroid follicles.)

The inferior gland developed from the Third pouch-(in conjunction with the Thymus); but displaced caudally by the descent of the thymus (which developed from the same pouch.) Thus the inferior gland developed higher up, however it has to migrate caudally than the superior gland and is more liable to end up in unusual positions.

The Para-Thyroid Glands remain closely associated with their respective branchial pouch derivatives.

The Superior Para-Thyroid Gland; tends to lie in constant position with 90% of Superior Para-Thyroid glands being found in the posterio-medial aspect of upper 1/3 of thyroid lobe; at the level of the cricoid cartilage.

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The Inferior Para-Thyroid Gland; tends to lie in an aberrant positions; 10% lies within the capsule of the thymus, and other aberrant glands may be found in the lower neck or the posterior superior mediastinum.

Ectopic Para-thyroid Glands:-

The inferior parathyroid glands, because of their extensive caudal embryologic migration, are more often present in ectopic locations:-

Ectopic Inferior Parathyroids:

10% of inferior parathyroid glands lie in the anterior mediastinum: within the capsule of the thymus ;( A) (The commonest ectopic location of inferior parathyroid.)-Or in the lower neck- in the thyro-thymic ligament; (fibro-fatty tissue connecting the inferior poles of thyroid lobes with the thymus).

Other sites include:-

Retro-sternal location; (B) in superior mediastinum ;( the majority are located below the innominate vein and aortic arch).

Submandibular location (C) anterior to CCA bifurcation.

Intra-thyroidal; within the thyroid gland - in 2% of cases (D)

Ectopic Superior Para thyroid Gland:

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Ectopic superior parathyroid glands are rare.

The commonest ectopic location of Superior Parathyroid; is in the tracheoesophageal groove (E).

Other ectopic sites include:-

Retropharyngeal (F), retro- esophageal (G), para- esophageal in posterior superior mediastinal (H), or intra -thyroidal (D) location.

However; Enlarged superior glands may descend in the trachea-esophageal groove and come to lie caudal to the inferior glands.

In most cases; the Superior Parathyroid Gland (even when located quite low in the posterior superior mediastinum), they can still be retrieved through a collar incision.

Accessory-(Supernumerary) Parathyroid Glands:-

Accessory parathyroids can be located in the peri-thyroid adipose tissue; most frequentl y ; located at the level of the inferior poles of the thyroid lobes, within the thyro-thymic fibro-fatty tissue; or thymus; less frequently in the middle mediastinum, at the level of the aorto-pulmonary window, or laterally in relation to carotid sheath.

The growth of the accessory parathyroid gland(s) germs is caused by Long term stimulation of parathyroid growth in patients with primary or secondary parathyroid hyperplasia.

Surgical Anatomy of Para-Thyroid Glands

There are 4 para-thyroid glands; two- Superior; and two- Inferior related to posterior surface of each thyroid lobe.

Autopsy Studies; showed that 85% of individules have 4-parathyroid glands; 13% have Supernumerary gland most often in association with the thymus; and only 2% have less than four glands.

Each Gland; is spherical or oval-Shape; the gland may be flattened in appearance, especially when it lies within the capsule of the thyroid gland.

Dimension; The average normal gland (5-7 mm; (length) x 2-3 mm; (width) x 1-2 mm ;( thickness)

Weight; Each Gland 30-50 mg.

Para-thyroid Gland is usually surrounded by adipose tissue; (encased in a small amount of fatty tissue); and it may be difficult to be distinguished from surrounding fat.

They are golden yellow to yellowish- brown in Color, (and only slightly darker or brown compared to adjacent fat); which should help to distinguish them from thyroid tissue and surrounding fat envelop.

Position of Para-Thyroid Glands:--

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Para-Thyroid Gland; lies in contact to posterior surface of thyroid lobe and may be either within or outside the thyroid's pretracheal facial capsule; i.e. In-between pretracheal facial capsule and the true fibrous capsule; / or behind the facial capsule and adherent to it.

Sometimes (especially the inferior glands in 2% of individules); the gland may be embedded within the thyroid tissues; (Intra-thyroidal); rather than being behind it.

The Superior Gland; is more constant in position; usually in contact with posterio-medial surface of upper

1/3 of the thyroid lobe; (usually; at the level of cricoid cartilage); at approximately 1-2 cm above the entrance of inferior thyroid artery-(the crossing point of ITA and RLN.)

Usually; at the level of cricoid cartilage ; where the RLN enters deep to the lower border of inferior pharyngeal constrictor muscle; the Superior Gland lie in a plane posterior to RLN.

Often; the superior glands lie beneath the facial capsule and adherent to it.

The Inferior Gland; is the more variable in position; but usually-(in 70% of individules); lies on the posterio-lateral surface of the lower pole, (or along the small branches of the inferior thyroid vein emerging from the lower pole of thyroid lobe;) and 1-2 cm below the entrance of the inferior thyroid artery-(the crossing point of ITA and RLN.); and usually in a plane anterior to the RLN.

Often; the inferior parathyroid glands; lie within the thyroid's pretracheal facial capsule; but rarely ;( in 2% of individules); it may be embedded within the thyroid lobe parenchyma rather than being behind it.

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The inferior parathyroid glands may be located on the surface of the thyroid gland; beneath the fibrous capsule but in clefts of the thyroid parenchyma-seems to be Intra-thyroidal; however rarely they may be found to be completely Intrathyroidal.

The glands are not necessarily on the same level on both sides. The chance of symmetrical positions of the parathyroids of both sides is approximately 70-80%.

Ectopic Para-thyroid Glands:-

The inferior parathyroid glands, because of their extensive caudal embryologic migration, are more often present in ectopic locations:-

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Ectopic Inferior Parathyroids:

10% of inferior parathyroid glands lie in the superio- anterior mediastinum: within the capsule of the thymus; or at/ within the thyro-thymic ligament; (fibro-fatty tissue connecting the inferior poles of thyroid lobes with the thymus) (A) (The commonest ectopic location of inferior parathyroid.).

Other Ectopic sites include:-

Retro-sternal location; (B) in superior mediastinum ;( the majority are located adjacent to the innominate vein and the origin of great vessels from aortic arch).

Submandibular location (C) – anterior to CCA bifurcation; the combined descent of inferior parathyroid

and thymus can be trapped within the carotid sheath ;( the undescended parathymus); which might become relevant and evident only when the parathyroid gland is enlarged and Hyperfunctioning.

Intra-thyroidal; within the thyroid gland - in 2% of cases. (D)

Ectopic Superior Para thyroid Gland:

Ectopic superior parathyroid glands are rare.

The commonest ectopic location of Superior Parathyroid; is in the tracheoesophageal groove (E).

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Other Ectopic sites include:-

Retropharyngeal (F), retro- esophageal (G), para- esophageal in posterior superior mediastinal (H), or intra-thyroidal location (D).

However; Enlarged superior glands may descend in the trachea-esophageal groove and come to lie caudal to the inferior glands.

In most cases; the Superior Parathyroid Gland (even when located quite low in the posterior superior mediastinum), they can still be retrieved through a collar incision.

Acquired Ectopic position of Parathyroid glands :-( Adenoma/Hyperplasia of the Glands)

In case of Adenoma/Hyperplasia of Parathyroid glands; they can leave their usual locations under the weight of the growing tumor tissues or the hyperplastic glands; (acquired parathyroid ectopia); especially the Superior parathyroids.

The growing Adenoma/Hyperplasia of Superior Parathyroid Gland; descends behind the inferior thyroid artery along the trachea-esophageal groove. So; usually Adenomatous/Hyperplastic Superior Parathyroid is located at the level of the inferior pole of the thyroid lobe near inferior thyroid artery. Adenomatous Superior Parathyroid may also descends; Posteriorly; in the tracheoesophageal groove; or into the posterior superior mediastinum in para-esophageal or retro-esophageal position.

Superior Parathyroid Adenoma receives blood supply from inferior thyroid A.; so; identification of descending blood vessel; may be a clue that superior parathyroid is located in the posterio-superior mediastinum.

The growing Adenoma/Hyperplasia of Inferior Parathyroid Gland ;( initially located on posterio-lateral surface of the inferior pole of the thyroid lobe); descends along the anterior path outlined by the thyro-thymic ligament in association of the thymus; or into the anterior superior mediastinum in association to major vessels.

Accessory (Supernumerary) Parathyroid Glands:

Approximately 13% of individules have Supernumerary Parathyroid-however only ½ of these supernumerary glands are proper glands; the others are tiny, rudimentary bits of parathyroid tissue, usually located near another normal gland.

Accessory- (Supernumerary) parathyroids can be located in the peri-thyroid adipose tissue; most frequently they are located at the level of the inferior poles of the thyroid lobes, within the thyro-thymic fibro-fatty tissue; or thymus; less frequently in the middle mediastinum, at the level of the aorto-pulmonary window, or laterally in relation to carotid sheath.

The growth of the accessory parathyroid gland germs is caused by Long term stimulation of parathyroid tissue in patients with primary or secondary HPT.

Supernumerary Parathyroid Glands; are significant surgically in 4-conditions:-

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(1)--Primary Hyperparathyroidism -P-HP; caused by MEN- especially Type I , and Familial Hyperparathyroidism, when all glands are abnormal.

(2)-- Secondary Hyperparathyroidism, most typically that: results from chronic renal failure; in which all glands are stimulated to enlarge and Hyperfunction.

(3)—Sporadic cases of Primary Hyperparathyroidism in which:-

The four usual glands are normal and only the supernumerary gland is abnormally enlarged and responsible for Hyperfunction; and

The supernumerary gland is enlarged in addition to another usual gland-(both are enlarged and Hyperfunctioning);-Double Adenoma.

Operative Identification (Normal Glands):-

Superior parathyroid gland; after the superior pole of the thyroid lobe is dissected; and mobilized the lobe is rolled medially exposing the posterior surface of the upper 1/3 of thyroid lobe, an area containing fat deep to; (posterior to) the pretracheal fascia is apparent. The superior parathyroid gland almost always lies within this fatty area-(envelop), beneath the thyroid sheath; and can be exposed by gentle

dissection to strip thin fascial layers overlying the gland; and manipulate the surrounding fat to expose the gland.

Inferior parathyroid gland; during dissection of inferior pole of thyroid lobe; and as the position of the inferior parathyroid is more variable, it may be located along the posterio-lateral surface of lower pole of the thyroid lobe; or along the small branches of the inferior thyroid vein emerging from the lower pole of thyroid lobe.

10% of inferior parathyroid glands lie within the capsule of the thymus in lower neck ;( concealed within its variably atrophic and fat-replaced thymic substance); or within/or at thyro-thymic ligament.

2% of the Parathyroid; especially the Inferior Gland; lies Intra-thyroidal; the gland may be located on the surface of the thyroid gland; beneath the fibrous true capsule but in clefts of the thyroid parenchyma-seems to be Intra-thyroidal; however rarely they may be found to be completely Intrathyroidal.

Normal Glands; are soft; pliable; non- palpable, and may have different shapes; spherical; or it may be flattened when lies within the facial capsule; (that flatten it against thyroid surface).Brownish-yellow in color-(golden yellow-to light brown); distinguished from its fat envelop ;( brighter yellow) and thyroid tissue; however sometimes; it may be difficult to distinguish parathyroid from surrounding fat.Following the branches of the inferior thyroid artery upwards and downwards; a small branch enters each gland. The tiny arterial branch enters the gland from medial side ;( hilum / vascular pedicle); and

has a leaf-like branching pattern.When a tiny biopsy taken from the non-hilar portion of the parathyroid gland; the entire parenchymal surface bleeds from pinpoint capillaries (in contrast to fat with its single bleeding vessel). Operative Identification-Abnormal Glands:-

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Abnormal Parathyroid Glands are generally enlarged in all dimensions; have a darker brown or reddish-brown color-(hyperplastic gland may appear reddish in color secondary to the associated

hyperemia); firmer than usual when gently probed; may have an irregular and knobby shape, and

there may be more prominent vascular pedicles; or a plexus of vasculature.

In the presence of a parathyroid neoplasm or adenoma; normal parathyroid tissue; often appears as a rim of compressed normal parathyroid tissue to one side of the adenoma; yellowish-tan in color as a result of atrophy.

Blood supply :-

Both Superior and Inferior parathyroids are usually supplied by small branches from the inferior thyroid artery.

Inferior thyroid artery is the principal blood supply to the parathyroid glands.

Each gland is supplied by a single small end artery from the inferior thyroid artery that often traveling in a course parallel; if not slightly anterior to the parathyroid glands before reaching the vascular hila from medial side.

Sometimes; (in 20% of cases); small branches from the superior thyroid artery-(from its Posterior Branch); supply 20% of Superior para-thyroids.

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In Thyroidectomy; if the main trunk of the inferior thyroid artery is sacrificed for dissection, both parathyroids on that side become devascularized because there is usually no collateral blood supply to maintain viability. So; even when total thyroidectomy is performed; individual ligation and division of only the branches of the inferior thyroid artery entering the thyroid capsule will maintain good vascular supply to the superior and inferior parathyroid glands.

Ectopic mediastinal parathyroid gland usually receives its blood supply from either the internal mammary

artery or small arteries within the thymus; however, an enlarged parathyroid gland; (Acquired Ectopic) that grows into the mediastinum usually carries with it the corresponding branch of the inferior thyroid artery.

Venous Drainage: - The parathyroid glands drain ipsilaterally into the superior, middle, and inferior thyroid veins.

Lymph drainage ; as in Thyroid Gland. Sympathetic vasoconstrictor fibres; follow branches of inferior thyroid A.

Histological Structure:-

The Gland; is a mass of small closely-packed chief or (principal) and oxyphil cells; surrounded by fibro-fatty vascular stroma containing large numbers of blood capillaries.

Chief Cells which secrete the parathyroid hormone (PTH).

Oxyphil cells; - Acidophilic; (mitochondria-rich) Cells which; are derived from chief cells and start to appear around puberty, and increase in numbers in adulthood.

Water-clear cells; - Glycogen-laden third group of cells; derived from chief cells, and present in small numbers.

Although Oxyphil and Water-clear cells are probably derived from the chief cells and secrete PTH, their functional significance is not known.