Thyroid Anatomy - الأهلية

Embryology

• The thyroid gland begins development around the 3rd to 4th week of gestation.
• The thyroid gland is derived from the thyroglossal duct and ultimobranchial bodies.
• The parathyroid glands develop from the third and fourth pharyngeal pouches.

Thyroglossal Duct

• Originates from endodermal tissue at the median bud of the pharynx.
• An initially hollow structure migrates caudally and passes in close continuity with, and sometimes through the developing hyoid cartilage.
• The foramen cecum at the base of the tongue shows the vestigial remnant of the duct.

Ultimobranchial Bodies

• Derived from the fourth and fifth pharyngeal pouches.
• They give rise to the parafollicular cells (C cells) that produce calcitonin.

Participants in Embryogenesis

Median Pharyngeal Diverticulum

• Arises from the floor of the pharynx and descends to the neck.
• It is connected to the foramen cecum via a tube known as the thyroglossal duct.
• After the thyroid reaches its definitive location in the lower neck, the connecting thyroglossal duct undergoes obliteration, forming the "levator glandulae thyroidae".

Parafollicular Cells (C-cells)

• Derived from the neural crest.
• Present mainly in the upper pole.
• Secrete calcitonin.

Development of Follicles and C Cells

• The thyroid gland organizes into follicles as functional units producing thyroxine hormone.
• The parafollicular cells (C cells) become embedded in the thyroid tissue
• Calcitonin is produced and is a hormone involved in calcium homeostasis.

Anatomy of Thyroid Gland

• The thyroid is H-shaped (Butterfly).
• It consists of 2 lateral lobes; the isthmus and the pyramidal lobe.
• The thyroid site is in front of the neck opposite C5, C6, C7, T1.
• It typically weighs 20-25 grams.

Extent

• The apex of the lobe lies on the thyroid cartilage below the oblique line.
• The base of the lobe is at the level of the 6th tracheal cartilage.
• The isthmus is opposite the 2nd, 3rd, and 4th tracheal cartilages.
• The pyramidal lobe arises from the isthmus and is connected to the base of the tongue.

Capsules

• It consists of a true capsule and a false capsule

Capsules

True Capsule

• Surrounds the gland completely.

False Capsule (Pre Tracheal Fascia)

• Lies outside the true capsule.
• Derived from the deep cervical fascia.

Attachment

• Above: To the oblique line of the thyroid cartilage.
• Below: To the fibrous pericardium.
• On Both Sides: To the deep fascia of the neck.
• It fixes the gland to the cricoid and thyroid cartilage.

Surgical Importance

• Responsible for the movement of the gland up and down with deglutition.
• Contributes to the compression manifestation of huge goiter.
• Causes retrosternal extension.
• Aids spread of infection downward.

Relations

Superficial Relations

• Skin, Superficial fascia (contains platysma & anterior jugular vein), Cervical deep fascia
• 4 muscles: Sternomastoid, Sternohyoid, Sternothyroid, Superior belly of omohyoid

Medial Relations

• Two tubes: Trachea, Esophagus
• Two cartilages: Thyroid cartilage, Cricoid cartilage
• Two muscles: Inferior constrictor, Cricothyroid
• Two nerves: External laryngeal, Recurrent laryngeal

Posterior relations

• Carotid sheath & its content (Common carotid art., Vagus nerve, IJV)
• Sympathetic chain is behind the carotid sheath.
• Ansa cervicalis is embedded in the anterior wall of the carotid sheath.
• Thoracic duct is on the left side.

Arterial Supply of the Thyroid

Superior Thyroid Artery

• The main supply of the thyroid gland.
• Originates from the external carotid artery.
• Runs downwards and forwards in relation to the external laryngeal nerve and is ligated close to the gland.
• Divides into anterior and posterior branches.
• Supplies the upper 1/3 of the lobe and the upper 1/2 of the isthmus.

Inferior Thyroid Artery

• It provides the main supply to the parathyroid gland.
• Comes from the thyrocervical trunk (from the first part of the Subclavian).
• Runs first upwards, then medially and finally downwards to reach the lower pole of the thyroid lobe.
• Terminal part is related to the recurrent laryngeal nerve
• Supplies the lower 2/3 of the lobe, the lower 1/2 of the isthmus, and the parathyroids

Thyroid ima artery

• In 3% of individuals
• Arises from the arch of the aorta or innominate artery
• Supplies the isthmus

Accessory Arteries

• Esophageal & tracheal arteries

Venous Drainage of the Thyroid

Superior Thyroid Vein

• Drains into the IJV.

Middle Thyroid Vein

• Drains into the IJV.

Inferior Thyroid Veins

• Drains into the left innominate vein.

Kocher Vein (inconstant)

• Fourth thyroid vein.
• Drains into the IJV.
• Lies () middle and inferior thyroid vein.

Lymphatic Drainage of the Thyroid

• Mainly medial part of the gland and isthmus.
• Drain into pre-tracheal L.N. (Delphic L.N.), para-tracheal L.N., pre-laryngeal L.N. (L.N. of Poirier)
• Lower part of the gland drains into superior mediastinal L.N.

Nerves Related to the Thyroid Gland

Recurrent Laryngeal Nerve

• Branch from the vagus nerve.
• Called recurrent as it arches on :
  • Left side: Arch of aorta & pulmonary artery
  • Right side: Right Subclavian artery.
• Lies close to the inferior thyroid artery close to the gland.
  • Commonly deep to the artery (40%) or between its branches (35%).
  • So, the inferior thyroid artery is ligated away from the gland to avoid injury of the recurrent nerve.
• Motor:
  • All intrinsic muscles of the larynx except the cricothyroid muscle supplied by the external laryngeal nerve.
• Sensory:
  • Mucus membrane of the larynx below the vocal cord.
• Injury:
  • Usually unilateral or bilateral.

External Laryngeal Nerve

• Branch from the superior laryngeal nerve which arises from the vagus.
• Lies close to the superior thyroid artery away from the gland.
  • So, the superior thyroid artery is ligated close to the gland.
• Supply:
  • Cricothyroid muscle tensor of the vocal cord.
• Injury:
  • Loss of high-pitched voice.

Clinical Manifestations of Nerve Damage:

Damage to the Recurrent Laryngeal Nerve

• Unilateral: Dyspnea on exertion, one cord adducted, other cord normal.
• Bilateral: Stridor or suffocation (urgent tracheostomy), both cords adducted.
• Complete:
• Unilateral: Horsiness of voice, one cord in cadaveric position (midway abduction & adduction), other cord normal.
  • Bilateral: Aphonia, both cords in cadaveric position.

Surgical Anatomy of the Thyroid

• The situation after surgery includes;
  • mobilization of the right lobe.
  • the relationships of the recurrent laryngeal nerve, and inferior thyroid artery.
  • the parathyroid glands.
• 1 & 2 are common sites for the superior parathyroid gland.
• The arrow shows the tendency for an enlarged gland to migrate from position 1 to position 2.
  • In an inferior direction, to lie Posterior to the inferior thyroid artery (5) and oesophagus (8).
• 3 & 4 are common sites for the inferior parathyroid gland.
• The upper horn of the thymus points like an index finger to the inferior parathyroid, which may lie under the ‘fingernail’.
• 5 inferior thyroid artery
• 6 recurrent laryngeal nerve
• 7 thymus
• 8 oesophagus

Congenital Anomalies (Extremely Rare)

• Congenital aplasia or hypoplasia:
  • Congenital hypothyroidism "Cretinism":
    • Failure of the thyroid gland to develop or function properly can lead to severe developmental and growth issues if not treated promptly.

Hemiagensis

• The left lobe of the gland is absent in 80% of cases.
• The remaining lobe is hypertrophied, but normal in shape.

Ectopic Thyroid Tissue

• Definition: Improper migration can result in thyroid tissue remaining at the base of the tongue or along the migration path.
  • Types: Lingual thyroid, Thyroglossal ectopic thyroid, Aberrant thyroid.
  • Sites: Larynx, low in neck, mediastinum (intrathoracic or mediastinal thyroid) OR Ovarian teratoma (Struma ovarii).

Thyroglossal Duct Cysts

• Definition: Remnants of the thyroglossal duct can persist and form cysts which may require surgical removal.
• Etiology: Mid line neck cyst due to patency of the thyroglossal duct.
• Pathology and "Site": A cyst may occur at any point in the course of the thyroglossal track from foramen caecum to the isthmus of the thyroid gland.
  • The commonest site is just below the hyoid bone.
  • It lies in the midline EXCEPT at the level of the thyroid cartilage.
    • Where it is usually pushed to the left because anatomically the levator glandulae thyroidae is shifted to the left at these sites.
• Clinical Picture:
  • Number: Single.
  • Site: midline of the neck (EXCEPT at the level of the thyroid cartilage usually pushed to the left).
  • Shape: Rounded or oval.
  • Surface: Smooth.
  • Edge: Well defined.
  • Tenderness: Not tender unless complicated.
  • Consistency: Cystic.
  • Movement: -Move up & down with deglutition -Move on protrusion of the tongue while the mouth is open -Moves from side to side & not vertical
• Investigation: Thyroid ultrasound or scan to ensure that the thyroid gland in its place and cervical lymph nodes.
• Complication: Thyroglossal fistula. Malignancy (1%): commonly papillary carcinoma.
• Treatment
  • Sistrunk Operation
    • Horizontal elliptical incision placed over the cyst or enclosing the fistulous opening.
• Cyst or fistula + the whole track + Central part of the hyoid bone + part of the base of the tongue may be excised.

Goiter (Enlarged Thyroid Gland)

• Cretenoid Goiter (Hypothyroidism)
• Simple Goiter (Euothyroidism)
  • Diffuse Hyperplastic Goiter: Physiological, Colloid.
• Simple Nodular Goiter: Solitary thyroid nodule, Multi-nodular goiter.
• Toxic Goitre (Hyperthyroidism) 1ry = Diffuse Toxic (Grave's)., 2ry = Toxic Nodular (Plummer's)., Toxic Nodule.
• Inflammatory Goiter (Thyroiditis)
  • Acute bacterial thyroiditis., Subacute (De Quervains)., Autoimmune (Hashimoto's)., Riedl’s thyroditis., Chronic as tuberculosis, and syphilis
• Neoplastic Goiter
  • Benign: adenomas.
  • Malignant: Either 1ry Papillary - Follicular - Medullary - Anaplastic OR 2ry.
• Miscellaneous
  • Amyloidosis