Thyroid Anatomy - المنصورة

• The thyroid gland comes from the thyroglossal duct and the ultimobranchial body
• Function begins during the 4th month of intrauterine life

Thyroglossal Duct

• Originates from the endoderm of the embryonic pharynx at the foramen cecum
• Starts at the foramen caecum
• Travels midline anterior to the hyoid bone
• Curves posterior to the hyoid bone
• Proceeds down the front of the thyroid and cricoid cartilage
• The cranial end location is indicated by the foramen cecum of the tongue

Fate of the cranial end

• May remain as:
  • Pyramidal lobe that extends from the isthmus
  • Levator glandulae thyroideae which is a fibromuscular band that connects the pyramidal lobe to the hyoid bone
• The lower end gives rise to the follicular cells of the thyroid gland
• The rest of the duct obliterates

Ultimobranchial Body

• Joins the lower end of the thyroglossal duct
• Gives rise to parafollicular cells

Congenital Anomalies

• Ectopic thyroid results from the thyroglossal duct failing to descend
• Thyroglossal cyst results from persistence of the thyroglossal duct
• With thyroglossal cysts, a Midline swelling is present in the neck near the hyoid bone, which is sometimes removed to prevent recurrence
• Lingual thyroid: Gland remains on the dorsum of the tongue
• When the cyst breaks through the skin it creates a thyroglossal fistula which is never congenial

Surgical Anatomy General

• Located in the lower front of the neck
• Weighs about 25 grams in adults, slightly heavier in females
• The vertebral level of the thyroid is at C5-T1
• It has two lateral lobes (right and left), which connect to the trachea via the isthmus, and a small pyramidal lobe

Extent of lobes

• Apex lies opposite the oblique line of the thyroid cartilage
• The base is at the level of the 5th or 6th tracheal ring
• Vertebral level is C5 to T1
• Against the level from the 2nd to (3rd or 4th) tracheal rings
• Pyramidal lobe is on the left side of the upper margin of isthmus
• Pyramidal lobe may connect to hyoid bone via levator glandulae thyroideae ms
• Has a true capsule (fibrous) and a false capsule (fascial)

Clinical Correlation

• Enlarged thyroid glands shift downward instead of upward
• Upward shift is stopped by the attachment of the sternothyroid muscle
• Downward shift occurs when short neck and strong strap muscles push the enlarged gland downward into the superior mediastinum (retrosternal goiter)

Thyroid Capsule

• True capsule: completely surrounds the gland, derived from the connective tissue at the periphery, and the venous plexus lies deep to it
• During surgical removal of the gland care is taken to remove the gland along with its true capsule to avoid injury to venous plexus
• False capsule: derived from the pretracheal layer of deep cervical fascia

Pretracheal Fascia attachment

• Superiorly: to the oblique line of the thyroid cartilage
• Laterally: continuous with the deep cervical fascia
• Inferiorly: continuous with the fibrous pericardium
• Importance: Responsible for movement of the thyroid gland up with swallowing

Pretracheal Fascia Clinical Correlation

• Movements of the thyroid during swallowing (deglutition) & speech
• Retrosternal extension
• Compression manifestation of huge goitre
• Suspensory ligament of Berry: a thickening of pretracheal fascia
  • Site: Posteromedial aspect of thyroid gland lobe & cricoid cartilage
  • Importance: Secures the gland to the larynx preventing downward movement and during surgical removal of the enlarged gland the recurrent laryngeal nerve (RLN) lies behind it & it is liable to injury during operation

Relations of Lobes

• Each lobe of the thyroid gland have the following:
  • Three surfaces: Superficial (anterolateral), Medial, Posterior (posterolateral)
  • Two boarders: Anterior, Posterior
  • Two poles: Superior "apex", Inferior "base"

Relations of surfaces

• Superficial Surface: Infrahyoid muscles: Sternothyroid, Sternohyoid, Omohyoid "Superior belly", laterally covered by the anterior margin of Sternomastoid
• Medial Surface:
  • Two Tubes: Larynx & Pharynx above, Trachea & Esophagus below
  • Two Muscles: Inferior constrictor, Cricothyroid
  • Two Cartilages: Cricoid, Thyroid
  • Two Nerves: External laryngeal, Recurrent laryngeal

Recurrent laryngeal nerve

• Passes upwards in the groove between the trachea and esophagus
• Lies immediately behind the cricothyroid joint and the ligament of Berry
• Posterior Relations: Carotid sheath (Vagus nerve - Carotid artery - Internal jugular vein), sympathetic chain "behind the carotid sheath", prevertebral fascia & longus colli. "directly behind", and on left side, the lower pole may be related to arch of thoracic duct

Relations of boarders

• Anterior Boarder is related to: The anterior branch of the superior thyroid artery
• Posterior Boarder is related to: The parathyroid glands and the anastomosis between the superior & inferior thyroid arteries

Relations of the Isthmus

• Anteriorly: Covered from within outwards by pretracheal fascia, strap muscles, investing layer of deep fascia, superficial fascia with anterior jugular veins, and the skin
• Posteriorly: It sits on the 2nd to (3rd or 4th) tracheal rings
• Upper margin: Terminal parts of anterior branches of both superior thyroid arteries
• Lower margin: Inferior thyroid veins emerge and the thyroidea ima (when present) enters

Relations of poles

• Superior Pole "Apex": limited by the insertion of thyrohyoid muscle to thyroid cartilage and related to the superior thyroid artery and the external laryngeal nerve
• Inferior Pole "Base": Related to the inferior thyroid artery, the recurrent laryngeal nerve, and the arch of the thoracic duct is on the left side

Surface Anatomy

• Lobes (Left & Right):
  • Site: In the anterior triangles in the lower neck, and inferior to the position of the oblique line of the thyroid cartilage
  • Relation to sternothyroid: Sternothyroid at arches superiorly to the oblique lines & lies anterior to the lobes of the thyroid gland
  • Prevents the upward movement of the lobes
  • Palpation of the lobes: By finding the thyroid prominence & arch of the cricoid cartilage, then feeling posterolateral to the larynx
• Isthmus:
  • Site: Crosses anterior to the upper end of the trachea
  • Palpation of the isthmus: In the midline inferior to the arch of the cricoid

Isthmus Clinical Importance

• Presence of the thyroid Isthmus & the associated vessels crossing the midline
  • Makes tracheal cartilage palpation difficult
  • Complicates anterior airway entry during surger tracheostomy

Venous drainage

• Thyroid veins do not accompany the thyroid arteries
• 3 Pairs of veins arise from venous plexus that lies deep to the true capsule
  • Superior thyroid vein from the upper pole on each side drains into the corresponding internal jugular vein
  • Middle thyroid vein (very short) which corresponds to the lobe and corresponding Internal jugular vein
  • Inferior thyroid vein from the lower margin of Isthmus drains on the anterior surface of trachea, right and left drain into left innominate (brachio-cephalic) vein, sometimes each vein may drain into respective brachio-cephalic veins
• Right and left inferior thyroid veins connect before trachea, their injury causes bleeding in tracheostomy
• Occasionally Kocher's vein "Another vein may be present", site is between the inferior & middle thyroid veins, and opens into the internal jugular vein

Arterial Supply

• Pairs of superior and inferior thyroid arteries
• The thyroidea ima
• The accessory thyroid arteries

Superior thyroid arteries

• Arises from the external carotid artery
• Descends to the superior pole of the lateral lobe of the gland, “Accompanying the external laryngeal nerve”, and enters the gland at the upper pole
• The upper pole is the safest site for tying the superior thyroid artery during thyroidectomy

Inferior thyroid arteries

• Arises from the thyrocervical trunk of the 1st part of the subclavian artery
• Forms a loop with upward convexity to sixth cervical vertebra
• Descends to reach the base of the lateral lobe of the gland
• Enters the lower pole of the gland "Where it is very close to the recurrent laryngeal nerve"
• Safer to tie the inferior thyroid artery as away from the lower pole of the gland as possible during thyroidectomy

Vascular Supply

• Inferior thyroid artery's ascending branch joins superior thyroid artery's descending branch at gland's posterior margin
• Ascending branch of inferior thyroid artery is preserved during ligation as it is the main supply to parathyroid glands
• The Thyroidea Ima (Not Always Present): Comes from arch of aorta (or) Brachiocephalic trunk then ascends in front of the trachea & enters the isthmus.
• The Accessory Thyroid Arteries: Comes from twigs from Esophageal & Tracheal branches of inferior thyroid arteries

Lymphatics (lymph from upper and lower parts)

• Lymph from the upper part from the upper deep cervical nodes (jugulo-digastric) and directly from the prelaryngeal nodes
• Lymph from the lower part from the lower deep cervical group (jugulo-omohyoid) and directly through the pretracheal (delphic) & paratracheal nodes
• A few lymph vessels may drain into brachiocephalic nodes in the superior mediastinum, while others may directly reach the thoracic duct
• Lymphatic spread of papillary carcinoma of the thyroid gland manifests in an interesting way; The thyroid gland looks normal but the deep cervical LNs of the jugular chain is enlarged & contains thyroid tissue

Nuclear Scan of Thyroid Gland

• Used to study the function of the thyroid gland by isotope scanning steps
  • Radioactive iodine (131I) is given orally
  • Photographs of thyroid gland are taken
• Hot spots indicate hyperactive areas
• Cold spots indicate hypoactive areas
• Thyroid scans find ectopic thyroid tissue outside the neck (lingual or retrosternal)

Pathology: Goiter

• General Characteristics: Abnormal enlargement of the thyroid gland, a common cause of a midline swelling in the neck, thyroid swelling moves with deglutition, may present as a smooth swelling or a nodular swelling
• One of the complications is Pressure Effects and varies depending on structure or structures compressed, structures compressed anatomical relations

Pressure Effects Caused by Goiter

• Trachea → Dyspnea (shortness of breath)
• Esophagus → Dysphagia (difficulty in swallowing)
• Recurrent laryngeal nerve → Hoarseness of voice
• Carotid sheath → Posterior displacement
• Sympathetic chain → Horner's syndrome

B) Thyroid Stimulating Antibodies

• A family of IgG immunoglobulins, collectively known as Thyroid Stimulating Antibodies (TsAB)
• Activates the TSH receptors on the follicular cell membrane
• More protracted action than TSH (TsAB = 16 - 24 hours vs TSH = 1.5 - 3 hours)
• Responsible for almost all cases of thyrotoxicosis ("Which is not due to autonomous toxic nodules")
• Serum concentrations are very low & not routinely measured

Histological Structure (Thyroid Follicles)

• It is the structural & functional unit of the thyroid gland and made of about 30 million units
• General Characteristics
  • Number is about 30 million
  • Size 0.05 - 0.5 mm
  • Shape rounded or oval and filled with colloid (Thyroglobulin)
  • Wall lined by low cubical epithelial cells

Types of Cells Lining the Follicles

• Follicular cells which make up 98% and are responsible for synthesis & secretion of thyroid hormones
• Parafollicular cells (C-cells, clear cells, or light cells) which make up 2% and secrete calcitonin hormone which causes reduced serum calcium level

Physiology (Thyroid Hormones Synthesis)

• Main hormones secreted are T3 and T4 and their synthesis is controlled by several enzymes
• Steps:
  • Trapping of inorganic Iodide from the blood by the thyroid gland
  • Oxidation by thyroid peroxidase enzyme (TPO), Iodide → lodine
  • Binding of iodine to Tyrosine to form Mono-iodotyrosine (MIT), Di-iodotyrosine (DIT)
  • Di + Di → T4 (Tetra iodothyronine), Mono + Di → T3 (Tri iodothyronine), coupling occurs within the thyroglobulin molecule
  • Under TSH control
    • Complex is resorbed into the cell
    • Thyroglobulin is broken down into T3 & T4 & liberated into blood
• Thyroid hormones circulate in 2 forms
  • Protein bound which is mainly Thyroxin Binding Globulin (TBG) and is 99% of the bounded part
  • Free part which is in equilibrium with the protein bound hormone and is the active part which is less than 1% Bounded
• Peripheral conversion: part of T4 can be converted to T3 in the peripheral tissue and as T3 it is more important & quick acting (within a few hours) whereas T4 acts more slowly (4-14 days)