Anatomy of the Thyroid Gland

ANATOMY OF THE THYROID AND PARATHYROID GLANDS

Location and Dimensions

• Thyroid Gland
  • Situated between the larynx and trachea medially, and the carotid sheath and sternocleidomastoid muscles laterally.
  • Weighs approximately 15–25 g.
  • Lateral Lobes: ~3–4 cm long and 1.5–2 cm wide.
  • Isthmus: ~1.2–2 cm long, ~2 cm wide, crossing the trachea between the first and second rings.
  • Typically asymmetric (right lobe often larger or extends higher).

Surgical Exposure of the Neck Structures

1. Superficial Layer
   • Skin, subcutaneous fat, and platysma muscle can be reflected as flaps for wide exposures.
   • External and anterior jugular veins and transverse cervical nerves run within this superficial layer.
2. Deeper Exposure
   • The sternocleidomastoid muscle, omohyoid muscle, sternohyoid, and sternothyroid muscles lie beneath.
   • Ansa hypoglossal nerve (motor to infrahyoid muscles) courses near the carotid sheath. Preserving it helps maintain swallowing function.
3. Anterior Jugular Veins
   • Supplement venous return from pharynx/upper neck.
   • Important to retract (rather than divide) these vessels during thyroid/parathyroid exposure to reduce postoperative edema of the neck/larynx.
4. Facial Nerve Branches (Lower Divisions)
   • Injury to the marginal mandibular branch can cause drooping of the lower lip on that side (no spontaneous regeneration).
5. Midline Lymph Node
   • A solitary node can be found just in front of the thyroid isthmus, between pretracheal muscles.
   • Enlarged in pharyngitis or laryngitis, but not typically in thyroiditis or tracheitis.

Relation to the Esophagus

• The esophagus often lies slightly to the right of midline, adjacent to the typically larger right thyroid lobe.

Exposure of Thyroid, Parathyroid, and Thymus

• Achieved by retracting or transecting pretracheal (infrahyoid) muscles.
• Maximum exposure sometimes requires a transverse cut of these muscles to reflect them superiorly and inferiorly.

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THYROID GLAND: ADDITIONAL DETAILS AND ANOMALIES

1. Shape and Position
   • Usually shaped like a horseshoe (“U” or “H”) with two lateral lobes connected by an isthmus.
   • Right lobe often larger; in dextrocardia, the left lobe may be larger.
2. Common Developmental Variations
   • Pyramidal Lobe (~15% of people): Extends upward from the isthmus; may enlarge with goiter or be a site of neoplasia.
   • Accessory Thyroid Tissue (≥5%): Small nodules separate from main thyroid; can mimic a tumor on exam.
   • Isthmus Agenesis (<1%): Medial aspects of lobes may feel like separate nodules.
   • Partial Lobe Absence (rare): Often involves lower half of a lobe.
   • Ectopic Thyroid (lingual thyroid, substernal thyroid).
   • Complete Agenesis: Extremely rare; usually discovered postnatally when congenital hypothyroidism develops.

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PARATHYROID GLANDS

1. Number and Typical Location
   • Usually four (two upper, two lower), located on the posterior surface of the thyroid lobes.
   • Each parathyroid has its own capsule but is attached to the back of the thyroid’s proper capsule.
2. Upper Parathyroid Glands
   • More constant in position, often slightly larger.
   • Lie near the upper thyroid pole to the region of the inferior thyroid artery.
3. Lower Parathyroid Glands
   • More variable in location (can be found from upper thyroid region to anterior mediastinum).
   • Embryologically associated with the thymus descent.
4. Surgical Relevance
   • A fifth parathyroid gland may exist, especially important if searching for hyperparathyroidism sources (e.g., adenoma or hyperplasia).

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LYMPHATIC DRAINAGE OF THE THYROID REGION

1. Anterior Midline Nodes
   • Delphian Node: Above the thyroid isthmus, in front of the cricoid cartilage; may be palpable if enlarged (e.g., thyroid cancer, Hashimoto thyroiditis).
   • Pretracheal Nodes: Below the thyroid isthmus, variable position.
2. Lateral and Deep Cervical Nodes
   • Along the lateral thyroid veins, near the recurrent laryngeal nerve, at the carotid sheath (jugular chain), and the supraclavicular fossa.
   • “Sentinel” or Virchow Nodes are the low jugular nodes near the thoracic duct inlet.

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LARYNGEAL MOTOR NERVES

1. External Branch of the Superior Laryngeal Nerve
   • Supplies the cricothyroid muscle (tenses vocal cords).
   • Injury can lead to subtle voice changes or “fuzziness,” more pronounced if bilateral.
2. Recurrent (Inferior) Laryngeal Nerves
   • Right Recurrent Nerve: Hooks under the subclavian artery, then ascends diagonally.
   • Left Recurrent Nerve: Hooks under the aortic arch, often ascending in the tracheoesophageal groove.
   • Important to identify and preserve during thyroid/parathyroid surgery to avoid vocal cord paralysis.

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DEVELOPMENT OF THE THYROID AND PARATHYROID GLANDS

Overview of Pharyngeal Development

• During early embryonic life (~second month), the pharynx forms from the foregut, with four pairs of pharyngeal pouches protruding laterally.
• Branchial Clefts align externally; anomalies may lead to fistulas or cysts along the neck.

Thyroid Gland Embryology

1. Thyroid Diverticulum
   • Appears in the midline floor of the primitive pharynx (~end of fourth week).
   • Rapidly becomes bilobed; initially attached to the pharynx by the thyroglossal duct.
2. Descent to Neck
   • By ~seventh week, the thyroid primordium descends to the tracheal level.
   • Thyroglossal Duct usually obliterates; a remnant at the tongue base is the foramen cecum.
   • Residual duct segments can form thyroglossal duct cysts or ectopic thyroid tissue.
3. Thyroid Follicles
   • Start forming around the eighth week; contain colloid by the third month.
4. Congenital Anomalies
   • Lingual Thyroid: Gland remains at tongue base.
   • Pyramidal Lobe: Retention of the distal thyroglossal duct.
   • Absent or Hypoplastic Gland: Can result in congenital hypothyroidism if untreated after birth.

Parathyroid and Thymus Embryology

1. Parathyroid Glands
   • Third and Fourth Pharyngeal Pouches: Dorsal portions form parathyroid primordia.
   • “Inferior” Parathyroids: Usually from third pouch, migrating caudally with the thymus.
   • “Superior” Parathyroids: From fourth pouch, remain more cranial.
   • Variable locations possible (e.g., mediastinal, near thymus).
2. Thymus Gland
   • Ventral parts of the third (and sometimes fourth) pouches.
   • Descends into the superior mediastinum, merging into a bilobed structure.
   • Undergoes involution after puberty, replaced largely by adipose tissue in adults.