Anatomy of the Thyroid Gland

ANATOMY OF THE THYROID AND PARATHYROID GLANDS

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).

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.
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.
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.
Facial Nerve Branches (Lower Divisions)
- Injury to the marginal mandibular branch can cause drooping of the lower lip on that side (no spontaneous regeneration).
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.

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

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

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.
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.

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.
Upper Parathyroid Glands
- More constant in position, often slightly larger.
- Lie near the upper thyroid pole to the region of the inferior thyroid artery.
Lower Parathyroid Glands
- More variable in location (can be found from upper thyroid region to anterior mediastinum).
- Embryologically associated with the thymus descent.
Surgical Relevance
- A fifth parathyroid gland may exist, especially important if searching for hyperparathyroidism sources (e.g., adenoma or hyperplasia).

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.
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.

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.
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.

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 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.
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.
Thyroid Follicles
- Start forming around the eighth week; contain colloid by the third month.
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 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).
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.