Thyroid Anatomy and Surgery for Thyroid Swelling

PART 1: THYROID ANATOMY

General Description

• Craniocaudal: ~5 cm
• Anteroposterior: 2-3 cm
• Width: ~3 cm

Anatomical Relations

Embryology

1. Median thyroid anlage - a thickening of pharyngeal floor endoderm. Descends caudally as a bilobed diverticulum connected to the tongue by the thyroglossal duct. The duct becomes solid by week 5, leaving behind the foramen cecum superiorly and the pyramidal lobe inferiorly. Thyroid reaches final position by week 7; follicle formation and thyroid hormone production begin by week 10.
2. Lateral thyroid anlage - arises from pharyngeal endoderm and fuses to the median anlage during descent at week 5. Contains ultimobranchial body cells from the 4th and 5th pharyngeal pouches, which become the calcitonin-secreting parafollicular C cells. Contributes approximately one-third of the final gland mass.

Blood Supply

• Superior thyroid artery - first branch of the external carotid artery; divides into anterior and posterior branches at the apex of each lobe
• Inferior thyroid artery - arises from the thyrocervical trunk (branch of subclavian artery); courses medially deep to the carotid artery and enters the thyroid at its midpoint. Intimately and variably associated with the RLN
• Thyroidea ima artery - arises directly from the aorta or innominate in 1-10% of individuals; enters the isthmus or replaces a missing inferior thyroid artery

• Superior thyroid vein - drains to internal jugular vein
• Middle thyroid vein - drains to internal jugular vein (may be absent or duplicated)
• Inferior thyroid vein - forms a plexus draining into the brachiocephalic vein

Nerves

• Left RLN: arises from the vagus at the level of the aortic arch, loops around the ligamentum arteriosum, ascends in the tracheoesophageal groove
• Right RLN: arises from the vagus at the right subclavian artery, loops posteriorly, and ascends more obliquely than the left
• Both cross the inferior thyroid artery (critical surgical landmark) and may branch
• The tubercle of Zuckerkandl (lateral/posterior extension of the thyroid at the cricoid level) marks where the RLN must be identified
• The RLN is most vulnerable at the ligament of Berry where it enters the larynx
• Nonrecurrent RLN: right side in 0.5-1% of individuals (associated with aberrant right subclavian artery)

• Runs with the superior thyroid artery to supply the cricothyroid muscle (voice pitch control)
• At risk during ligation of the superior thyroid artery; injury causes loss of high-pitched phonation

Lymphatics

• Pre-laryngeal (Delphian) lymph nodes - midline above the isthmus; their involvement signals adverse tumor characteristics
• Pretracheal nodes
• Paratracheal nodes

PART 2: SURGERY FOR THYROID SWELLING (THYROIDECTOMY)

Indications

1. Hyperthyroidism - when nonsurgical management has failed or is not preferred (Graves disease, toxic multinodular goiter, solitary toxic adenoma)
2. Goiter with or without compressive symptoms - dyspnea, dysphagia, hoarseness, superior vena cava syndrome
3. Thyroid nodules and thyroid cancer
4. Symptomatic goiter or failure of medical treatment

Extent of Resection (Nomenclature)

Preoperative Preparation

• Biochemical assessment: TSH, T3, T4; calcium in patients at risk for concurrent PHPT (MEN2A)
• Imaging: Neck ultrasound mandatory; CT if substernal extension or tracheal compression suspected
• FNA biopsy for nodular disease as indicated
• Euthyroid state: For hyperthyroidism, antithyroid drugs ± beta-blockade before surgery. For Graves disease, Lugol solution or SSKI within 10 days of surgery reduces vascularity
• Voice/laryngeal assessment: Mandatory history; selective laryngoscopy for high-risk patients (voice changes, prior neck surgery, posterior extrathyroidal extension, bulky nodal metastases)
• Airway assessment: For large or substernal goiter - indirect laryngoscopy + CT to assess tracheal deviation/compression before intubation

Anesthesia

• General endotracheal anesthesia is standard
• Neuromonitoring ETT with embedded electrodes placed at vocal cords when intraoperative nerve monitoring (IONM) is planned; muscle relaxation is contraindicated with IONM
• Laryngeal mask with spontaneous breathing is an alternative allowing real-time endoscopic vocal cord monitoring
• Deep and superficial cervical plexus blocks can supplement analgesia

Surgical Technique (Key Steps)

1. Positioning: Supine with neck extended (shoulder roll), reverse Trendelenburg to reduce venous engorgement
2. Incision: Kocher (transverse cervical) incision ~2 cm above sternal notch, within skin crease
3. Subplatysmal flaps raised superiorly and inferiorly
4. Midline raphe divided between strap muscles; Delphian nodes encountered first
5. Superior pole dissection: superior thyroid vessels ligated close to the thyroid capsule to protect the EBSLN
6. RLN identification: nerve found in the tracheoesophageal groove; traced to the ligament of Berry - the most dangerous point
7. Inferior thyroid artery branches ligated close to capsule (preserves parathyroid blood supply); the RLN crosses this artery and must be visualized before ligation
8. Parathyroid glands identified and preserved in situ with their vascular pedicles; inadvertently devascularized glands are auto-transplanted to the sternocleidomastoid or forearm
9. Medial tracheal attachments divided under the isthmus; isthmus is included in the specimen
10. Closure with or without drain; wound closed in layers

Postoperative Care and Complications

Indications for Surgery in Thyroid Swelling Specifically

• Compression symptoms: dyspnea (tracheal compression), dysphagia (esophageal compression), hoarseness (RLN involvement), facial flushing/SVC syndrome
• Substernal extension with airway compromise
• Cosmetic concerns or rapid growth
• Suspicion of malignancy (hard, fixed, irregular nodule; associated lymphadenopathy; rapid growth)
• Failure of medical management in hyperthyroid states