Thyroid and parathyroid surgery — thyroidectomy technique, RLN injury, hypoparathyroidism, thyroid cancer, hyperparathyroidism localisation

Definition / Overview

Thyroid surgery encompasses a spectrum of resections, from hemithyroidectomy (lobectomy ± isthmusectomy) to total thyroidectomy, performed for benign and malignant thyroid disease. The thyroid gland is the largest purely endocrine organ, synthesising thyroxine ($T_4$) and triiodothyronine ($T_3$), whose actions drive basal metabolic rate, cardiac output, thermoregulation, and protein synthesis. Surgical decision-making requires precise knowledge of thyroid physiology, regional anatomy, and the mechanisms underlying the key complications of recurrent laryngeal nerve (RLN) injury and hypoparathyroidism.

Applied Thyroid Physiology

The Hypothalamic-Pituitary-Thyroid Axis

• Hypothalamus secretes thyrotropin-releasing hormone (TRH) → anterior pituitary releases thyroid-stimulating hormone (TSH).
• TSH binds TSHR on follicular cells, driving iodide uptake, thyroglobulin synthesis, and cleavage/release of $T_3$/$T_4$.
• $T_4$ is the dominant secreted product; peripheral deiodination converts it to active $T_3$ (primarily in liver and kidney).
• Negative feedback: rising free $T_3$/$T_4$ suppresses both TRH and TSH.

Calcitonin and Parafollicular (C) Cells

• Parafollicular C cells secrete calcitonin in response to hypercalcaemia.
• Calcitonin opposes PTH: inhibits osteoclast activity and promotes renal calcium excretion.
• Clinically relevant as a tumour marker in medullary thyroid carcinoma (MTC).

Preoperative Metabolic State

• Euthyroid state is mandatory before elective thyroid surgery to avoid thyroid storm.
• Hyperthyroid patients require preoperative preparation with antithyroid drugs (carbimazole, propylthiouracil), beta-blockade (propranolol), and, for Graves' disease, Lugol's iodine for 10-14 days pre-operatively to reduce gland vascularity.
• Hypothyroid patients undergoing elective surgery should ideally be rendered euthyroid with levothyroxine first; in urgent settings, active replacement is commenced perioperatively.

Indications for Thyroid Surgery

Malignant / Suspected Malignant Disease

Benign Disease

• Compressive goitre: dysphagia, stridor, tracheal deviation, SVC obstruction, particularly with retrosternal extension.
• Hyperthyroidism: Graves' disease unresponsive to antithyroids or radioiodine; toxic multinodular goitre; toxic adenoma.
• Cosmetic / social: large visible goitre.
• Concurrent primary hyperparathyroidism requiring neck exploration.
• Women planning pregnancy within 6 months who require definitive hyperthyroid treatment.

Extent of Resection, Decision Principles

• Hemithyroidectomy: solitary benign nodule, diagnostic excision, low-risk PTC ≤1 cm confined to one lobe.
• Total thyroidectomy: bilateral disease, malignancy requiring post-operative radioiodine (RAI), large or compressive goitre, Graves' disease.
• Prophylactic central lymph node dissection (CLND): controversial; central compartment nodal metastases occur in up to 80% of PTC with clinically negative nodes, yet remedial central dissection carries higher complication risk, decision must weigh staging benefit against morbidity.

Surgical Anatomy, Applied to Thyroid Surgery

Gland and Its Relations

• The thyroid lies in the pretracheal space between vertebral levels C5-T1, enveloped by the pretracheal fascia.
• Lobes are connected by the isthmus overlying the 2nd-4th tracheal rings.
• Closely related posteriorly to the trachea, oesophagus, and prevertebral fascia; laterally to the carotid sheath.

Vascular Supply

Recurrent Laryngeal Nerve, Anatomy Critical for Examination

• The right RLN loops around the right subclavian artery and ascends obliquely in the tracheo-oesophageal groove, entering the larynx posterior to the cricothyroid joint.
• The left RLN loops around the aortic arch (ligamentum arteriosum) and runs more vertically in the tracheo-oesophageal groove.
• The RLN innervates all intrinsic laryngeal muscles except the cricothyroid; it carries sensation to the subglottis.
• The nerve crosses the ITA at variable points, the relationship is not constant and cannot be predicted.
• Non-recurrent RLN occurs on the right side in ~0.5-1% of patients (associated with a right-sided aortic arch or aberrant right subclavian artery); extremely rare on the left.
• Berry's ligament (posterior suspensory ligament of the thyroid) is the most hazardous zone, the RLN passes in close proximity or occasionally within this ligament.

External Branch of the Superior Laryngeal Nerve (EBSLN)

• Innervates the cricothyroid muscle (tenses/lengthens vocal cord, affecting pitch).
• Travels with the STA in its superior course, but descends medially near the superior pole.
• Injury causes loss of high-pitched voice, subtle but significant for singers/voice professionals.
• Individual ligation of superior pole vessels at the thyroid capsule protects the EBSLN.

Parathyroid Glands, Anatomical Relationships

• Typically four glands, each weighing 30-35 mg.
• Superior parathyroids (from 4th pharyngeal pouch): relatively constant position posterior to the upper thyroid lobe, dorsal to the RLN, superior to the ITA-RLN crossing point.
• Inferior parathyroids (from 3rd pharyngeal pouch with the thymus): more variable, typically ventral to the RLN, inferior to the ITA-RLN crossing.
• Vascular supply predominantly from terminal branches of the ITA, hence truncal ITA ligation risks parathyroid ischaemia.

Operative Technique, Key Principles

1. Positioning: supine, neck extended on a shoulder roll; horizontal Kocher incision placed 2-3 cm above the sternal notch in a natural skin crease.
2. Subplatysmal flaps raised superior to thyroid cartilage and inferior to sternal notch.
3. Midline strap muscle separation; lateral retraction (not division unless gland is very large).
4. Superior pole dissection: individually ligate STA branches at capsule to protect EBSLN.
5. Lateral dissection: divide middle thyroid vein; identify and protect RLN by direct visualisation throughout its course, visual identification is the standard of care.
6. RLN identification: traced from its entry at the thoracic inlet to the cricothyroid joint; the nerve lies in or near the tracheo-oesophageal groove but its exact position is variable.
7. Parathyroid preservation: identify all four glands; preserve on vascular pedicle by dissecting the thyroid capsule away from the gland; avoid truncal ligation of ITA.
8. Parathyroid autotransplantation: if a gland is devascularised or inadvertently excised, confirm it is parathyroid tissue (frozen section), mince into 1-3 mm pieces, implant into sternocleidomastoid muscle pocket; mark with permanent suture.
9. Berry's ligament division: last step of liberation of the lobe; meticulous haemostasis with fine clips or bipolar; avoid thermal energy near the RLN.
10. Haemostasis, drain (if indicated), layered closure.

Intraoperative Nerve Monitoring (IONM)

• Electrodes on the endotracheal tube (or needle electrodes in the cricothyroid) detect evoked muscle action potentials on RLN stimulation.
• Useful particularly in reoperative cases to aid anatomical identification.
• No level 1 evidence demonstrating statistically significant reduction in permanent nerve injury rates.
• IONM complements but does not replace meticulous technique.

Complications

Recurrent Laryngeal Nerve Injury

• Bilateral RLN injury is a surgical emergency; both vocal cords adduct to the midline causing inspiratory stridor. Secure the airway immediately (re-intubation or emergency surgical airway).
• Post-operative hoarseness mandates formal assessment: indirect or fibreoptic laryngoscopy to document cord mobility.
• Patients at elevated risk: reoperative surgery, malignancy invading the tracheo-oesophageal groove, Graves' disease (increased vascularity), large substernal goitre.

Hypoparathyroidism and Hypocalcaemia

Mechanism

• Transient hypocalcaemia is the most common complication of total thyroidectomy.
• Caused by devascularisation or venous congestion of parathyroid glands during mobilisation, not necessarily physical removal.
• Hungry bone syndrome is an additional contributor in patients with pre-existing hyperthyroidism (Graves' disease) where increased bone turnover leads to rapid calcium uptake by bone once euthyroidism is restored after surgery.

Classification

Clinical Features of Hypocalcaemia

• Perioral and fingertip paraesthesiae (earliest symptom)
• Carpopedal spasm (Trousseau's sign: carpal spasm with BP cuff inflated above systolic for 3 minutes)
• Facial twitching (Chvostek's sign: tap facial nerve at angle of jaw)
• Tetany, seizures, laryngospasm, cardiac arrhythmias (QT prolongation)

Management

• PTH measured 6-24 hours post-operatively is highly predictive: PTH <1.0-1.5 pmol/L suggests impending permanent hypoparathyroidism and warrants prophylactic replacement.
• Hypomagnesaemia potentiates hypocalcaemia and impairs PTH secretion, always check and replace magnesium.

Haematoma and Haemorrhage

• Incidence ~1-2%; can be life-threatening due to tracheal compression.
• Typically presents within 6-8 hours of surgery but can be delayed to 24 hours.
• Expanding haematoma with respiratory distress: open the wound at the bedside immediately (undo skin closure and release strap muscle sutures) to decompress the trachea, then transfer to operating theatre for definitive haemostasis.

Other Complications

Perioperative Management

Preoperative

• Euthyroid state: confirmed biochemically (normal TSH, free $T_4$, free $T_3$) before elective surgery.
• Graves' disease: carbimazole ± propranolol for 4-6 weeks; Lugol's iodine 5 drops TDS for 10-14 days immediately pre-operatively to reduce vascularity and decrease intraoperative blood loss.
• Baseline assessment: fibreoptic laryngoscopy to document pre-existing vocal cord function (especially reoperative cases), ultrasound ± CT for retrosternal extension, calcium/PTH/magnesium levels.
• Counsel patient regarding risks: RLN injury (transient and permanent), hypoparathyroidism, haemorrhage, need for lifelong thyroxine after total thyroidectomy.

Intraoperative

• Communication with anaesthetist regarding IONM endotracheal tube placement if monitoring is planned.
• Avoid excessive neck extension in patients with known cervical spine disease.
• Meticulous haemostasis; caution with ultrasonic/bipolar energy devices within 3-5 mm of the RLN (thermal injury risk at tissue temperatures that exceed safe thresholds).

Postoperative

• Post-operative observation for haematoma: minimum 6-8 hours; overnight stay after total thyroidectomy is standard practice in most Australian institutions.
• Serial calcium monitoring: 6 h and 24 h post-operatively; PTH at 6 h.
• Commence levothyroxine after total thyroidectomy (standard replacement ~1.6 µg/kg/day; dose adjusted by TSH at 6-8 weeks).
• If RAI is planned post-operatively for differentiated thyroid carcinoma, levothyroxine is withheld (or recombinant TSH used) to achieve TSH stimulation for remnant ablation.

Thyroid Storm, Perioperative Emergency

• Precipitated by surgical stress in an inadequately prepared hyperthyroid patient.
• Features: hyperthermia, tachyarrhythmia, agitation, cardiac failure, altered consciousness, Burch-Wartofsky score assists diagnosis.
• Management: propranolol IV (to block adrenergic effects and peripheral $T_4$→$T_3$ conversion), propylthiouracil (PTU) via nasogastric tube (blocks synthesis and peripheral conversion), Lugol's iodine (1 hour after PTU to prevent iodine fuelling hormone synthesis), hydrocortisone 100 mg IV 8-hourly (blocks conversion, treats relative adrenal insufficiency), active cooling, IV fluids, ICU admission.

High-Yield Summary for GSSE

• The RLN is at greatest risk at Berry's ligament and at its crossing with the ITA, direct visualisation throughout is mandatory.
• Inferior parathyroids lie ventral to the RLN; superior parathyroids lie dorsal to the RLN.
• Truncal ligation of the ITA without parathyroid identification is a modifiable cause of permanent hypoparathyroidism.
• Transient hypocalcaemia resolves within days-weeks; permanent hypoparathyroidism is defined by persistence beyond 6 months.
• Bilateral RLN injury = airway emergency; unilateral = hoarseness ± aspiration.
• IONM aids RLN identification but does not replace surgical technique and has not been proven to reduce injury rates in randomised trials.
• A devascularised parathyroid gland should be minced and autotransplanted to the SCM, mark with permanent suture.

Sources

• RACS position statements & curriculum (Royal Australasian College of Surgeons)
• Therapeutic Guidelines (eTG)