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THYROID - Detailed Surgery Answers

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1. Surgical Anatomy of the Thyroid Gland

Gross Anatomy

• Lateral: medial aspects of the carotid sheath
• Anterior: sternothyroid and sternohyoid (strap) muscles
• Posterior: trachea and esophagus (especially on left)

• Anterior suspensory ligament - above the isthmus
• Ligament of Berry (posterior suspensory ligament) - firm attachment to trachea posteromedially; the RLN is intimately related here and is most vulnerable to iatrogenic injury at this point. Branches of RLN may traverse the ligament in 25% of individuals.
• Tubercle of Zuckerkandl - most lateral and posterior extent of thyroid lobe; the last segments of the RLN course below this structure.

Blood Supply

• Superior thyroid vein - drains into internal jugular vein
• Middle thyroid vein - least consistent; drains into internal jugular
• Inferior thyroid veins - form a plexus draining into brachiocephalic veins

Nerve Supply (Surgically Critical)

• Left RLN: arises from vagus at the aortic arch, loops around ligamentum arteriosum, ascends in tracheoesophageal groove
• Right RLN: arises from vagus at the right subclavian artery, usually passes posterior to artery, more oblique course
• Non-recurrent RLN: occurs in 0.5-1% on right (associated with vascular anomaly); extremely rare on left (only with situs inversus)
• Most vulnerable at Ligament of Berry

• Approximately 20% of patients are at risk of injury if superior pole vessels are ligated en masse
• Injury causes loss of cricothyroid muscle function → loss of high-pitched voice (singer's nerve)

Lymphatic Drainage

• Central compartment (Level VI) - primary drainage
• Lateral neck (Levels II-V)
• Superior mediastinal nodes

Embryology

• Thyroid arises from median and lateral thyroid anlagen; originates from the floor of the pharynx at the foramen cecum and descends through the neck via the thyroglossal duct to its final position

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2. Physiology of Thyroid Hormones and TFTs

Thyroid Hormone Synthesis

1. Iodide trapping - Na/I symporter (NIS) on basolateral membrane co-transports 2Na+ and 1I- using energy from Na-K ATPase. Concentrates iodide ~30x (up to 250x when maximally active).
2. Transport to apical membrane - via pendrin (chloride-iodide counter-transporter)
3. Oxidation of I- to I0 - by thyroid peroxidase (TPO) in presence of H2O2
4. Organification - iodination of tyrosyl residues on thyroglobulin → MIT (monoiodotyrosine), DIT (diiodotyrosine)
5. Coupling - DIT + DIT → T4; MIT + DIT → T3 (by TPO)
6. Pinocytosis and hydrolysis - thyroglobulin endocytosed, lysosomes release T3/T4
7. Secretion - T4 and T3 released; MIT/DIT deiodinated and recycled

Hypothalamic-Pituitary-Thyroid Axis

• Hypothalamus → TRH → Anterior pituitary → TSH → Thyroid → T3/T4
• T3 is primarily responsible for inhibition (feedback) at both hypothalamus and pituitary
• TSH stimulates virtually all steps of thyroid hormone synthesis and iodide trapping

Thyroid Function Tests (TFTs)

• High TSH + Low fT4 = Primary hypothyroidism
• Suppressed TSH + High fT4 = Hyperthyroidism
• T3 thyrotoxicosis = high T3, normal T4 and TSH

• Anti-TPO + Anti-Tg antibodies - elevated in Hashimoto's thyroiditis (~80%)
• TSI (thyroid stimulating immunoglobulin) - elevated in Graves' disease
• Serum Thyroglobulin (Tg) - tumor marker for monitoring differentiated thyroid cancer recurrence post-thyroidectomy; elevated Tg after total thyroidectomy = recurrence
• Serum Calcitonin - marker for Medullary Thyroid Carcinoma (MTC)
• TRH stimulation test - 500 μg TRH IV; normal: TSH rises ≥6 μIU/mL at 30-60 min

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3. Thyroiditis

Classification

Subacute Thyroiditis - Key Details

• ~50% have preceding URI
• Thyrotoxicosis due to T4/T3 release from disrupted follicles (not new synthesis) → transient, self-limiting (4-6 weeks)
• Radionuclide scan: LOW uptake (distinguishes from Graves')
• Treatment: NSAIDs/salicylates (2.4-3.6 g/day); severe → Prednisone 40 mg/day x 3-4 weeks, then taper; β-blockers for thyrotoxic symptoms if troublesome

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4. Thyrotoxicosis and Its Management (High Yield - QQ 2017)

Definition

Causes

• Graves' disease (most common) - diffuse toxic goiter; TSI stimulates TSH receptor
• Toxic multinodular goiter (Plummer's disease)
• Solitary toxic adenoma (Plummer's disease variant)
• Subacute/silent thyroiditis
• Exogenous thyroid hormone intake
• Struma ovarii, TSH-secreting pituitary adenoma (rare)

Clinical Features

• Symptoms: weight loss, heat intolerance, palpitations, tremor, anxiety, diarrhea, oligomenorrhea
• Signs: tachycardia, AF, warm moist skin, fine tremor, exophthalmos (Graves' only), lid lag, lid retraction, pretibial myxedema (Graves' only), thyroid bruit (Graves')
• Graves' specific: exophthalmos, pretibial myxedema, thyroid acropachy (triad)

Three Treatment Modalities

A. Antithyroid Drugs (Thionamides)

• Drugs: Carbimazole (converted to methimazole), Propylthiouracil (PTU)
• Mechanism: Inhibit TPO → block organification and oxidation of iodide → block T3/T4 synthesis; PTU also inhibits peripheral conversion of T4 to T3
• Doses: Carbimazole 20-40 mg/day; PTU 100-150 mg TDS
• Duration: 12-18 months; remission in 30-50% of Graves' patients
• Side effects: Agranulocytosis (0.1-0.5%; most serious - check WBC if sore throat/fever), hepatotoxicity (PTU), rash, arthralgia
• PTU preferred in: pregnancy (1st trimester), thyroid storm, breast-feeding

B. Radioactive Iodine (¹³¹I)

• Definitive, non-invasive treatment
• Taken up by thyroid cells → beta emission → follicular cell destruction
• Contraindicated in pregnancy, children, active severe ophthalmopathy
• Side effects: permanent hypothyroidism (desired endpoint), radiation thyroiditis, worsening ophthalmopathy
• Defer pregnancy for 6-12 months after treatment

C. Surgery (Thyroidectomy)

• Definitive treatment with immediate effect
• Indications: large goiter with compressive symptoms, failed antithyroid drugs, patient preference, significant ophthalmopathy, pregnancy (second trimester), children, suspected malignancy, toxic multinodular goiter
• Preparation: make euthyroid with thionamides first; then Lugol's iodine for 10 days pre-op (reduces vascularity - Wolff-Chaikoff effect)
• Procedure: Total or near-total thyroidectomy
• Complications: RLN injury, hypoparathyroidism, thyroid storm (if inadequately prepared)

D. Additional Medications (Surgical Pharmacology)

Treatment Summary Table (Scott-Brown's)

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5. Pathology of Thyroid Tumors

Classification of Thyroid Tumors

• Follicular adenoma (most common benign tumor)
• Toxic adenoma (Plummer's adenoma)

Papillary Thyroid Carcinoma (PTC)

• 80% of all thyroid malignancies; F:M = 2:1; mean age 30-40 years
• Histology: Papillary projections, characteristic nuclear features - pale "ground glass" cytoplasm, nuclear grooves, intranuclear cytoplasmic inclusions ("Orphan Annie" nuclei), psammoma bodies (calcified deposits)
• Molecular: BRAF V600E mutation (most common); RET/PTC rearrangements (especially post-radiation)
• Multifocality in up to 85% on microscopy
• Lymph node mets: common (especially in young); "lateral aberrant thyroid" = nodal metastasis
• Prognosis: Excellent; >95% 10-year survival
• Microcarcinoma: ≤1 cm, non-invasive; often incidental; associated with 25% occult LN mets
• NIFTP (Noninvasive Follicular Thyroid Neoplasm with Papillary-like nuclear features) - encapsulated follicular variant PTC without invasion; indolent behavior

Follicular Carcinoma (FTC)

• 10% of thyroid cancers; F:M = 3:1; mean age 50 years; iodine-deficient areas
• Presents as solitary nodule; FNAC cannot distinguish benign follicular lesion from FTC (need capsular/vascular invasion on histology)
• Spread: hematogenous (lungs, bone, liver, brain) - NOT lymphatic
• Cervical lymphadenopathy at presentation: only ~5%
• Diagnosis confirmed by capsular invasion and/or vascular invasion on histology
• Treatment: Total thyroidectomy + TSH suppression + RAI ablation

Medullary Thyroid Carcinoma (MTC)

• Arises from parafollicular C-cells (at junction of upper 1/3 and lower 2/3 of thyroid)
• 1-2% of thyroid cancers
• Sporadic (75%) vs Hereditary (25%)
• RET proto-oncogene mutations - all hereditary MTC (MEN2A, MEN2B, FMTC); 50% of sporadic MTC
• MEN2A (95% of MEN2): MTC + Primary Hyperparathyroidism + Pheochromocytoma; RET mutations exon 10/11
• MEN2B (5% of MEN2): MTC + Pheochromocytoma + Mucosal neuromas + Marfanoid habitus; RET codon 918 (exon 16) - most aggressive
• Marker: Calcitonin (diagnostic + follow-up); also CEA
• All patients with MTC should undergo genetic testing for RET (even "sporadic" cases - up to 7% may have hereditary form)
• Treatment: Total thyroidectomy + bilateral central neck dissection; screen family members

Anaplastic Carcinoma

• 1-2%; most aggressive thyroid malignancy; average age 71 years
• ~50% survive 6 months; often arise in longstanding goiter/well-differentiated cancer
• Treatment: multimodal (surgery rarely curative); chemoradiation (doxorubicin + radiation); dabrafenib for BRAF V600E-mutated cases

Familial Cancer Syndromes Associated with Thyroid Cancer

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6. Pathology of Thyroid Malignancy (Summary points)

AMES / MACIS Prognostic Scoring (for PTC/FTC)

• Age (males >40, females >50)
• Metastasis (distant)
• Extent (extrathyroidal invasion)
• Size (>4 cm)

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7. Thyroglossal Duct Cyst (QQ 2018)

Embryology

Features

• Most common congenital neck mass in children
• Location: midline, just below the hyoid bone (most common site); can be at any point along the thyroglossal tract
• Classic sign: Mass moves upward with swallowing AND with tongue protrusion (distinguishes from other neck masses)
• Size: 1-4 cm
• Lining: Stratified squamous epithelium (near tongue base) or pseudostratified columnar epithelium (lower sites); transitional patterns also occur
• Fibrous wall often includes lymphoid aggregates or thyroid remnants
• Malignant transformation: exceedingly rare (<1%)
• Can become infected → painful, red, fluctuant

Diagnosis

• Clinical (characteristic midline mass + tongue protrusion sign)
• Ultrasound/CT scan to confirm and check for normal thyroid tissue below (essential before surgery)
• FNAC if required to exclude malignancy

Treatment

• Excision of the cyst + central portion of hyoid bone + the entire tract up to the base of the tongue
• Removing the hyoid prevents recurrence (thyroglossal duct passes through/near it)
• Recurrence rate without Sistrunk: ~50%; with Sistrunk: ~5%
• Complete excision is mandatory

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8. Solitary Thyroid Nodule

Epidemiology

• Present in ~4% of the US population
• Thyroid cancer incidence: ~40 new cases per million
• Therefore, most nodules are benign; key task is identifying which need surgery

Features Suggesting Malignancy

• History of ionizing radiation exposure (40% chance of cancer)
• Age <20 or >60 years; Male sex
• Rapid growth, hard consistency, fixation to adjacent structures
• Hoarseness (RLN involvement) - worrisome sign
• Dysphagia, dyspnea - locally advanced disease
• Cervical lymphadenopathy
• Family history of thyroid cancer/MEN2
• Single firm nodule in MNG
• Pain (suggests hemorrhage, thyroiditis, or rarely malignancy)

Features Suggesting Benign

• Soft, cystic, smooth
• Positive radionuclide uptake ("hot" nodule - rarely malignant)
• History of Hashimoto's thyroiditis
• Rapid regression
• TSH normal or elevated

Workup

1. History and examination
2. TSH (first test) - if suppressed → radionuclide scan (to identify autonomously functioning "hot" nodule)
3. Ultrasound - assess echogenicity, borders, calcifications, lymph nodes; ACR TI-RADS classification
4. FNAC (Fine Needle Aspiration Cytology) - most important diagnostic test; Bethesda classification system (I-VI)
5. Radionuclide scan - hot nodule (autonomous), cold nodule (higher malignancy risk), warm nodule
6. CT scan - for retrosternal extension, airway compression

Bethesda Classification and Management

Features of a Hot Nodule vs Cold Nodule

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9. Complications of Thyroid Surgery (Mx)

Intraoperative Complications

Postoperative Complications

1. RLN Injury

• Unilateral: hoarseness (temporary if neuropraxia; permanent if severed); <1% permanent in experienced hands
• Bilateral (catastrophic): acute airway obstruction → immediate reintubation + tracheostomy
• Both vocal cords adduct to midline with bilateral injury

2. Hypoparathyroidism / Hypocalcemia (Most Common Significant Complication)

• Transient hypocalcemia: up to 50% of cases (temporary stunning/devascularization)
• Permanent hypoparathyroidism: <2% of cases
• Higher risk in: Graves' disease, total thyroidectomy + central neck dissection
• Symptoms: perioral tingling, carpopedal spasm (Trousseau's sign), facial spasm (Chvostek's sign), tetany, laryngospasm
• Management: IV calcium gluconate (acute); oral calcium + vitamin D (long-term)

3. Thyroid Storm (Thyrotoxic Crisis)

• Life-threatening complication if patient inadequately prepared for surgery
• Precipitated by: surgery, infection, trauma, emotional stress
• Features: High fever (>38.5°C), tachycardia/AF, agitation, delirium, diarrhea, vomiting → coma
• Burch-Wartofsky Score used for diagnosis
• Management (ABCDE + PTISS):
  • Propranolol (IV/oral) + PTU 600 mg stat then 200-250 mg q4h
  • Thionamides (PTU preferred - also blocks T4→T3)
  • Iodine (Lugol's or SSKI) - 1 hour AFTER thionamides
  • Steroids (hydrocortisone 300 mg/day or dexamethasone)
  • Supportive: IV fluids, cooling, treat precipitant

4. Hematoma

• Postoperative neck hematoma can cause rapid airway compromise
• Emergency bedside opening of wound to evacuate hematoma if airway threatened

5. Wound Complications

• Seroma (aspiration if symptomatic)
• Wound cellulitis/infection (rare)
• Keloid/hypertrophic scar

6. Horner's Syndrome

• Injury to cervical sympathetic chain
• In invasive cancers or retroesophageal goiters
• Ptosis + miosis + anhidrosis

7. Air Embolism (rare)

• Injury to large neck veins

Complications of Thyroid Surgery - Quick Summary Table

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10. Pharmacology of Antithyroid Drugs

Thionamides

Beta-Blockers

• Propranolol (non-selective): blocks adrenergic symptoms + inhibits T4→T3 peripheral conversion
• Atenolol/metoprolol: cardioselective, useful in asthma
• Used in all thyrotoxicosis for rapid symptomatic relief; also pre-operatively

Iodine Preparations

• Lugol's Iodine (5% I2 + 10% KI): 0.5 mL (16 drops) TDS for 10 days pre-op
• SSKI (Saturated Solution Potassium Iodide): thyroid storm
• Effect: Wolff-Chaikoff effect (high iodine transiently inhibits synthesis) + Plummer effect (reduces gland vascularity + hormone release)
• Critical: must give thionamides FIRST; iodine given alone can worsen thyrotoxicosis (iodine load)

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Sources

• Schwartz's Principles of Surgery, 11th Ed. - Ch. 38 (Thyroid, Parathyroid, and Adrenal)
• Sabiston Textbook of Surgery - Ch. 36 (Thyroid)
• Mulholland & Greenfield's Surgery, 7th Ed. - Ch. 75
• Fischer's Mastery of Surgery, 8th Ed. - Well-Differentiated Thyroid Carcinoma
• Guyton & Hall Medical Physiology - Ch. 77
• Cummings Otolaryngology, Head & Neck Surgery - Thyroiditis chapter
• Scott-Brown's Otorhinolaryngology - Thyrotoxicosis treatment modalities

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Topic-wise Weightage Summary