Applied Anatomy of the Thyroid Gland
Goitre
Definition: Any enlargement of the thyroid gland, regardless of cause or functional status.
| Type | Description |
|---|---|
| Simple (non-toxic) | Physiological enlargement — puberty, pregnancy, iodine deficiency |
| Toxic | Hyperfunctioning — Graves’ disease (diffuse toxic), toxic nodular goitre |
| Non-toxic nodular | Single or multiple nodules — colloid cyst, adenoma, multinodular goitre |
| Malignant | Carcinoma of thyroid |
Iodine-deficiency goitre (endemic goitre): Most common cause worldwide. Low dietary iodine → low T3/T4 → increased TSH → diffuse hyperplasia of thyroid. Prevention: iodised salt.
Thyroid Swellings — Clinical Assessment
Key clinical sign: Thyroid swellings move upward on swallowing (attached to pretracheal fascia which is fixed to larynx/trachea). This is the hallmark of a thyroid swelling.
Distinguishing thyroglossal cyst: Also moves on tongue protrusion (connected to foramen caecum via thyroglossal duct remnant).
Thyroid Carcinoma
| Type | Frequency | Origin | Behaviour | Lymph Spread | Haematogenous Spread |
|---|---|---|---|---|---|
| Papillary | ~60% | Follicular cells | Good prognosis; slow | Early, extensive | Rare |
| Follicular | ~25% | Follicular cells | Good prognosis | Rare | Common (bone, lung) |
| Medullary | ~5–10% | Parafollicular C cells | Intermediate | Yes | Yes |
| Anaplastic | ~5% | Follicular cells | Very poor; rapid | Extensive | Yes |
| Lymphoma | Rare | B lymphocytes | Variable | — | — |
Papillary carcinoma: Most common; may arise in pre-existing goitre; associated with prior radiation exposure; spreads to regional lymph nodes; excellent prognosis (20-year survival >90%).
Follicular carcinoma: Spreads by blood (haematogenous); bone and lung metastases; cannot be distinguished from follicular adenoma on FNAC alone (capsular/vascular invasion needed on histology).
Medullary carcinoma: Secretes calcitonin — used as tumour marker; also secretes CEA. Associated with MEN 2A (medullary thyroid carcinoma + phaeochromocytoma + hyperparathyroidism) and MEN 2B (medullary thyroid carcinoma + phaeochromocytoma + mucosal neuromas). Caused by RET proto-oncogene mutations.
Anaplastic carcinoma: Most aggressive; rapidly fatal; presents as hard, rapidly growing mass causing dyspnoea, dysphonia, dysphagia.
Nerve Injuries in Thyroid Surgery
Recurrent Laryngeal Nerve Injury
| Type | Cause | Effect |
|---|---|---|
| Unilateral | Ligation/traction near inferior thyroid artery or ligament of Berry | Hoarseness; voice fatigue; cord lies paramedian |
| Bilateral | Total thyroidectomy without nerve identification | Stridor; respiratory distress; emergency tracheostomy may be needed |
The RLN must be identified and protected throughout its course during thyroidectomy — not just at one point.
External Branch of Superior Laryngeal Nerve Injury
- Damage during ligation of the superior thyroid artery at upper pole
- Loss of high-pitched vocalisation; cricothyroid paralysis
- Often subclinical unless patient is a singer/professional voice user
Sympathetic Trunk Injury
- May occur during extensive dissection
- Causes Horner’s syndrome: ptosis, miosis, anhidrosis, enophthalmos on the affected side
Hypoparathyroidism after Thyroidectomy
- Most common complication of thyroid surgery (especially total thyroidectomy)
- Caused by inadvertent removal or devascularisation of parathyroid glands
- Tetany occurs within 24–48 hours postoperatively
- Chvostek’s sign: tapping over the facial nerve at the parotid → facial muscle twitch
- Trousseau’s sign: inflating BP cuff above systolic pressure for 3 minutes → carpal spasm (main d’accoucheur)
- Treatment: IV calcium gluconate acutely; long-term oral calcium and vitamin D
Thyrotoxicosis (Hyperthyroidism)
Most common causes:
- Graves’ disease (diffuse toxic goitre) — autoimmune; TSH receptor antibodies (thyroid-stimulating immunoglobulins)
- Toxic multinodular goitre (Plummer’s disease)
- Toxic adenoma (single hot nodule)
Features: Weight loss despite increased appetite, heat intolerance, sweating, palpitations, tremor, anxiety, diarrhoea, goitre, exophthalmos (Graves’ only).
Pretibial myxoedema: Infiltrative dermopathy — raised, non-pitting skin thickening over shins; pathognomonic of Graves’ disease.
Treatment: Antithyroid drugs (carbimazole/propylthiouracil), radioiodine (¹³¹I), surgery.
Surgical Landmarks — Thyroidectomy
- Kocher’s incision: Transverse skin crease incision 2 cm above the sternal notch
- Sternothyroid muscle must be divided or retracted — limits upward mobilisation
- Middle thyroid vein ligated first (no accompanying artery) — allows lobe mobilisation
- Superior thyroid artery ligated at capsule of upper pole (protects EBSLN)
- Inferior thyroid artery ligated laterally — not at the gland (protects RLN and parathyroids)
- RLN identified in tracheo-oesophageal groove before ligation of any posteromedial structures
- Parathyroid glands identified and preserved with their blood supply
Thyroid Isthmus and Tracheostomy
- The isthmus lies in front of the 2nd–4th tracheal rings
- In low tracheostomy (below the isthmus): isthmus must be divided and ligated
- The thyroidea ima artery (if present) lies in front of the trachea — risk of haemorrhage if divided unrecognised
Investigations in Thyroid Disease
| Investigation | Use |
|---|---|
| TSH | Best initial screen; low in hyperthyroidism, high in hypothyroidism |
| Free T3, Free T4 | Confirm functional status |
| Thyroid antibodies (TPO, TSH-R) | Graves’ disease, Hashimoto’s |
| Ultrasound | Structure, nodule characterisation, guided FNAC |
| FNAC (fine needle aspiration cytology) | First-line investigation for nodules; distinguishes benign from malignant (except follicular) |
| Radioisotope scan (⁹⁹ᵐTc / ¹²³I) | Hot nodule (toxic), cold nodule (higher malignancy risk), ectopic thyroid |
| Serum calcitonin | Medullary carcinoma (screening in MEN2) |
| CEA | Medullary carcinoma monitoring |