Thyroid and parathyroid anatomy
Thyroid and parathyroid anatomy and the laryngeal nerves: FCPS Part 1
Thyroid and parathyroid anatomy, blood supply and development, and the superior and recurrent laryngeal nerves, for FCPS Part 1. Explains nerve injury at thyroidectomy, post-operative hypocalcaemia, goitre signs and tumour spread.
One of the most tested topics in past papers
Thyroid gland
- Two lateral lobes joined by an isthmus in front of the 2nd to 4th tracheal rings. A pyramidal lobe may rise from the isthmus (lower thyroglossal duct remnant).
- Lobes extend from the oblique line of the thyroid cartilage to about the 6th tracheal ring.
- Enclosed in the pretracheal fascia, which attaches to the larynx, so the gland moves up on swallowing. The posteromedial capsule is fixed to the cricoid and upper trachea by the suspensory ligament of Berry.
- Relations: in front, strap muscles (sternothyroid, sternohyoid, omohyoid) and sternocleidomastoid; medially, larynx, trachea, pharynx, oesophagus and recurrent laryngeal nerves; posterolaterally, the carotid sheath.
- Development: endodermal diverticulum at the foramen caecum of the tongue, descending as the thyroglossal duct. A thyroglossal cyst moves up on swallowing and on tongue protrusion. C cells come from the ultimobranchial body (4th pouch, neural crest) and make calcitonin.
- Retrosternal goitre can compress the thoracic inlet: raising both arms (narrowing the inlet further) causes facial plethora, distended neck veins and stridor (Pemberton sign).
- Physiology link: the gland releases mostly T4; the active T3 is made mainly in peripheral tissues by deiodinases, which also inactivate hormone (reverse T3). Hyperthyroidism raises heart rate partly through increased β-adrenergic receptor sensitivity.
Blood supply
| Vessel | Origin | Surgical point |
|---|---|---|
| Superior thyroid artery | First branch of the external carotid | External laryngeal nerve runs close to it at the upper pole: ligate close to the gland |
| Inferior thyroid artery | Thyrocervical trunk of the subclavian | Recurrent laryngeal nerve crosses its branches; it also supplies both parathyroids: ligate branches close to the gland capsule (capsular dissection) to save nerve and parathyroid supply |
| Thyroidea ima (about 10%) | Brachiocephalic trunk or arch | Runs up in front of the trachea; tracheostomy hazard |
| Superior and middle thyroid veins | Internal jugular vein | |
| Inferior thyroid veins | Brachiocephalic veins | In front of the trachea below the isthmus |
Laryngeal nerves
| Nerve | Course | Supplies | Injury |
|---|---|---|---|
| Internal laryngeal (superior laryngeal branch) | Pierces thyrohyoid membrane | Sensation above the vocal cords | Loss of cough reflex, aspiration |
| External laryngeal | With the superior thyroid artery | Cricothyroid (tenses the cords) | Weak, tiring voice; loss of high pitch |
| Recurrent laryngeal | Right: under the right subclavian artery. Left: under the aortic arch beside the ligamentum arteriosum. Both rise in the tracheo-oesophageal groove | All intrinsic muscles except cricothyroid; sensation below the cords | Unilateral: hoarseness. Bilateral: cords near the midline, stridor and airway obstruction |
- The recurrent nerve is most at risk at the ligament of Berry, where it passes close to (or through) the ligament as the lobe is lifted off the trachea.
- A non-recurrent right laryngeal nerve occurs when there is an aberrant right subclavian artery (retro-oesophageal): the nerve goes straight from the vagus to the larynx.
- The long left nerve can be compressed in the chest by lung cancer, nodes, aortic aneurysm or a large left atrium.
- Posterior cricoarytenoid is the only abductor of the vocal cords.
Parathyroid glands
- Usually four, yellowish-brown, about 5 mm, behind the thyroid lobes.
- Superior parathyroids come from the 4th pouch; their position is fairly constant (near the cricothyroid junction, above the inferior thyroid artery).
- Inferior parathyroids come from the 3rd pouch with the thymus; they migrate further, so their position varies (lower pole, thyrothymic ligament, mediastinal thymus).
- Both pairs are supplied mainly by the inferior thyroid artery.
Hypocalcaemia after thyroidectomy
- Usually temporary parathyroid ischaemia from handling and ligation; calcium falls on day 1–2, PTH is low, and function returns within weeks (as when no parathyroid tissue is found in the specimen). Removal of all glands causes permanent hypoparathyroidism.
- Why it causes tetany: low extracellular calcium raises nerve membrane permeability to sodium and lowers the threshold, so nerves fire spontaneously: perioral and fingertip tingling, Chvostek sign (tap over the facial nerve in front of the ear twitches the lip), Trousseau sign (carpopedal spasm with a BP cuff). Alkalosis lowers ionised calcium and can cause tetany with normal total calcium.
- Check calcium; treat with calcium and alfacalcidol (IV calcium gluconate if severe).
Hyperparathyroidism
| Type | Calcium | Phosphate | PTH | Cause |
|---|---|---|---|---|
| Primary | High | Low | High or inappropriately normal | Single adenoma (about 85%); localise with technetium-99m sestamibi and ultrasound before focused surgery |
| Secondary | Low or normal | High (in CKD) | Very high | CKD: phosphate retention and low calcitriol drive all four glands to enlarge |
| Tertiary | High | Variable | Very high | Autonomous glands after long secondary disease |
| Familial hypocalciuric hypercalcaemia | Mildly high | Low-normal | Slightly high | Calcium-sensing receptor mutation; urinary calcium/creatinine clearance ratio below 0.01; surgery does not help |
Thyroid nodules and tumours
- Solitary nodule, normal TSH: ultrasound with fine-needle aspiration cytology. Low TSH: isotope scan first ("hot" nodules are rarely malignant).
- Toxic multinodular goitre: older woman, long-standing goitre, thyrotoxicosis; scan shows several hot nodules with suppressed background.
| Tumour | Spread and features |
|---|---|
| Papillary | Commonest; lymphatic, first to central compartment (level VI) nodes; psammoma bodies, Orphan Annie nuclei; radiation exposure |
| Follicular | Blood-borne to bone (skull, lytic) and lung; FNAC cannot separate adenoma from carcinoma, which needs capsular or vascular invasion on histology |
| Medullary | C cells, calcitonin, amyloid; MEN 2 |
| Anaplastic | Elderly, rapid, poor prognosis |
| Primary lymphoma | Rapidly enlarging goitre in an older woman with long-standing Hashimoto thyroiditis; B-cell (CD20); needs core biopsy |
Complications of thyroidectomy
Haematoma under the strap muscles compressing the airway (open the wound at the bedside), recurrent and external laryngeal nerve injury, hypocalcaemia, hypothyroidism, thyroid storm, tracheomalacia after long-standing goitre.
Remember Upper pole and superior thyroid artery: external laryngeal nerve (pitch). Lower pole and inferior thyroid artery: recurrent laryngeal nerve (hoarseness).
Exam trap Inferior parathyroids come from the 3rd pouch; superior from the 4th.
Exam trap Bilateral recurrent nerve injury threatens the airway; unilateral injury affects the voice.
How FCPS asks about this
- Level of the isthmus: 2nd to 4th tracheal rings; moves on swallowing because of the pretracheal fascia.
- Where the recurrent laryngeal nerve is most at risk: at the ligament of Berry; non-recurrent with an aberrant right subclavian.
- Nerve supplying cricothyroid: external laryngeal; injury loses high-pitched voice.
- Tingling and Chvostek sign on day 2 after thyroidectomy: hypocalcaemia from parathyroid ischaemia; check calcium.
- Facial plethora and stridor on raising arms: retrosternal goitre (Pemberton sign).
- First nodes for papillary carcinoma, and skull metastasis with follicles: level VI; follicular carcinoma.
- Localising a parathyroid adenoma: sestamibi scan; low urinary calcium with familial hypercalcaemia means FHH.
Test yourself on this topic
5 of the 61 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
During an elective tracheostomy through the third and fourth tracheal rings, there is troublesome venous bleeding from vessels running down in front of the trachea below the thyroid isthmus. Which of the following veins is most likely injured?
Answer: Inferior thyroid veins
The inferior thyroid veins leave the lower border of the isthmus and run down in front of the trachea, often forming a plexus, to drain into the brachiocephalic veins. They are the commonest source of venous bleeding when the trachea is opened below the isthmus. The middle thyroid vein runs laterally to the internal jugular vein and the superior thyroid vein leaves the upper pole, away from the tracheostomy site.
Bleeding at tracheostomy
- Below isthmus: inferior thyroid veins
- Drain to brachiocephalic veins
- Rarely: thyroidea ima artery
Remember Venous plexus in front of the lower trachea: inferior thyroid veins.
Exam trap In children the brachiocephalic vein can rise above the sternal notch and be injured.
Why the other options are wrong
- Middle thyroid vein Runs laterally from the lobe to the IJV.
- Superior thyroid vein Leaves the upper pole with the superior thyroid artery.
- External jugular vein Lies superficial to the SCM, lateral to the operative field.
- Internal jugular vein Lies in the carotid sheath, lateral to the trachea.
A 7-year-old has a midline neck cyst below the hyoid that rises when he protrudes his tongue. In the planned operation, the central part of the hyoid body is removed with the cyst. What is the reason for this?
Answer: The duct tract runs through the hyoid body
The thyroid gland descends from the foramen caecum at the back of the tongue, leaving the thyroglossal duct. As the hyoid bone forms, the duct becomes closely related to its body, passing in front of, behind or through it. A thyroglossal cyst arises from a persistent part of the duct. The Sistrunk operation removes the cyst, the central hyoid and a core of tissue up towards the foramen caecum, because leaving any duct remnant leads to recurrence.
Sistrunk operation
- Duct from foramen caecum
- Tract runs with the hyoid body
- Remove cyst, central hyoid, tract
- Simple excision recurs
Remember Thyroglossal cyst: take the middle of the hyoid.
Exam trap Check that normal thyroid is present before excision; the mass could be ectopic thyroid.
Why the other options are wrong
- The hyoid is usually fractured by the cyst The cyst does not fracture the hyoid.
- It gives access to the thyroid isthmus The isthmus lies lower and is not reached through the hyoid.
- It protects the hypoglossal nerves The nerves lie laterally and are not protected by this step.
- It prevents injury to the lingual artery The lingual arteries lie laterally, away from the midline.
Papillary thyroid cancer most often spreads first to which neck node group?
Answer: Central compartment (level VI)
The thyroid drains first to the central compartment (level VI): pretracheal (Delphian), paratracheal and prelaryngeal nodes, and then to the lateral deep cervical chain (levels II–IV) and upper mediastinum (level VII). Papillary carcinoma spreads by lymphatics, so central nodes are checked and cleared when involved. Level V involvement indicates more extensive lateral disease.
Thyroid lymph drainage
- First: level VI (central)
- Includes pretracheal, paratracheal
- Then levels II–IV laterally
- Below: level VII mediastinum
Remember Thyroid drains to the centre first.
Exam trap Follicular carcinoma spreads mainly by blood (bone, lung), not lymph.
Why the other options are wrong
- Submental nodes (level IA) Drains the chin and lower lip.
- Posterior triangle (level V) Involved later, if at all.
- Upper deep cervical (level II) Lateral spread comes after central.
- Parotid nodes Drain the face and scalp.
A woman with a large goitre develops facial plethora, distended neck veins and stridor when she raises both arms above her head. What does this sign indicate?
Answer: Retrosternal goitre
A retrosternal goitre extends through the thoracic inlet (bounded by T1, the first ribs and the manubrium) behind the sternum. Raising the arms narrows the inlet further, compressing the great veins and trachea: facial congestion, venous distension and stridor (Pemberton's sign). Retrosternal goitres can cause dysphagia and airway obstruction and usually require surgery, often through a cervical incision. Lower border cannot be felt above the clavicle.
Retrosternal goitre
- Extends through thoracic inlet
- Pemberton's sign: arms up → plethora, stridor
- Cannot feel the lower border
Remember Arms up, face goes red: goitre in the inlet.
Why the other options are wrong
- Thyroid storm Fever, tachycardia, agitation; not positional.
- Recurrent laryngeal palsy Hoarseness, not positional plethora.
- Hypocalcaemia Tetany, not plethora.
- Unrelated SVC thrombosis The sign points to inlet compression by the goitre.
A 68-year-old woman with Hashimoto thyroiditis on levothyroxine for 15 years has a thyroid swelling that has grown rapidly over 6 weeks, with pressure symptoms. Core biopsy shows sheets of large atypical lymphoid cells positive for CD20. Which of the following is the most likely diagnosis?
Answer: Primary thyroid lymphoma
Chronic autoimmune (Hashimoto) thyroiditis fills the gland with lymphoid tissue and is the main risk factor for primary thyroid lymphoma, usually a B-cell non-Hodgkin lymphoma such as diffuse large B-cell or MALT lymphoma. It presents in older women as a rapidly enlarging goitre. CD20-positive large lymphoid cells confirm a B-cell lymphoma. Unlike anaplastic carcinoma, it responds well to chemotherapy and radiotherapy.
Thyroid lymphoma
- Background Hashimoto
- Rapid goitre in older woman
- B-cell: CD20
- Treat with chemo-radiotherapy
Remember Fast goitre in Hashimoto: biopsy for lymphoma.
Exam trap Core biopsy, not FNAC alone, is usually needed to type the lymphoma.
Why the other options are wrong
- Anaplastic carcinoma Also grows rapidly in older people, but cells are epithelial, not CD20-positive.
- Riedel thyroiditis Causes a woody fibrous gland, not sheets of lymphoid cells.
- Papillary carcinoma Shows papillae and characteristic nuclei.
- Subacute thyroiditis Causes a painful gland with granulomas.
Where this sits in the CPSP syllabus
- Paper 1: Head, neck and back, Thyroid and parathyroid glands; laryngeal nerves
- Surgery & Allied, Paper 2: Head and neck, Thyroid and parathyroid glands; laryngeal nerves
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