Larynx and thyroid
Larynx, thyroid and parathyroid glands: anatomy, embryology and imaging
The larynx is read on CT and MRI by its cartilages, levels and fat spaces; the thyroid and parathyroids by ultrasound, CT and nuclear medicine. CPSP links each structure to its nerve, its pharyngeal arch or pouch, and the physiology behind each scan finding.
One of the most tested topics in past papers
Laryngeal skeleton and levels
| Cartilage | Key facts |
|---|---|
| Thyroid | Unpaired, open behind; ossifies irregularly with age, mimicking erosion |
| Cricoid | Only complete ring (C6); narrowest part of the infant airway |
| Arytenoids | Paired; their vocal processes mark the level of the true cords on axial CT |
| Epiglottis | Elastic cartilage; does not ossify |
- Supraglottis: epiglottis, aryepiglottic folds, false cords, ventricle. Rich lymphatics, so tumours present late with nodes.
- Glottis: true cords. The vocal folds have very few lymphatics, so small cord cancers rarely spread to nodes and present early with hoarseness.
- Subglottis: croup gives a steeple narrowing on a frontal film; a subglottic haemangioma gives one-sided narrowing in an infant.
- Pre-epiglottic and paraglottic spaces are fat-filled; loss of their bright T1 fat signals deep tumour invasion. A laryngocele is an air-filled dilatation of the saccule of the ventricle.
Pharyngeal arches and the neck
| Arch | Nerve | Muscles | Cartilage derivatives |
|---|---|---|---|
| 1st | V3 (mandibular) | Muscles of mastication, mylohyoid, anterior digastric, tensors | Meckel cartilage: malleus, incus |
| 2nd | VII | Facial expression, stapedius, posterior digastric | Stapes, styloid process, lesser horn of hyoid |
| 3rd | IX | Stylopharyngeus | Greater horn of hyoid |
| 4th and 6th | Vagus (superior and recurrent laryngeal) | Pharyngeal constrictors, cricothyroid, intrinsic laryngeal muscles | Thyroid, cricoid, arytenoid cartilages |
- Eagle syndrome: an elongated styloid process (over about 3 cm) or calcified stylohyoid ligament irritates the glossopharyngeal nerve in the tonsil bed, giving throat pain on swallowing and turning the head, often after tonsillectomy, with referred ear pain.
- Preauricular pit: incomplete fusion of the six auricular hillocks (first and second arches), not a branchial sinus.
The pharynx below: pouch and foreign bodies
- Zenker (pharyngeal) pouch: mucosa herniates through Killian's dehiscence, between the thyropharyngeus and cricopharyngeus parts of the inferior constrictor, posteriorly. Elderly patient with halitosis, regurgitation of undigested food and a gurgling neck swelling; barium swallow shows the pouch.
- Swallowed coins lodge most often at the cricopharyngeus (C6), the narrowest part of the oesophagus.
Nerves of the larynx
| Nerve | Supplies | Imaging and clinical link |
|---|---|---|
| Internal laryngeal | Sensation above the cords | Pierces the thyrohyoid membrane; injured in the piriform fossa |
| External laryngeal | Cricothyroid (tenses cords, high pitch) | Runs with the superior thyroid artery at the upper pole |
| Recurrent laryngeal | All other intrinsic muscles; sensation below the cords | Runs with the inferior thyroid artery; at risk at the ligament of Berry |
- Posterior cricoarytenoid is the only abductor. One RLN palsy leaves the cord paramedian (breathy voice); both give stridor.
- The left RLN hooks under the arch at the ligamentum arteriosum, so CT for a left cord palsy must reach the aortopulmonary window. The right hooks round the right subclavian artery.
Thyroid: anatomy and embryology
- The isthmus lies over tracheal rings 2–4. Pretracheal fascia attaches the gland to the larynx, so it moves on swallowing.
- Arteries: superior thyroid (external carotid), inferior thyroid (thyrocervical trunk), occasional thyroidea ima. Veins: superior, middle (no companion artery) and inferior thyroid veins.
- Development: from pharyngeal floor endoderm at the foramen caecum, descending along the thyroglossal duct (remnant: pyramidal lobe). A lingual thyroid may be the only thyroid tissue, so scan before removing it. C cells come via the ultimobranchial body.
Thyroid hormone physiology
- The gland secretes mainly T4 (thyroxine); most active T3 is made peripherally by deiodinases. Hormones are inactivated by deiodination (to reverse T3) and by hepatic conjugation.
- Tachycardia in hyperthyroidism reflects increased cardiac beta-adrenergic receptor number and sensitivity, hence the benefit of beta blockers. Hypothyroidism is treated with levothyroxine.
- Congenital hypothyroidism: thyroid hormone is needed for bone maturation, so the distal femoral and proximal tibial epiphyseal ossification centres are absent at term (delayed bone age). Prolonged jaundice, large tongue, umbilical hernia and hoarse cry follow; most babies look normal at birth, hence screening.
Thyroid imaging physiology
| Finding | Explanation |
|---|---|
| Thyroid denser than muscle on unenhanced CT | Its iodine content |
| Technetium-99m pertechnetate uptake | Trapped by the sodium–iodide symporter but not organified |
| Radioiodine reduced after contrast CT | Excess stable iodide floods the gland |
| Hot nodule with the rest of the gland faint | Autonomous nodule makes hormone, suppressed TSH switches off normal tissue; rarely malignant, so FNA is not usually needed |
| High uptake with normal function in an endemic goitre valley | Iodine deficiency: TSH-driven gland traps avidly, and the small iodide pool means each tracer dose is a larger fraction |
| Very low uptake in a toxic patient | Subacute thyroiditis or amiodarone type 2: hormone leaks from damaged follicles |
| Diffuse marked Doppler flow, soft goitre | Graves disease |
- Iodine-131 destroys cells mainly by beta particles; its gamma rays allow imaging.
Thyroid ultrasound and pathology
| Condition | Imaging and pathology |
|---|---|
| Papillary carcinoma | Hypoechoic nodule with microcalcifications (psammoma bodies); RET/PTC (radiation) or BRAF V600E; level VI nodes first |
| Colloid nodule | Bright foci with comet tail artefact; benign |
| Hashimoto thyroiditis | Enlarged, diffusely hypoechoic and coarse with micronodules and bright fibrous strands; the dark gland reflects dense lymphocytic infiltration replacing reflective colloid follicles; raised TSH. A rapidly enlarging very dark mass may be lymphoma |
| Riedel thyroiditis | Stony-hard fixed gland; low-density infiltrative mass extending into strap muscles and around the carotid sheath; dense fibrosis, IgG4-positive plasma cells, obliterative phlebitis. Mimics cancer |
| Anaplastic carcinoma | Elderly woman with long-standing goitre, rapid growth, stridor; necrotic mass invading trachea and oesophagus; bizarre giant and spindle cells; TP53 inactivation |
| Medullary carcinoma | C cells, calcitonin, amyloid; RET point mutation (MEN 2) |
Parathyroids
| Gland | Origin | Position |
|---|---|---|
| Superior | Fourth pouch | Fairly constant, behind the upper thyroid, posterior to the RLN |
| Inferior | Third pouch (with the thymus) | Variable; may descend with the thymus into the anterior mediastinum |
- Ultrasound: oval hypoechoic nodule behind the thyroid with a polar feeding vessel. Sestamibi: retained because adenoma cells are rich in mitochondria. 4D-CT: avid arterial enhancement then washout.
- Hyperparathyroid bone: many tiny lucencies giving a granular (salt-and-pepper) skull vault, loss of lamina dura, subperiosteal resorption of the radial side of the middle phalanges, and brown tumours. These appear only with long-standing disease.
Remember Superior parathyroid from the fourth pouch, inferior from the third: the lower gland comes from the higher pouch because it travels with the thymus.
Exam trap Upper pole surgery risks the external laryngeal nerve (pitch); lower pole surgery risks the recurrent laryngeal nerve (hoarseness).
Exam trap Infiltration beyond the thyroid does not always mean cancer: Riedel thyroiditis does the same.
How FCPS asks about this
- Hot nodule, rest of gland invisible: suppressed TSH; rarely malignant.
- Hashimoto gland dark on ultrasound: dense lymphocytic infiltration.
- Endemic goitre, normal function: increased radioiodine uptake.
- Rapidly growing goitre in an elderly woman with giant cells: anaplastic carcinoma; TP53 loss.
- Zenker diverticulum: through Killian's dehiscence.
- First arch nerve and muscles: V3; muscles of mastication.
- Long styloid process with throat pain: glossopharyngeal nerve irritation (Eagle syndrome).
Test yourself on this topic
5 of the 90 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 6-year-old with a goitre and hypothyroidism has a radioiodine study. Uptake at 2 hours is high, but when potassium perchlorate is given, more than half of the gland's radioactivity is released within an hour. What does this result indicate?
Answer: Iodide organification defect
Perchlorate competitively blocks the sodium-iodide symporter, so iodide that has been trapped but not yet bound to thyroglobulin leaks out of the cell. In a normal gland iodide is rapidly organified by thyroid peroxidase and stays in the gland, so little is discharged. A large discharge means trapped iodide is not being organified, as in thyroid peroxidase deficiency or Pendred syndrome. The high initial uptake reflects TSH-driven trapping in a gland that cannot complete synthesis.
Perchlorate discharge test
- Perchlorate blocks the symporter
- Unbound iodide leaks out
- Large discharge: organification defect
- Causes: TPO deficiency, Pendred
Remember Iodide that washes out was never bound.
Exam trap High uptake with hypothyroidism points to a synthesis defect, not a gland that is absent.
Why the other options are wrong
- Defective sodium-iodide symporter Would give very low uptake from the start, with nothing to discharge.
- Absent thyroid tissue Would show no uptake at all.
- TSH receptor resistance Would give low uptake because the gland is not stimulated.
- Excess dietary iodine Would lower uptake, not cause a large discharge.
A radiologist is identifying the level of the true vocal cords on axial CT of the larynx. At this level the paraglottic fat is minimal and the vocalis muscle is seen. Which bony or cartilaginous landmark is also seen at this level?
Answer: Arytenoid vocal processes
On axial CT, the level of the true vocal cords is identified by the vocal processes of the arytenoid cartilages pointing forwards, to which the vocal ligaments attach, with the thyroarytenoid (vocalis) muscle filling the fold and the top of the cricoid lamina behind. At the false cord level above, the paraglottic space contains more fat and the arytenoid bodies or corniculates are seen. This distinction matters when staging tumours and judging spread across the ventricle. The anterior commissure should be thin, under about 1-2 mm, at the cord level.
True cord level on CT
- Vocal processes of arytenoids
- Vocalis muscle in the fold
- Top of cricoid lamina behind
- Little paraglottic fat
Remember Vocal processes mark the true cords.
Exam trap Mistaking the false cords for true cords leads to wrong staging of glottic tumours.
Why the other options are wrong
- Body of the hyoid bone Lies higher, above the thyroid cartilage, at the supraglottic level.
- Tip of the epiglottis The epiglottic tip lies well above, in the oropharynx.
- Corniculate cartilages Sit on the apex of the arytenoids at the false cord level and above.
- Lower border of the cricoid Lies below the glottis, at the subglottic level.
A 26-year-old woman has palpitations, heat intolerance, gritty eyes and a smooth, soft enlargement of the whole thyroid with a bruit over it. TSH is suppressed. What is the most likely cause?
Answer: Graves disease
In Graves disease, TSH receptor-stimulating antibodies drive the gland to grow and to make hormone. The hyperplastic gland is highly vascular, which causes the audible bruit; colour Doppler shows diffusely increased flow, the thyroid inferno. Eye signs come from the same antibodies acting on orbital fibroblasts. Destructive thyroiditis, by contrast, has a quiet, poorly perfused gland.
Graves disease
- TSH receptor-stimulating antibodies
- Hyperplastic, vascular gland
- Bruit over the thyroid
- Eye signs: orbital fibroblasts
Remember Diffuse goitre with a bruit and eye signs: Graves.
Exam trap A bruit means a hyperplastic, vascular gland; it is absent in destructive thyroiditis.
Why the other options are wrong
- Subacute thyroiditis Painful, tender gland after a viral illness, with no bruit.
- Hashimoto thyroiditis Usually causes hypothyroidism with a firm, rubbery goitre.
- Iodine-deficient goitre Usually euthyroid, without a bruit or eye signs.
- Postpartum thyroiditis A painless destructive thyroiditis after delivery, without eye signs.
A 62-year-old smoker has a laryngeal carcinoma. Axial CT shows tumour spreading deep to the mucosa, replacing the fat between the thyroid cartilage laterally and the false and true vocal folds medially, crossing the ventricle from supraglottis to glottis. Which space is involved?
Answer: Paraglottic space
The paraglottic space is a fat-containing space on each side of the larynx, between the thyroid cartilage laterally and the quadrangular membrane and conus elasticus medially. It runs vertically from the supraglottis past the ventricle to the glottis. Tumour that enters it can spread submucosally across the ventricle, becoming transglottic and often fixing the cord. Involvement upstages the tumour.
Paraglottic space
- Fat lateral to the folds
- Between thyroid cartilage and membranes
- Allows transglottic spread
- Involvement upstages tumour
Remember Fat beside the cords carries tumour up and down.
Exam trap Deep paraglottic spread may be missed on endoscopy, which sees only the surface.
Why the other options are wrong
- Pre-epiglottic space Lies in front of the epiglottis, not lateral to the cords.
- Reinke space A thin superficial layer within the vocal fold itself.
- Retropharyngeal space Lies behind the pharynx, outside the larynx.
- Piriform fossa A mucosal recess of the hypopharynx, not a deep laryngeal space.
A 45-year-old man on long-term dialysis has a painful expansile lytic lesion in the mandible. Biopsy shows many multinucleated giant cells in a vascular fibrous stroma with haemorrhage and haemosiderin. PTH is very high. What is the lesion?
Answer: Brown tumour
A brown tumour is a localised lesion of osteitis fibrosa cystica in severe hyperparathyroidism, primary or secondary to renal failure. Intense osteoclastic resorption, microfractures and bleeding produce a mass of giant cells, fibrous tissue and haemosiderin, which gives the brown colour. It is a reactive lesion, not a true tumour, and heals when PTH is controlled. Histology is identical to central giant cell granuloma, so biochemistry is essential.
Brown tumour
- Severe hyperparathyroidism
- Giant cells + haemosiderin
- Reactive, not neoplastic
- Heals when PTH is controlled
Remember Giant cell lesion with high PTH: brown tumour.
Exam trap Giant cell lesions of the jaw always need calcium and PTH checked before surgery.
Why the other options are wrong
- Giant cell tumour of bone Arises at the ends of long bones in young adults with normal PTH.
- Aneurysmal bone cyst Shows blood-filled spaces with fluid levels and occurs mostly in the young.
- Ameloblastoma Shows odontogenic epithelium, not giant cells with haemosiderin.
- Multiple myeloma deposit Shows sheets of plasma cells.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Head and neck, Larynx, thyroid and parathyroid glands
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