Imaging of endocrine organs (to assess structure) - ultrasound
Structural, not functional
- Structural test only. It cannot tell you thyroid function
- TSH decides whether you image with US or scintigraphy at all
The rule
| Biochemistry | Imaging |
|---|---|
| Euthyroid + nodule/goitre | Ultrasound |
| TSH suppressed (hyperthyroid) | Scintigraphy (Tc-99m/I-123) |
| Hypothyroid | Neither routinely - antibodies |
- US indications
- Palpable nodule or goitre in a euthyroid patient
- Incidentaloma on CT/MRI/PET
- Cervical lymphadenopathy
- Thyroid cancer surveillance (neck US +/- Tg)
- Guidance for FNA
- Parathyroid localisation (with sestamibi)
- Can assess vascularity on Doppler, but cannot reliably separate thyroiditis from active Graves' - both can be hypoechoic and hypervascular. Use TRAb or scintigraphy.
Epidemiology
- Nodules on US in ~50-68% of adults (palpable in only ~5%)
- Prevalence rises with age, female sex, iodine deficiency, prior neck irradiation
- ~5-10% of nodules are malignant - the number does not rise with nodule number
- Incidental thyroid findings on carotid US/CT/PET ~2-16%
- The epidemic of small papillary carcinoma is an imaging epidemic - mortality is flat
Sonographic features map to histology
- Sonographic features map to histology
- Cystic/spongiform -> colloid nodule, virtually never malignant
- Solid hypoechoic -> higher cellularity -> papillary carcinoma
- Taller-than-wide -> growth across tissue planes
- Irregular/lobulated margin, extrathyroidal extension -> invasion
- Punctate echogenic foci (microcalcification) -> psammoma bodies, papillary carcinoma
- Peripheral rim calcification -> usually benign, but can conceal malignancy
- Diffuse patterns
- Hashimoto's: heterogeneous, hypoechoic, micronodular, pseudonodules
- Graves': diffusely enlarged, hypoechoic, "thyroid inferno" on Doppler
- Subacute thyroiditis: focal hypoechoic areas, reduced vascularity, tender
ACR TI-RADS - points across 5 categories
ACR TI-RADS - points across 5 categories
Composition, echogenicity, shape, margin, echogenic foci. Sum the points.
| Category | Points | FNA if | Follow-up US if |
|---|---|---|---|
| TR1 benign | 0 | - | - |
| TR2 not suspicious | 2 | - | - |
| TR3 mildly susp. | 3 | >=2.5 cm | >=1.5 cm |
| TR4 moderately susp. | 4-6 | >=1.5 cm | >=1 cm |
| TR5 highly susp. | >=7 | >=1 cm | >=0.5 cm |
- Size alone never justifies FNA - the pattern sets the threshold
- Biopsy suspicious nodes at any size in preference to the nodule
Node features that change management
- Loss of fatty hilum, rounded shape, microcalcification, cystic change, peripheral vascularity
- -> FNA node + wash-out thyroglobulin on the needle rinse (far more sensitive than cytology alone)
Bethesda cytology (what the FNA returns)
Bethesda cytology (what the FNA returns)
| Malignancy risk | ||
|---|---|---|
| I | Non-diagnostic | repeat |
| II | Benign | 0-3% |
| III | AUS/FLUS | ~13-30% |
| IV | Follicular neoplasm | ~23-34% |
| V | Suspicious | ~67-83% |
| VI | Malignant | ~97-100% |
- FNA cannot diagnose follicular carcinoma - capsular/vascular invasion needs the whole specimen -> hemithyroidectomy
- Molecular testing (ThyroSeq, Afirma) for Bethesda III/IV to avoid diagnostic surgery - limited availability and no MBS funding in Australia
What the report should drive
- TR1-2, or below the size threshold -> no FNA
- Reassure; no routine repeat imaging for TR1/TR2
- Benign FNA (Bethesda II) -> repeat US 12-24 months; re-biopsy only if growth (>20% in 2 dimensions, or >50% volume) or new suspicious features
- Bethesda III/IV -> repeat FNA, molecular testing, or diagnostic hemithyroidectomy
- Bethesda V/VI -> surgery; pre-operative neck US mapping of central and lateral compartments is mandatory
- Active surveillance is an accepted alternative for low-risk papillary microcarcinoma (<1 cm, no extrathyroidal extension, no nodal disease)
Symptomatic benign nodules
- Large/compressive or cosmetic -> hemithyroidectomy, or ethanol ablation (cystic), or radiofrequency/microwave ablation (solid, increasingly available)
Post-thyroidectomy cancer surveillance
- Neck US + thyroglobulin + Tg antibodies - the mainstay
- Tg-Ab positive -> thyroglobulin uninterpretable; trend the antibody instead
Parathyroid
- US + sestamibi concordant -> minimally invasive parathyroidectomy
- Discordant or negative -> 4D-CT, choline PET
Associations
- Iodine deficiency, prior head/neck irradiation (esp. childhood)
- Hashimoto's thyroiditis - primary thyroid lymphoma risk
- Family history of thyroid cancer; MEN2 (medullary), FAP, Cowden, Carney complex, DICER1
- Acromegaly, obesity
- Multinodular goitre - toxic and non-toxic
- Incidental FDG-avid thyroid nodule on PET (~30% malignant - always biopsy)
Natural history
- Most benign nodules stay stable or grow slowly; ~10-15% regress
- False-negative FNA rate ~1-3% with adequate sampling
- Papillary microcarcinoma under active surveillance: <5% grow >3 mm and <2% develop nodal disease at 5-10 yr
- Overdiagnosis is the principal harm of unselected thyroid imaging
- -> do not order thyroid US to investigate a normal TSH, fatigue or weight change
- US-detected recurrence after thyroidectomy is usually nodal and salvageable
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12 more sections, plus exam facts
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