Quick Reference — Size Thresholds
CT or MRI
| Patient | Condition | Action |
|---|---|---|
| Any | Suspicious features present | Thyroid US (any size) |
| Any | Limited life expectancy / major comorbidities | No further evaluation |
| General, age <35 yr | No suspicious features | Thyroid US if ≥1 cm |
| General, age ≥35 yr | No suspicious features | Thyroid US if ≥1.5 cm |
Ultrasound
| Patient | Condition | Action |
|---|---|---|
| Any | Suspicious sonographic features present | Dedicated thyroid US (any size) |
| Any | Limited life expectancy / major comorbidities | No further evaluation |
| General, age <35 yr | No suspicious features | Thyroid US if ≥1 cm |
| General, age ≥35 yr | No suspicious features | Thyroid US if ≥1.5 cm |
FDG-PET
| Uptake Pattern | Patient | Action |
|---|---|---|
| Focal | Limited life expectancy / major comorbidities | No further evaluation |
| Focal | General population | Thyroid US (malignancy risk ~30–35%) |
| Diffuse | Any | Thyroid function tests; no routine imaging follow-up |
| No metabolic activity (PET/CT or PET/MRI) | Any | Apply CT/MRI size thresholds above |
Incidental on CT or MRI
First: check for suspicious features. If any of the following are present, refer for thyroid ultrasound regardless of nodule size or patient age:
- Irregular margins
- Microcalcifications
- Extrathyroidal extension
- Cervical lymphadenopathy
If no suspicious features are present:
- Limited life expectancy or significant comorbidities → no further evaluation, regardless of size
- General population, age <35 yr → thyroid ultrasound if nodule ≥1 cm; no evaluation if <1 cm
- General population, age ≥35 yr → thyroid ultrasound if nodule ≥1.5 cm; no evaluation if <1.5 cm
The age split at 35 years reflects the lower prior probability of clinically significant thyroid cancer in younger patients relative to the high background prevalence of incidental nodules. The higher threshold in older patients reduces unnecessary workup.
Incidental on Ultrasound
This algorithm applies when a thyroid nodule is found incidentally while imaging extra-thyroidal structures (e.g., carotid duplex, neck soft tissue, parathyroid scintigraphy).
First: check for suspicious sonographic features. If any of the following are present, refer for dedicated thyroid ultrasound regardless of size or age:
- Microcalcifications
- Irregular or lobulated margins
- Taller-than-wide shape
- Extrathyroidal extension
- Abnormal cervical lymph nodes
If no suspicious features are present:
- Limited life expectancy or significant comorbidities → no further evaluation
- General population, age <35 yr → dedicated thyroid US if nodule ≥1 cm
- General population, age ≥35 yr → dedicated thyroid US if nodule ≥1.5 cm
Size thresholds are identical to CT/MRI. The modality changes the suspicious feature checklist, not the size cutoffs.
Incidental on FDG-PET
Diffuse thyroid FDG uptake: Thyroid function tests. Diffuse uptake is consistent with thyroiditis (most commonly Hashimoto's). Routine imaging follow-up is not required.
Focal thyroid FDG uptake: Malignancy risk is approximately 30–35% — this is a clinically significant finding.
- Limited life expectancy or significant comorbidities → no further evaluation
- General population → thyroid ultrasound
Nodule identified on PET/CT or PET/MRI without metabolic activity: Apply the CT/MRI age-stratified size thresholds above.
ACR TI-RADS — After Thyroid Ultrasound
When thyroid ultrasound is performed following any of the above triggers, the ACR Thyroid Imaging Reporting and Data System (TI-RADS) is used to stratify findings and guide biopsy decisions.
TI-RADS assigns points based on five sonographic features:
| Category | Features Assessed |
|---|---|
| Composition | Cystic, spongiform, mixed, solid |
| Echogenicity | Anechoic, hyperechoic/isoechoic, hypoechoic, very hypoechoic |
| Shape | Wider-than-tall vs. taller-than-wide |
| Margin | Smooth/ill-defined vs. lobulated/irregular vs. extrathyroidal extension |
| Echogenic foci | None/large comet-tail, macrocalcifications, peripheral calcifications, punctate echogenic foci |
Point totals map to TR1–TR5 categories, with size-dependent biopsy thresholds at each category. A TI-RADS calculator is available for point-by-point scoring.
TI-RADS applies to dedicated thyroid ultrasound — it is not used to characterize nodules on CT or MRI, which use the separate incidental findings algorithm above.
Why This Matters
Thyroid incidentalomas are found in approximately 16–18% of neck CTs performed for other reasons. The vast majority are benign, and aggressive workup of all incidental nodules leads to unnecessary procedures, cost, and patient anxiety without improving outcomes. The ACR algorithm is explicitly designed to filter out nodules where the clinical benefit of further evaluation is negligible — hence the age stratification and the comorbidity override.
The suspicious features shortcut is the most important rule to internalize: any one of those CT/MRI features (irregular margin, microcalcification, extrathyroidal extension, adenopathy) eliminates size as a consideration entirely.
Reference
Hoang JK, Langer JE, Middleton WD, et al. Managing Incidental Thyroid Nodules Detected on Imaging: White Paper of the ACR Incidental Thyroid Findings Committee. J Am Coll Radiol. 2015;12(2):143–150.