Yes, ultrasound is the essential first-line imaging study for any suspected thyroid nodule.
If you suspect a thyroid nodule, you should absolutely get an ultrasound—it is the only appropriate initial imaging modality for thyroid nodule characterization and provides superior visualization compared to CT or MRI. 1, 2
Initial Diagnostic Pathway
Start with thyroid function tests (TSH) before imaging, as the TSH result will guide your subsequent management approach. 2, 3 However, ultrasound remains indicated regardless of TSH status when a thyroid nodule is suspected. 3
If TSH is Normal or Elevated:
- Proceed directly to thyroid ultrasound for high-resolution morphological evaluation 2, 3
- Ultrasound will confirm the nodule is within the thyroid and characterize malignancy risk 3
- Radionuclide scanning has no role in euthyroid patients and wastes resources 2
If TSH is Low (Suppressed):
- Perform ultrasound first to evaluate thyroid morphology and nodules 2
- Follow with radioiodine uptake scan to determine if the nodule is hyperfunctioning ("hot") 2, 3
- Hyperfunctioning nodules rarely require biopsy due to low malignancy risk 3
Why Ultrasound is Essential
High-frequency ultrasound can detect nodules as small as 5mm and is the most accurate, safe, and cost-effective method for initial evaluation. 1 Ultrasound is sensitive for detecting clinically impalpable thyroid nodules and can characterize them as cystic, solid, or complex. 4
Key Ultrasound Features That Guide Management:
Suspicious features requiring fine-needle aspiration include: 1, 3
- Microcalcifications (highly specific for papillary thyroid carcinoma)
- Marked hypoechogenicity (darker than surrounding thyroid)
- Irregular or microlobulated margins
- Absence of peripheral halo
- Solid composition
- Central hypervascularity
- Taller-than-wide shape
Reassuring features suggesting benign pathology include: 1
- Smooth, regular margins with thin halo
- Peripheral vascularity only
- Cystic or spongiform appearance
What Happens After Ultrasound
Fine-needle aspiration biopsy (FNA) should be performed for: 1, 3, 5
- Any nodule >1 cm with suspicious ultrasound features
- Nodules <1 cm with suspicious features plus high-risk clinical factors (head/neck irradiation, family history, suspicious lymph nodes)
- Any nodule >2 cm even without suspicious features (due to increased malignancy risk) 1
- Nodules >4 cm regardless of ultrasound appearance 1
Ultrasound-guided FNA is the preferred diagnostic method due to its accuracy, economy, safety, and effectiveness. 1 Ultrasound guidance improves FNA accuracy compared to palpation-guided biopsy. 3
Critical Clinical Context
Size alone is inversely related to malignancy risk—nodules <2 cm have the highest malignancy rate (30%), while larger nodules ≥2 cm have lower rates (20%). 6 However, size should not be considered an independent risk factor in isolation; the constellation of ultrasound features and clinical findings should guide FNA decisions. 6
Ultrasound size correlates well with final pathologic size (R² = 0.748), making it reliable for surgical planning. 6
Common Pitfalls to Avoid
- Never proceed directly to radionuclide scanning without checking TSH first 2
- Never use radionuclide scanning to determine malignancy in euthyroid patients—it has low positive predictive value and cannot distinguish benign from malignant nodules 2
- Never skip ultrasound in favor of other imaging modalities like CT or MRI for initial thyroid nodule evaluation 1, 2
- Never assume larger nodules are more dangerous—smaller nodules actually have higher malignancy rates 6
Special Populations
For pregnant women and children, the evaluation approach is similar to nonpregnant adults, with ultrasound as the first-line imaging and FNA for suspicious nodules. 5 The exception is that molecular testing has not been validated in these populations. 5