What is the recommended ultrasound Thyroid Imaging Reporting and Data System (TIRADS) approach for evaluating and managing a thyroid nodule, including indications for fine‑needle aspiration (FNA) and observation?

TIRADS and Thyroid Nodule Management

Core TIRADS Principle and FNA Indications

TIRADS systems standardize ultrasound reporting and use nodule size combined with suspicious features to determine FNA thresholds, with the fundamental rule that nodules <1 cm generally do not require FNA unless they are subcapsular or have suspicious cervical lymphadenopathy—even when classified as high-risk by TIRADS. 1

The primary goal of TIRADS is to minimize unnecessary FNA procedures while maintaining diagnostic accuracy for clinically significant thyroid malignancies. 1

Size-Based FNA Thresholds by TIRADS Category

For Nodules ≥1 cm:

• TIRADS 2 (benign pattern): No FNA indicated; surveillance only 2
• TIRADS 3 (low suspicion): Surveillance recommended rather than immediate FNA unless additional high-risk clinical features present 2
• TIRADS 4 (intermediate-to-high suspicion): Proceed with ultrasound-guided FNA when solid composition and hypoechoic appearance are present 2
• TIRADS 5 (high suspicion): FNA recommended for nodules ≥1 cm 2

For Nodules <1 cm:

• General rule: FNA not recommended for non-subcapsular nodules classified as cT1a cN0, even if high-risk by TIRADS 1
• Exceptions requiring FNA despite size <1 cm: 2
  • Subcapsular location
  • Suspicious metastatic cervical lymphadenopathy
  • History of head and neck irradiation (increases malignancy risk 7-fold) 2
  • Family history of thyroid cancer, particularly medullary carcinoma or familial syndromes 2
  • Age <15 years or male gender 2

Suspicious Ultrasound Features That Increase Malignancy Risk

When performing TIRADS classification, document these high-risk sonographic features: 2

• Microcalcifications: Highly specific for papillary thyroid carcinoma (psammoma bodies) 2
• Marked hypoechogenicity: Solid nodules darker than surrounding thyroid parenchyma 2
• Irregular or microlobulated margins: Infiltrative borders rather than smooth contours 2
• Absence of peripheral halo: Loss of thin hypoechoic rim normally surrounding benign nodules 2
• Central hypervascularity: Chaotic internal vascular pattern (peripheral vascularity only is reassuring) 2
• Solid composition: Higher malignancy risk compared to cystic nodules 2

Algorithmic Approach to FNA Decision-Making

Step 1: Measure nodule size and classify by TIRADS

• Use high-resolution ultrasound with high-frequency transducer 2
• Document all suspicious features systematically 2

Step 2: Apply size-specific FNA thresholds

• Nodules >4 cm: Perform FNA regardless of ultrasound appearance due to increased false-negative rate 2
• Nodules 1-4 cm: Perform FNA if ≥2 suspicious ultrasound features present (solid, hypoechoic, irregular margins, microcalcifications, central hypervascularity) 2
• Nodules <1 cm: Perform FNA only if suspicious features PLUS high-risk clinical factors present 2

Step 3: Assess clinical context that modifies risk

• History of head/neck irradiation 2
• Family history of thyroid cancer 2
• Rapidly growing nodule 2
• Firm, fixed nodule on palpation (suggests extrathyroidal extension) 2
• Vocal cord paralysis or compressive symptoms (suggests invasive disease) 2
• Suspicious cervical lymphadenopathy 2
• Focal FDG uptake on PET scan 2

Technical Aspects of FNA

• Ultrasound guidance is mandatory: Allows real-time needle visualization, confirms accurate sampling, and is superior to palpation-guided biopsy 2
• Target the solid portion in mixed solid-cystic nodules, as this carries highest malignancy risk 2
• Measure serum calcitonin as part of diagnostic workup to screen for medullary thyroid cancer, which has higher sensitivity than FNA alone (detects 5-7% of thyroid cancers that FNA may miss) 2
• For inadequate samples: Repeat FNA under ultrasound guidance; if repeat remains nondiagnostic, consider core needle biopsy 2

Management Based on Bethesda Classification Results

• Bethesda II (benign): Surveillance with repeat ultrasound at 12-24 months; malignancy risk only 1-3% 2
• Bethesda III (AUS/FLUS) or IV (follicular neoplasm): Consider molecular testing (BRAF, RAS, RET/PTC, PAX8/PPARγ) to refine malignancy risk; 97% of mutation-positive nodules are malignant 2
• Bethesda V (suspicious) or VI (malignant): Immediate referral for total or near-total thyroidectomy with pre-operative assessment of lymph node compartments 2

Critical Pitfalls to Avoid

• Do not perform FNA on nodules <1 cm without high-risk features: This leads to overdiagnosis and overtreatment of clinically insignificant papillary microcarcinomas 1, 2
• Do not override a reassuring FNA when worrisome clinical findings persist: False-negative results occur in 11-33% of cases 2
• Do not rely on thyroid function tests (TSH, T3, T4) for malignancy assessment: Most thyroid cancers present with normal thyroid function 2
• Do not use radionuclide scanning in euthyroid patients to determine malignancy risk: Ultrasound features are far more predictive 2

Comparison of TIRADS Systems

Multiple TIRADS systems exist (ACR TI-RADS, European TI-RADS, Korean TI-RADS, ATA guidelines), with varying performance characteristics: 3

• ATA guidelines: Highest diagnostic accuracy (AUC 0.715) with lowest false-negative rate (7.2%), but would avoid FNA in only 20% of nodules 3
• ACR TI-RADS: Would avoid FNA in 34.7% of nodules but with higher false-negative rate (24%) 3
• Korean TI-RADS: Most conservative approach, avoiding FNA in only 6% with lowest false-negative rate (1.9%) 3

The choice of system depends on institutional preference, but all appropriately classify nodules for malignancy risk. 3

The Paradox of Current Guidelines for Small Nodules

A significant clinical dilemma exists for nodules <1 cm: 1

• TIRADS recommends against FNA for most nodules <1 cm to avoid overdiagnosis 1
• Yet thermal ablation guidelines require confirmed malignancy before treatment 1
• Risk stratification (low-, intermediate-, high-risk) requires postoperative histology and cannot be applied preoperatively 1
• Cancer subtype cannot be reliably determined on cytology alone (papillary carcinoma is well-detected, but follicular carcinoma remains indeterminate, and medullary carcinoma is diagnosed in only 50% of cases) 1

This creates an impossible situation where alternative treatments to surgery require preoperative cancer confirmation, yet guidelines discourage obtaining that confirmation for small nodules. Coordinated guidelines are urgently needed to resolve this contradiction. 1