What is the Thyroid Imaging Reporting and Data System (TI‑RADS) score and how does it guide management of thyroid nodules?

Thyroid Imaging Reporting and Data System (TI-RADS) Score

What TI-RADS Is and How It Works

TI-RADS is a standardized ultrasound-based risk stratification system that assigns thyroid nodules a malignancy risk score (TR1-TR5 or categories 2-5) based on specific sonographic features, then uses size thresholds to determine which nodules warrant fine-needle aspiration biopsy. 1

The system evaluates five key ultrasound characteristics and assigns points or patterns based on their presence 2, 3:

• Composition: Solid nodules carry higher malignancy risk than cystic nodules; purely cystic nodules with <10% solid component are typically benign 1
• Echogenicity: Marked hypoechogenicity (darker than surrounding thyroid tissue) is highly suspicious; isoechoic nodules have intermediate risk 4, 1
• Shape: Taller-than-wide orientation (anteroposterior diameter exceeds transverse diameter) strongly suggests malignancy 2, 3
• Margin: Irregular, microlobulated, or infiltrative borders increase cancer probability, whereas smooth well-defined borders suggest benign disease 4, 1, 2
• Calcifications: Microcalcifications are highly specific for papillary thyroid carcinoma; coarse calcifications are less concerning 4, 2, 3

TI-RADS Categories and Malignancy Risk

The American College of Radiology system stratifies nodules into five categories with corresponding malignancy rates 1, 2, 3:

• TR2 (Category 2): Benign appearance—0% malignancy risk 5, 3
• TR3 (Category 3): Mildly suspicious—<2% malignancy risk 2, 5, 3
• TR4 (Category 4): Moderately suspicious—subdivided into 4A (2-5%), 4B (5-50%), and 4C (50-90%) based on number of suspicious features 2, 3
• TR5 (Category 5): Highly suspicious—≥90% malignancy risk when five suspicious features are present 2, 3

Research validation demonstrates that TI-RADS achieves 95-97% sensitivity, 68-90% specificity, and 99% negative predictive value for detecting thyroid cancer 6, 3.

How TI-RADS Guides FNA Biopsy Decisions

The American College of Radiology recommends performing ultrasound-guided FNA based on both TI-RADS category AND nodule size, using progressively lower size thresholds for higher-risk categories. 1

Size-Based FNA Thresholds by Category

• TR3 nodules: Perform FNA if ≥1.5 cm 1
• TR4 nodules: Perform FNA if ≥1.0 cm 4, 1
• TR5 nodules: Perform FNA if ≥0.5 cm 1
• Any nodule ≥4 cm: Perform FNA regardless of ultrasound appearance due to increased false-negative rate 4

Critical Exception for Nodules <1 cm

Do NOT perform FNA on nodules <1 cm based solely on suspicious ultrasound features, even if classified as TR4 or TR5, unless high-risk clinical factors are present. 4, 1 This prevents overdiagnosis of clinically insignificant papillary microcarcinomas that do not impact mortality or quality of life 7, 1.

High-risk clinical factors that lower the FNA threshold for small nodules include 4:

• History of head and neck irradiation (increases malignancy risk 7-fold)
• Family history of thyroid cancer, particularly medullary carcinoma or familial syndromes
• Suspicious cervical lymphadenopathy on ultrasound
• Subcapsular location (increases extrathyroidal extension risk)
• Age <15 years (higher baseline malignancy probability)

Management Algorithm Based on TI-RADS

For Nodules Meeting FNA Criteria

1. Perform ultrasound-guided FNA (superior to palpation-guided biopsy for accuracy) 4
2. Measure serum TSH to determine if the nodule is autonomously functioning 4, 1
3. Assess cervical lymph nodes for suspicious features on complete neck ultrasound 4
4. Consider serum calcitonin measurement to screen for medullary thyroid cancer (higher sensitivity than FNA alone) 4

For Nodules NOT Meeting FNA Criteria

• TR3 nodules <1.5 cm: Ultrasound follow-up at 6-12 months, then 12-24 month intervals if stable 1
• TR4 nodules <1.0 cm without high-risk factors: Ultrasound surveillance rather than immediate FNA 4, 1
• TR5 nodules <0.5 cm without high-risk factors: Active surveillance is safe; progression rates are manageable through monitoring 4

Management Based on FNA Results (Bethesda Classification)

• Bethesda II (Benign): Continue surveillance with repeat ultrasound at 12-24 months; malignancy risk only 1-3% 4
• Bethesda III/IV (Indeterminate): Consider molecular testing (BRAF, RAS, RET/PTC, PAX8/PPARγ) or repeat FNA 4, 1
• Bethesda V/VI (Suspicious/Malignant): Immediate referral for total or near-total thyroidectomy 4

Important Limitations and Pitfalls

TI-RADS has a 5-10% false-negative rate for FNA, meaning a reassuring biopsy should not override worrisome clinical findings. 4, 1 False-negative results occur in up to 11-33% of cases when clinical suspicion is high 4.

Additional limitations include 1, 8:

• Cannot distinguish follicular adenoma from follicular carcinoma (requires histologic assessment of capsular/vascular invasion)
• Insufficient sensitivity for follicular variant of papillary carcinoma 8
• Inter-observer variability in ultrasound interpretation persists despite standardization 8
• Cytology rarely provides reliable cancer subtype information preoperatively 7, 1

Common Diagnostic Pitfalls

• Degenerating shrinking nodules may mimic suspicious features 8
• Confusion between individual and coalescent nodules affects accurate categorization 8
• Central hypervascularity in Hashimoto's thyroiditis may be mistaken for malignancy unless associated with a discrete nodule with irregular borders and microcalcifications 4

The Paradox of Small High-Risk Nodules

Current guidelines create a clinical dilemma: TI-RADS does not recommend FNA for nodules <1 cm to avoid overdiagnosis, yet nonsurgical treatments like thermal ablation require confirmed malignancy before treatment. 7, 1 This means patients cannot be staged without surgery, and risk stratification requires postoperative histology, creating an impossible situation where alternative treatments require preoperative cancer confirmation 7.

For nodules <1 cm classified as TR5, active surveillance remains a safe alternative to immediate FNA, with progression rates manageable through monitoring 4. However, this approach requires coordinated guidelines that reconcile the competing recommendations from radiology, endocrinology, and surgical societies 7.