What is the purpose and application of the Thyroid Imaging Reporting and Data System (TI-RADS) classification in managing thyroid nodules?

TI-RADS Classification: Purpose and Clinical Application

Primary Purpose

TI-RADS is a standardized ultrasound-based risk stratification system designed to assess malignancy risk in thyroid nodules and determine which nodules require fine needle aspiration (FNA) biopsy, thereby reducing unnecessary biopsies of benign lesions. 1

The system emerged to standardize terminology in thyroid ultrasound reports and minimize unnecessary FNA procedures that would result in benign/nonneoplastic cytology 2. This addresses a critical clinical need, as thyroid nodules are extremely common (detected in up to 12% of populations) but thyroid cancer occurs in less than 3-5% of nodules 2, 3.

Risk Stratification Categories

TI-RADS classifies nodules into five risk categories (TR1-TR5) based on ultrasound features, with progressively increasing malignancy risk 1, 4:

• TR3 (Low suspicion): Malignancy risk 1.0-1.7% 5, 6
• TR4 (Intermediate suspicion): Malignancy risk 7.8-11.2% 5, 6
• TR5 (High suspicion): Malignancy risk 60.2-84.7% 5, 6

Key Ultrasound Features Assessed

The system evaluates specific sonographic characteristics that carry different malignancy weights 1, 5:

High-risk features (strongest predictors):

• Marked hypoechogenicity (compared to overlying strap muscle) 5
• Microcalcifications 2, 5
• Irregular or microlobulated margins 5

Intermediate-risk features:

• Solid composition (higher risk than cystic) 1
• Taller-than-wide shape 1

Low-risk features:

• Cystic or <10% solid component 1
• Smooth, well-defined borders 1
• Isoechoic appearance 1

FNA Biopsy Thresholds

The ACR TI-RADS uses size-dependent thresholds that vary inversely with risk category 1:

• TR3 nodules: FNA if ≥1.5 cm 1
• TR4 nodules: FNA if ≥1.0 cm 1
• TR5 nodules: FNA if ≥0.5 cm 1

This approach recognizes that smaller nodules require higher suspicion levels to warrant biopsy 2. Notably, non-subcapsular nodules <1 cm (cT1a cN0) are generally recommended for surveillance rather than FNA, even if high-risk by TI-RADS 2.

Exceptions Requiring FNA Despite Size

FNA should be performed regardless of size thresholds when 7:

• History of cervical or head/neck irradiation exists 7
• Suspicious cervical lymphadenopathy is present 7
• Subcapsular nodule location 2, 7
• Documented rapid growth during surveillance 7

Surveillance Protocol

For nodules not meeting FNA criteria 1, 7:

• Initial follow-up: Ultrasound at 6-12 months 1, 7
• Continued surveillance: 12-24 month intervals if stable 1, 7
• Benign FNA results: Continue surveillance at 12-24 month intervals 1

Diagnostic Performance

The system demonstrates strong clinical utility 4, 3:

• Sensitivity: 80-98% for malignancy detection 8, 3
• Specificity: 56-79% 8, 3
• Positive predictive value: 84% for TI-RADS ≥4 3
• Negative predictive value: 67-97.9% 8, 3

Critical Limitations and Pitfalls

Important caveats to recognize 2, 1:

• 5-10% false-negative rate for FNA exists 1
• Cannot distinguish follicular adenoma from follicular carcinoma (requires histologic assessment of capsular/vascular invasion) 1
• Cytology rarely provides reliable cancer subtype information 2, 1
• Ultrasound risk assessment is independent of nodule size - an 8mm nodule with microcalcifications carries the same risk as a 35mm nodule with identical features 2

Initial Workup Integration

Begin with thyroid function tests (TSH measurement) as the initial laboratory assessment 1. This identifies functioning nodules that may not require FNA regardless of ultrasound appearance. All FNA procedures should be ultrasound-guided 1.