What is the Thyroid Imaging Reporting and Data System (TIRADS) classification?

TIRADS Classification System

TIRADS (Thyroid Imaging Reporting and Data System) is a standardized ultrasound-based risk stratification system developed by the American College of Radiology to assess malignancy risk in thyroid nodules and guide decisions about fine needle aspiration biopsy. 1

Core Classification Structure

The ACR TIRADS system assigns points based on five ultrasound features to calculate a total score that determines the risk category: 2

• Composition: Solid nodules carry higher malignancy risk than cystic nodules; cystic nodules or those with <10% solid component are typically benign 1
• Echogenicity: Hypoechoic nodules are more suspicious, while isoechoic nodules have intermediate risk 1
• Shape: Taller-than-wide configuration increases suspicion for malignancy 2
• Margin: Smooth, well-defined borders are associated with benign nodules, while irregular or infiltrative margins suggest malignancy 1
• Echogenic foci: Presence of microcalcifications, macrocalcifications, or peripheral calcifications affects risk stratification 2

Risk Categories and Malignancy Rates

The system stratifies nodules into five categories with corresponding malignancy risks: 3, 4

• TR1 (Benign): 0% risk of malignancy
• TR2 (Not suspicious): 0-4.2% risk of malignancy
• TR3 (Mildly suspicious): 2.1-13.3% risk of malignancy
• TR4 (Moderately suspicious): 15.6-57.9% risk of malignancy
• TR5 (Highly suspicious): 68.9-100% risk of malignancy

FNA Biopsy Thresholds

The ACR TIRADS uses size-based thresholds that vary by risk category to determine when FNA is indicated: 1

• TR3 nodules: FNA recommended at ≥1.5 cm
• TR4 nodules: FNA recommended at ≥1.0 cm
• TR5 nodules: FNA recommended at ≥0.5 cm
• TR1 and TR2 nodules: No FNA recommended regardless of size

Research suggests that nodules with TIRADS 4-5 and diameter <12 mm are highly suspicious and should be considered for FNA even below standard thresholds. 5

Diagnostic Performance

The ACR TIRADS demonstrates strong diagnostic accuracy: 3, 6

• Sensitivity: 74.6-91.67% for detecting malignant nodules
• Specificity: 52.8-89.6%
• Positive predictive value: 66.6%
• Negative predictive value: 93.8%
• Inter-observer agreement: Substantial (kappa 0.77) 4

Nodules meeting FNA criteria have 3 times greater chance of malignancy compared to those that do not. 4

Clinical Application Algorithm

Initial assessment should include: 1

1. Thyroid function tests (TSH measurement) as the initial laboratory assessment
2. High-quality thyroid ultrasound with systematic evaluation of all five features
3. Assignment of TIRADS category based on cumulative point score
4. Determination of FNA indication based on category and size thresholds

For nodules not meeting FNA criteria: 1

• Ultrasound follow-up in 6-12 months is recommended
• Continued surveillance at 12-24 month intervals for stable nodules

For nodules undergoing FNA: 7

• US-guided FNA is more accurate, economical, and safer than palpation-guided approaches
• Local anesthesia (1-2% lidocaine) should be used
• Continued surveillance at 12-24 month intervals for benign FNA results

Important Limitations and Caveats

The system has several recognized limitations: 1, 8

• False-negative rate of 5-10% for FNA biopsies, requiring maintained clinical suspicion when other concerning features are present
• Cannot distinguish follicular adenoma from follicular carcinoma, which requires histologic assessment of capsular or vascular invasion
• Cytology rarely provides reliable cancer subtype information

Clinical risk factors may warrant more aggressive evaluation regardless of TIRADS score: 8

• History of head and neck irradiation
• Family history of thyroid cancer
• Suspicious physical exam findings (firm, fixed nodule, rapid growth)
• Presence of suspicious cervical lymphadenopathy

The overall rate of thyroid cancer in patients with nodules is less than 3-5%, with generally favorable prognosis, which supports the conservative size thresholds used in the system. 8