TIRADS Characteristics
Overview of TIRADS
TIRADS (Thyroid Imaging Reporting and Data System) is a standardized ultrasound-based risk stratification system that uses specific sonographic features to assess malignancy risk in thyroid nodules and guide decisions about fine needle aspiration (FNA) biopsy. 1
Core Ultrasound Features
TIRADS classification is based on five key sonographic characteristics that are scored to determine malignancy risk 2, 3:
1. Composition
- Solid nodules carry higher malignancy risk than cystic nodules 2
- Cystic nodules or those with <10% solid component are typically benign 1
- Mixed nodules require assessment of the solid component percentage 1
2. Echogenicity
- Hypoechoic or markedly hypoechoic nodules are more suspicious for malignancy 2, 3
- Isoechoic nodules have intermediate risk 4
- Hyperechoic nodules are generally associated with lower malignancy risk 2
3. Shape
- Taller-than-wide orientation (anteroposterior diameter greater than transverse diameter) is highly suspicious for malignancy 2, 3
- Wider-than-tall nodules are more likely benign 3
4. Margins
- Irregular, infiltrative, or microlobulated margins suggest malignancy 2, 3
- Smooth, well-defined borders are associated with benign nodules 4
5. Echogenic Foci (Calcifications)
- Microcalcifications are highly suspicious for papillary thyroid carcinoma 2, 3
- Macrocalcifications have intermediate risk 3
- Peripheral rim calcifications are typically benign 2
ACR TIRADS Categories and Malignancy Risk
The American College of Radiology TIRADS system stratifies nodules into five categories 5:
- TR1: Benign (0% malignancy risk) 3
- TR2: Not suspicious (0-13.2% malignancy risk) 5
- TR3: Mildly suspicious (3.6-21.7% malignancy risk) 3, 5
- TR4: Moderately suspicious (17-75% malignancy risk, mean 50.3%) 3, 5
- TR5: Highly suspicious (72.4-98% malignancy risk) 3, 5
FNA Biopsy Thresholds Based on Size and Category
The ACR TIRADS system uses nodule size thresholds that vary by risk category to determine when FNA is indicated 1, 6:
- TR3 nodules: FNA recommended only if ≥1.5 cm 6, 4
- TR4 nodules: FNA recommended if ≥1.0 cm 6
- TR5 nodules: FNA recommended if ≥0.5 cm 6
Important Size-Malignancy Relationship
Smaller thyroid nodules (<12 mm) with high TIRADS scores (4-5) are paradoxically at higher risk for malignancy than larger nodules with the same TIRADS category 7. This inverse relationship between size and malignancy risk means that a 9 mm TR5 nodule may warrant more aggressive evaluation than a 3 cm TR3 nodule 7.
Clinical Application Algorithm
Step 1: Perform thyroid function tests first
- TSH measurement is the initial laboratory assessment 1, 4
- If TSH is subnormal, radionuclide imaging may be indicated to assess for autonomously functioning nodules 1
Step 2: Ultrasound evaluation and TIRADS scoring
- Assess all five sonographic features systematically 2, 3
- Calculate total TIRADS score and assign category 2
- Measure nodule size accurately 7
Step 3: Determine FNA indication
- Apply size thresholds based on TIRADS category 6, 4
- For nodules meeting size criteria, perform US-guided FNA 1, 4
- For nodules below size threshold, schedule follow-up ultrasound in 6-12 months 6, 4
Step 4: Evaluate cervical lymph nodes
- Assess for suspicious features: microcalcification, cystic change, hyperechogenicity, abnormal blood flow, rounded shape, irregular margins 1
- If suspicious lymph nodes present, perform FNA regardless of primary nodule size 1
Diagnostic Performance
The ACR TIRADS system demonstrates 75% accuracy in predicting malignancy, with 80.3% sensitivity and 60.8% specificity when using TR4 as the optimal cutoff point 5. The system has an area under the ROC curve of 0.747, indicating good discriminatory ability 5.
Critical Pitfalls and Limitations
Size Threshold Controversy
TIRADS guidelines recommend surveillance rather than FNA for high-risk nodules <1 cm, creating a clinical dilemma when these small nodules appear highly suspicious 1. This is particularly problematic because nodules <12 mm with TIRADS 4-5 features have higher malignancy rates than larger nodules 7.
Subcapsular Location Exception
Even if a nodule is below the size threshold for FNA, subcapsular location warrants FNA regardless of size due to increased risk of extrathyroidal extension 1.
False-Negative Risk
FNA has a 5-10% false-negative rate, so continued clinical suspicion is warranted even with benign cytology if concerning features develop during follow-up 4.
Cytology Limitations
TIRADS cannot distinguish follicular adenoma from follicular carcinoma, as this requires histologic assessment of capsular or vascular invasion 1. Additionally, cytology rarely provides reliable cancer subtype information 1.