What is the TI‑RADS (Thyroid Imaging Reporting and Data System) score and how is it used to assess malignancy risk and guide management of thyroid nodules?

TI-RADS Score: Ultrasound-Based Risk Stratification for Thyroid Nodules

TI-RADS (Thyroid Imaging Reporting and Data System) is a standardized ultrasound classification that assigns thyroid nodules to risk categories (TR1-TR5) based on the number of suspicious sonographic features present, with each category carrying a specific malignancy probability that directly determines size thresholds for fine-needle aspiration biopsy. 1

Core Ultrasound Features That Define TI-RADS Categories

The system evaluates five key suspicious characteristics, with malignancy risk escalating as more features accumulate 2, 3:

• Solid composition – Solid nodules carry higher malignancy risk than cystic or mixed lesions 4, 1
• Marked hypoechogenicity – Nodules darker than surrounding thyroid parenchyma 4, 2
• Microcalcifications – Hyperechoic spots ≤1 mm representing psammoma bodies, highly specific for papillary thyroid carcinoma 4, 2
• Irregular or microlobulated margins – Infiltrative borders rather than smooth contours 4, 2
• Taller-than-wide shape – Anteroposterior dimension exceeds transverse diameter on transverse view 2, 3

TI-RADS Classification System and Malignancy Risk

The American College of Radiology TIRADS assigns nodules to categories based on the number of suspicious features present, with validated malignancy rates as follows 1, 2:

Category	Suspicious Features	Malignancy Risk	Management
TR1	Benign (purely cystic)	0%	No FNA needed [1]
TR2	Not suspicious	0%	No FNA needed [2]
TR3	Mildly suspicious (0 features)	1.3–5%	FNA if ≥1.5 cm [1,2]
TR4	Moderately suspicious (1–2 features)	5–30%	FNA if ≥1.0 cm [1,2]
TR5	Highly suspicious (≥3 features)	75–95%	FNA if ≥0.5 cm [1,2]

Research validation demonstrates that this stratification achieves 97% sensitivity, 90% specificity, and 99% negative predictive value when categories 4–5 are considered malignant 2.

Algorithmic Approach to FNA Decision-Making

The size threshold for biopsy decreases as TI-RADS category increases, balancing detection of clinically significant cancers against overdiagnosis of indolent microcarcinomas 5, 1:

For Nodules ≥1 cm

• TR4 or TR5 → Proceed to ultrasound-guided FNA 4, 5
• TR3 → FNA only if ≥1.5 cm or high-risk clinical factors present 5, 1
• TR2 → Surveillance only, no FNA 1

For Nodules <1 cm

• Do not perform FNA unless the nodule is subcapsular or suspicious cervical lymphadenopathy is present, even for TR5 classification 5, 1
• This threshold prevents overdiagnosis of papillary microcarcinomas that do not impact mortality or quality of life 5, 1

For Nodules >4 cm

• Perform FNA regardless of TI-RADS category due to increased false-negative rates in large lesions 4, 5

High-Risk Clinical Factors That Lower FNA Threshold

When any of these factors are present, consider FNA even for smaller nodules or lower TI-RADS categories 4, 5:

• History of head and neck irradiation – Increases malignancy risk approximately 7-fold 4, 1
• Family history of thyroid cancer, particularly medullary carcinoma or familial syndromes 4, 1
• Age <15 years or male gender – Higher baseline malignancy probability 4
• Suspicious cervical lymphadenopathy on ultrasound 4, 5
• Subcapsular location – Increases risk of extrathyroidal extension 4, 1
• Rapidly growing nodule – Growth ≥3 mm in any dimension during surveillance 4

Technical Approach to FNA When Indicated

• Ultrasound guidance is mandatory for real-time needle visualization and accurate sampling, superior to palpation-guided techniques 4, 5
• Target the solid component in mixed solid-cystic nodules, as it harbors the highest malignancy risk 4, 5
• Measure serum calcitonin as part of the diagnostic workup to screen for medullary thyroid cancer, which has higher sensitivity than FNA alone (detects 5–7% of cancers missed by cytology) 4, 5
• If initial sample is nondiagnostic, repeat FNA under ultrasound guidance; persistent inadequacy warrants core-needle biopsy 4, 5

Management Based on Bethesda Cytology Results

The Bethesda System stratifies FNA results into six categories that guide subsequent management 4, 5:

• Bethesda II (benign) – Surveillance with repeat ultrasound at 12–24 months; malignancy risk 1–3% 4, 5
• Bethesda III (AUS/FLUS) or IV (follicular neoplasm) – Molecular testing for BRAF, RAS, RET/PTC, PAX8/PPARγ; mutation-positive nodules have ~97% malignancy probability 4, 5
• Bethesda V (suspicious) or VI (malignant) – Immediate referral for total or near-total thyroidectomy with pre-operative cervical lymph node assessment 4, 5

Critical Pitfalls and Limitations

Avoid these common errors that lead to overdiagnosis or delayed diagnosis 5, 1:

• Do not perform FNA on sub-centimeter nodules without high-risk features, as this leads to unnecessary treatment of clinically insignificant papillary microcarcinomas 5, 1
• Do not override a reassuring FNA when worrisome clinical findings persist – false-negative results occur in 11–33% of cases 4
• Recognize that TI-RADS has a 5–10% false-negative rate and cannot distinguish follicular adenoma from carcinoma, which requires histologic assessment of capsular invasion 1
• Current guidelines create a paradox for nodules <1 cm: TI-RADS advises against FNA to prevent overdiagnosis, yet thermal ablation protocols require histologic confirmation, and risk stratification cannot be applied pre-operatively 5, 1

Surveillance Protocol for Nodules Not Meeting FNA Criteria

• Initial follow-up ultrasound at 6–12 months to assess for interval growth or development of suspicious features 1
• Continued surveillance at 12–24 month intervals for stable nodules 1
• Trigger for repeat FNA: Growth ≥3 mm in any dimension, development of new suspicious features, or emergence of high-risk clinical factors 4