Table 1. Summary of US FNAB recommendations of the 4 guidelines for thyroid nodules.

Guidelines	FNAB size cutoff
ACR-TIRADS 2017
TR1 (benign)	No biopsy
TR2 (not suspicious)	No biopsy
TR3 (mildly suspicious)	Recommend FNAB if ≥25 mm; follow if ≥15 mm
TR4 (moderately suspicious)	Recommend FNAB if ≥15 mm; follow if ≥10 mm
TR5 (highly suspicious)	Recommend FNAB if ≥10 mm; follow if ≥5 mm
ATA 2016
Benign	No biopsy
Very low suspicion	Consider FNAB at ≥20 mm; observation without FNAB is also a reasonable option
Low suspicion	Recommend FNAB if ≥15 mm
Intermediate suspicion	Recommend FNAB if ≥10 mm
High suspicion	Recommend FNAB if ≥10 mm
K-TIRADS 2016
K-TIRADS 2 (benign)	No biopsy
(I) Spongiform	Recommend FNAB if ≥20 mm
(II) Partially cystic nodule with comet-tail artifact	No biopsy
(III) Pure cyst	No biopsy
K-TIRADS 3 (low suspicion)	Recommend FNAB if ≥15 mm
K-TIRADS 4 (intermediate suspicion)	Recommend FNAB if ≥10 mm
K-TIRADS 5 (high suspicion)	Recommend FNAB if ≥10 mm; selective FNAB if >0.5
C-TIRADS 2020
C-TR 2	No biopsy
C-TR 3	No biopsy
C-TR 4A	Recommend FNAB if ≥15 mm; recommend FNAB if ≥10 mm when 1 occurs
C-TR 4B	Recommend FNAB if ≥10 mm; recommend FNAB if ≥5 mm when 1 occurs
C-TR 4C	Similar to C-TR 4B
C-TR 5	Similar to C-TR 4B; recommend FNAB of any size if 2 occur

1: multifocality, subcapsular nodule, trachea, and recurrent laryngeal nerve invasion. 2: there are typical cervical metastatic lymph nodes of thyroid cancer. US, ultrasound; FNAB, fine-needle aspiration biopsy; ACR-TIRADS 2017, the 2017 Thyroid Imaging Reporting and Data System of the American College of Radiology (10); ATA 2016, the 2016 American Thyroid Association guideline (11); K-TIRADS 2016, the 2016 Thyroid Imaging Reporting and Data System of the Korean Thyroid Association/Korean Society of Thyroid Radiology (5); C-TIRADS 2020, the 2020 Chinese Thyroid Imaging Reporting and Data System (3); TR, TIRADS.