Indications for Thyroid Fine-Needle Aspiration Biopsy
Fine-needle aspiration biopsy (FNAB) should be performed for any thyroid nodule >1 cm, and for nodules <1 cm if suspicious ultrasonographic features are present, including hypoechogenicity, microcalcifications, irregular borders, solid composition, or abnormal blood flow. 1
Size-Based Indications
Primary size threshold:
- Any nodule >1 cm warrants FNAB regardless of other features 1
- Nodules >4 cm require FNAB regardless of ultrasound appearance due to increased malignancy risk 1
- Nodules >2 cm should be evaluated even without suspicious features, as recommended by the American College of Surgeons and National Comprehensive Cancer Network 1
Small nodules (<1 cm):
- FNAB is indicated only when suspicious ultrasound features are present AND high-risk clinical factors exist 1
- Avoid performing FNAB on nodules <1 cm without high-risk features, as this leads to overdiagnosis and overtreatment of clinically insignificant cancers 1
Suspicious Ultrasound Features That Trigger FNAB
When any nodule >1 cm demonstrates ≥2 of the following features, proceed with ultrasound-guided FNAB 1:
- Microcalcifications (highly specific for papillary thyroid carcinoma) 1
- Marked hypoechogenicity (solid nodules darker than surrounding thyroid parenchyma) 1
- Irregular or microlobulated margins (infiltrative borders rather than smooth contours) 1
- Absence of peripheral halo (loss of the thin hypoechoic rim normally surrounding benign nodules) 1
- Solid composition (carries higher malignancy risk compared to cystic nodules) 1
- Central hypervascularity (chaotic internal vascular pattern) 1
High-Risk Clinical Factors That Lower the FNAB Threshold
The following clinical factors warrant FNAB even for smaller nodules or those with fewer suspicious ultrasound features 1:
- History of head and neck irradiation (increases malignancy risk approximately 7-fold) 1
- Family history of thyroid cancer, particularly medullary thyroid carcinoma or familial syndromes 1
- Age <15 years or male gender (higher baseline malignancy probability) 1
- Rapidly growing nodule (suggests aggressive biology) 1
- Firm, fixed nodule on palpation (indicates extrathyroidal extension) 1
- Vocal cord paralysis or compressive symptoms (suggest invasive disease) 1
- Suspicious cervical lymphadenopathy 1
- Focal FDG uptake on PET scan 1
Procedural Requirements
FNAB must be performed under ultrasound guidance, as this is the preferred method due to superior accuracy, economy, safety, and effectiveness compared to palpation-guided techniques 1, 2. The procedure should be performed by trained operators (radiologists, endocrinologists, or surgeons with expertise in ultrasound-guided needle procedures) working with an experienced cytopathologist 3.
Important Clinical Caveats
- A reassuring FNA should not override worrisome clinical findings, as false-negative results occur in up to 11-33% of cases 1
- TSH levels should ideally be known before FNA, as higher TSH levels are associated with increased risk for differentiated thyroid cancer 1
- Follicular neoplasms may yield indeterminate results requiring surgical excision for definitive diagnosis, as FNA cannot distinguish follicular adenoma from adenocarcinoma 2, 3
- In the presence of negative cytology with tumor size ≥4 cm, close follow-up is indicated with repeat FNAB in 6 months 4
- For indeterminate cytology results without factors associated with malignancy, a conservative approach with clinical follow-up and repeat FNAB in 6 months to 1 year is reasonable 4