What are the indications for fine needle aspiration (FNA) in thyroid nodules?

Indications for Fine Needle Aspiration in Thyroid Nodules

Fine needle aspiration cytology (FNAC) should be performed in any thyroid nodule >1 cm and in those <1 cm if there are suspicious clinical or ultrasonographic features. 1, 2

Primary Indications for FNAC

• FNAC is indicated for thyroid nodules >1 cm regardless of thyroid function test results, as it provides the most sensitive and specific method for differentiating between benign and malignant nodules 2
• Nodules <1 cm should undergo FNAC if they have suspicious clinical features:
  • History of head and neck irradiation 1
  • Positive family history of thyroid cancer 1
  • Suspicious features on palpation 1
  • Presence of cervical adenopathy 1
• Nodules <1 cm should also undergo FNAC if they have suspicious ultrasonographic features:
  • Hypoechogenicity 1, 3
  • Microcalcifications 1, 3
  • Absence of peripheral halo 1, 3
  • Irregular borders 1, 3
  • Solid composition 3
  • Abnormal blood flow 3

Size-Based Recommendations

• The American College of Surgeons and the National Comprehensive Cancer Network recommend evaluation of thyroid nodules larger than 2 cm even without suspicious features due to increased malignancy risk 3
• Nodules ≥3 cm warrant special attention as they have a 3-times greater risk of malignancy compared to smaller nodules 3
• For nodules ≥4 cm, FNAC remains reliable with similar test characteristics as smaller nodules (specificity 99% for ≥4 cm vs 98% for <4 cm) 4

Role of Ultrasound Guidance

• Ultrasound-guided FNAC is the preferred method for thyroid nodule evaluation due to its accuracy, economy, safety, and effectiveness 3
• Ultrasound guidance improves diagnostic yield by:
  • Ensuring accurate targeting of the nodule 5
  • Allowing sampling of the solid component in partially cystic nodules 5
  • Facilitating aspiration of difficult-to-palpate nodules 6

Management of FNAC Results

• If FNAC yields inadequate samples, the procedure should be repeated 1, 2
• In cases of follicular neoplasia with normal TSH and "cold" appearance on thyroid scan, surgery should be considered 1, 2
• Measurement of serum calcitonin should be part of the diagnostic evaluation as it has higher sensitivity than FNAC for medullary thyroid cancer (5-7% of all thyroid cancers) 1, 2

Follow-up After Initial Benign FNAC

• The best criterion for repeating FNAC in nodules with initially benign cytology is suspicious ultrasonographic features on initial or subsequent US examination 7
• Nodule growth alone has poor specificity and positive predictive value for malignancy if ultrasonographic features remain unsuspicious 7
• No repeat FNAC is necessary for nodules that remain unsuspicious on ultrasound and do not grow 7

Important Clinical Considerations

• Thyroid cancer is relatively rare (approximately 5% of all thyroid nodules), making FNAC crucial for appropriate risk stratification 2
• Thyroid function tests (TSH, T3, T4) are of limited value in diagnosing thyroid cancer, as most thyroid cancers present with normal thyroid function 2
• For nodules with indeterminate FNAC results, molecular testing for mutations (BRAF, RAS, RET/PTC, PAX8/PPARγ) may provide additional diagnostic information 2, 3
• The reliability of FNAC is not affected by nodule size, contradicting previous beliefs that larger nodules require surgical excision regardless of FNAC results 4