What are the Bethesda categories for thyroid fine‑needle aspiration cytology and their recommended management?

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Bethesda System for Thyroid Cytopathology: Categories and Management

The Bethesda System classifies thyroid fine-needle aspiration (FNA) cytology into six standardized diagnostic categories, each with an associated malignancy risk and specific management recommendations. 1

The Six Bethesda Categories

Category I: Nondiagnostic or Unsatisfactory

  • Malignancy risk: Variable, typically requires repeat FNA 1
  • Management: Repeat FNA with ultrasound guidance 1
  • Clinical pitfall: Despite recommendations for repeat FNA, many cases proceed directly to surgery in practice 2

Category II: Benign

  • Malignancy risk: 0-3% 3
  • Includes: nodular goiter, colloid goiter, hyperplastic/adenomatoid nodule, Hashimoto's thyroiditis 1
  • Management: Clinical and ultrasound surveillance 1

Category III: Atypia of Undetermined Significance (AUS) or Follicular Lesion of Undetermined Significance (FLUS)

  • Malignancy risk: 5-15% 1
  • Alternative terms: rule out neoplasm, atypical follicular lesion, cellular follicular lesion 1
  • Management options: 1
    • Consider molecular diagnostic testing to refine risk stratification
    • If molecular testing (combined with clinical and ultrasound features) predicts malignancy risk ≤5%, active surveillance is appropriate
    • If molecular testing suggests higher risk or is unavailable, consider lobectomy for definitive diagnosis
  • Important caveat: Molecular diagnostics should be interpreted cautiously and in context of clinical, radiographic, and cytologic features 1

Category IV: Follicular Neoplasm or Suspicious for Follicular Neoplasm

  • Malignancy risk: 15-40% 1
  • Includes Hürthle cell neoplasm 1
  • Key limitation: Diagnosis of follicular or Hürthle cell carcinoma requires evidence of vascular or capsular invasion, which cannot be determined by FNA 1
  • Management: 1
    • Consider molecular diagnostic testing for risk stratification
    • If molecular testing suggests papillary thyroid carcinoma (especially BRAF V600E mutation), manage as papillary carcinoma
    • If molecular testing predicts malignancy risk ≤5%, consider active surveillance
    • Otherwise, lobectomy or total thyroidectomy for definitive diagnosis
  • Critical exception: Molecular diagnostics are NOT recommended for Hürthle cell neoplasms as they perform poorly in this subtype 1

Category V: Suspicious for Malignancy

  • Malignancy risk: 60-75% 3
  • Management: Surgical resection (lobectomy or total thyroidectomy based on clinical factors) 1

Category VI: Malignant

  • Malignancy risk: 97-99% (virtually 100%) 3
  • Includes: papillary carcinoma, medullary thyroid carcinoma (MTC), anaplastic thyroid carcinoma (ATC), or lymphoma 1
  • Management: Surgical resection with extent determined by tumor characteristics 1

Surgical Decision-Making Algorithm for Differentiated Thyroid Cancer

Indications for total thyroidectomy (any present): 1

  • Known distant metastases
  • Cervical lymph node metastases
  • Extrathyroidal extension
  • Tumor >4 cm in diameter
  • Poorly differentiated histology
  • Prior radiation exposure (Category 2B)
  • Macroscopic multifocal disease
  • Positive resection margins
  • Vascular invasion

Indications for lobectomy (all must be present): 1

  • No prior radiation exposure
  • No distant metastases
  • No cervical lymph node metastases
  • No extrathyroidal extension
  • Tumor ≤4 cm in diameter

Key Updates and Considerations

The 2023 Bethesda revision provides single standardized names for each category and updated malignancy risks based on post-2017 data, including pediatric-specific risks and management algorithms 4

NIFTP (Noninvasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features): This reclassification impacts malignancy risk calculations; when diagnosed, only lobectomy is required with ongoing surveillance 1, 3

Molecular testing role: The NCCN panel emphasizes that molecular diagnostics are not mandatory and should not be overemphasized—clinical judgment, ultrasound features, and patient preference remain critical 1

Common pitfall: The AUS/FLUS category remains heterogeneous in usage and clinical outcomes across institutions, with malignancy rates sometimes overlapping with Category IV 2

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

3
Research

The 2017 Bethesda System for Reporting Thyroid Cytopathology.

Thyroid : official journal of the American Thyroid Association, 2017

4
Research

The 2023 Bethesda System for Reporting Thyroid Cytopathology.

Thyroid : official journal of the American Thyroid Association, 2023

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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