The Bethesda system for reporting thyroid cytopathology comprises six categories of pathological reporting of thyroid FNA, with each category linked to a malignancy risk.

Usage

The Bethesda system was first published in 2010 and has been revised twice, with the most recent and third edition published in 2023 to align with the 2022 WHO classification of thyroid tumours ³.

Classification

The adult diagnostic categories for the 2023 Bethesda system are as follows ⁴:

• category I

  • non-diagnostic

    • cyst fluid only

    • virtually acellular special

    • other, e.g. obscuring blood, clotting artifact, drying artifact

  • risk of malignancy ~13% (range 5-20%)

  • usual management: repeat US-guided thyroid FNA (which will be diagnostic in ~70% (range 60-80%) of cases)

• category II

  • benign

    • consistent with follicular nodular disease (includes adenomatoid nodule, colloid nodule, etc.)

    • consistent with chronic lymphocytic (Hashimoto) thyroiditis in the proper clinical context

    • consistent with granulomatous (subacute) thyroiditis

  • risk of malignancy: ~4% (range 2-7%)

  • usual management: clinical and ultrasound follow-up

• category III

  • atypia of undetermined significance; specify AUS-nuclear atypia or AUS-other

  • risk of malignancy: ~22% (range 13-30%)

  • usual management: repeat US-guided FNA, molecular testing (if available), diagnostic hemithyroidectomy, or surveillance

• category IV

  • follicular neoplasm;specify if oncocytic (Hürthle cell) type

  • risk of malignancy: ~30% (range 23-34%)

  • usual management: molecular testing (if available), diagnostic hemithyroidectomy

• category V

  • suspicious for suspicious for malignancy (e.g. papillary thyroid carcinoma, medullary thyroid carcinoma, metastatic carcinoma, lymphoma)

  • risk of malignancy: ~74% (range 67-83%)

  • usual management: molecular testing (if available), hemithyroidectomy/thyroidectomy

• category VI

  • malignant

    • papillary thyroid carcinoma

    • high-grade follicular-derived carcinoma

    • medullary thyroid carcinoma

    • undifferentiated (anaplastic) carcinoma

    • squamous cell carcinoma

    • carcinoma with mixed features (specify)

    • metastatic malignancy

    • non-Hodgkin lymphoma

  • risk of malignancy: ~97% (range 97-100%)

  • usual management: hemithyroidectomy/thyroidectomy

See also

• ultrasound-guided FNA of the thyroid

• McGill thyroid nodule score

• Bethesda criteria of hereditary non-polyposis colorectal cancer