Thyroid & parathyroid


Bethesda system

Diagnostic categories

Topic Completed: 1 August 2014

Minor changes: 30 August 2021

Copyright: 2014-2021,, Inc.

PubMed Search: Bethesda guidelines thyroid diagnostic categories

Phillip A. Williams, M.D., M.Sc.
Shahid Islam, M.D., Ph.D.
Page views in 2020: 10,882
Page views in 2021 to date: 11,802
Cite this page: Williams P, Islam S. Diagnostic categories. website. Accessed September 22nd, 2021.
Definition / general
  • Nondiagnostic or unsatisfactory (ND / UNS, < 10% of thyroid FNAs)
    • Cyst fluid only (CFO)
    • Virtually acellular specimen (need at least 6 groups of benign follicular cells, composed of at least 10 cells each for benign)
    • Other (obscuring blood, clotting artifact, overly thick smear, etc.)
    • Risk of malignancy: 1 - 4%
    • Management: repeat FNA; use with ultrasound guidance for ND / UNS results; repeat for CFO only if worrisome clinical or sonographic features

  • Benign (60 - 70% of thyroid FNAs)
    • Consistent with a benign follicular nodule (includes adenomatoid nodule, colloid nodule, etc.)
    • Consistent with lymphocytic (Hashimoto) thyroiditis in the proper clinical context
    • Consistent with granulomatous (subacute) thyroiditis
    • Other (abundant colloid, black thyroid, reactive changes, radiation changes, cyst lining cells, etc.)
    • Risk of malignancy: 0 - 3%
    • Management: clinical followup

  • Atypia of undetermined significance (or follicular lesion of undetermined significance) (3 - 6% of thyroid FNAs)
    • Broad category of not easily classified FNAs, including:
      • Predominant population of microfollicles in an aspirate that does not fulfill criteria for a follicular neoplasm or suspicious for follicular neoplasm
      • Predominance of Hürthle cells in a sparsely cellular aspirate
      • Interpretation obscured by preparation artifact (e.g., air drying or clotting artifact)
      • Moderate to markedly cellular specimen composed exclusively of Hürthle cells in a clinical setting of benign Hürthle cell nodule (e.g., lymphocytic (Hashimoto) thyroiditis or multinodular goiter)
      • Nuclear features of papillary thyroid carcinoma (nuclear grooves, enlarged nuclei with pale chromatin, and alterations in nuclear contour and shape) in an otherwise benign appearing sample
      • Cyst lining cells with nuclear grooves, prominent nucleoli, elongated nuclei and variable intranuclear inclusions in an otherwise benign appearing sample
      • Minor population of follicular cells with nuclear enlargement, variable prominent nucleoli (e.g., history of radioactive iodine treatment, carbimazole or repair of an involuted cyst or hemorrhage)
      • Atypical lymphoid infiltrate, NOS
    • Risk of malignancy: 5 - 15%
    • Management: repeat FNA

  • Follicular neoplasm (or suspicious for a follicular neoplasm)
    • Specify if exclusively Hürthle cell (oncocytic) type, which have different genetic changes and are more likely to be benign (in 16 - 25% of cases)
    • Risk of malignancy: 15 - 30%
    • Management: surgical lobectomy

  • Suspicious for malignancy
    • Suspicious for papillary carcinoma (only 1 - 2 features of PTC present, focal changes or sparsely cellular)
    • Suspicious for medullary carcinoma
    • Suspicious for metastatic carcinoma
    • Suspicious for lymphoma
    • Other
    • Risk of malignancy: 60 - 75%
    • Management: near total thyroidectomy or surgical lobectomy

  • Malignant (3 - 7% of thyroid FNAs)
    • Papillary thyroid carcinoma
    • Poorly differentiated carcinoma
    • Medullary thyroid carcinoma
    • Undifferentiated (anaplastic) carcinoma
    • Squamous cell carcinoma
    • Carcinoma with mixed features (specify)
    • Metastatic carcinoma
    • Non-Hodgkin lymphoma
    • Other
    • Risk of malignancy: 97 - 99%
    • Management: near total thyroidectomy
Cytology images

Contributed by Ayana Suzuki, C.T.


Hemorrhagic background


Respiratory epithelium

Air dried smear

Cyst fluid only


Watery colloid

Cracking colloid

Follicular clusters

3D structures


FLUS - cellular

FLUS - architectural

FLUS - Hürthle

Atypical lymphocytes



FN - Hürthle

Suspicious for malignancy:

Suspicious for PTC

Suspicious for lymphoma

Hyalinizing trabecular tumor


Papillary carcinoma

Medullary carcinoma

Insular carcinoma

Anaplastic carcinoma



Thyroid cytology I: approach

Thyroid cytology II: cases

Additional references
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