Thyroid & parathyroid

Contributed by Ayana Suzuki, Ph.D. and Andrey Bychkov, M.D., Ph.D.

• TBS I: nondiagnostic
  • Includes inadequate by cellularity, unsatisfactory by quality and cyst fluid only specimens
  • TBS criteria for adequacy of thyroid FNA specimens is ≥ 6 groups of well visualized follicular cells (≥ 10 per cluster)
  • Frequency 3 - 34%, resection rate 7 - 15%, ROM 10 - 13% of all nodules and up to 30% of resected nodules
  • Management: reaspiration, except for pure cyst
• TBS II: benign
  • Cytology sample that is adequate for evaluation and consists of colloid and benign appearing follicular cells
  • Frequency 60 - 70%, resection rate 10%, ROM < 3%
  • Follicular nodular disease is the most common lesion
  • Management: follow up based on ultrasound pattern
• TBS III: atypia of undetermined significance (AUS)
  • Aspirates with few cells that have distinct but mild nuclear atypia or with more extensive cell numbers but subtle nuclear atypia
  • Frequency 1 - 20%, resection rate 43 - 65%, ROM 13 - 30% (NIFTP = malignant) or 6 - 20% (NIFTP ≠ malignant) and up to 40% of resected TBS III nodules
  • AUS is subdivided into AUS with nuclear atypia and other atypia
    • ROM: AUS with nuclear atypia 36 - 44%, AUS with other atypia 15 - 23%
  • On resection / histopathology diagnosed as follicular nodular disease, follicular adenoma and papillary thyroid carcinoma (PTC)
  • Management: reaspiration or molecular testing (Thyroid 2016;26:1)
• TBS IV: follicular neoplasm (FN)
  • Cases with most of the follicular cells arranged in cell crowding or microfollicle formation
  • Frequency 7%, resection rate 42 - 45%, ROM 23 - 34% (NIFTP = malignant) or 23% (NIFTP ≠ malignant)
  • Histopathology: follicular adenoma, follicular nodular disease, NIFTP, follicular variant of papillary thyroid carcinoma and follicular carcinoma
  • Management: diagnostic thyroid lobectomy or molecular testing
• TBS IV: follicular neoplasm (oncocytic follicular neoplasm [FN-OFN])
  • Cases with most of the follicular cells showing abundant fine granular cytoplasm (oncocytes)
  • Frequency 1.2 - 9%, resection rate 30%, ROM 25 - 50%
  • Histopathology: oncocytic adenoma and carcinoma
  • Management: diagnostic thyroid lobectomy or molecular testing
• TBS V: suspicious for malignancy
  • Used when cytology strongly suggests malignancy but is not sufficient for a conclusive diagnosis
  • Frequency 3%, resection rate 70%, ROM 74% (NIFTP = malignant) or 65% (NIFTP ≠ malignant)
  • Histopathology: usually PTC
  • Management: surgery (usually)
• TBS VI: malignant
  • Used when cytology strongly suggests malignancy
  • Frequency 5%, resection rate 65 - 80%, ROM 97 - 100% (NIFTP = malignant) or 94% (NIFTP ≠ malignant)
  • Histopathology: wide spectrum of thyroid malignancies, from PTC (most common) to medullary thyroid carcinoma, anaplastic thyroid carcinoma, lymphoma, etc.
  • Management: surgery (usually)
• TBS 2023 continues to advise using the category names and optionally, their corresponding category numbers (e.g., benign [Bethesda II], follicular neoplasm [Bethesda IV])

• Single distinct name for each category
• Each category is associated with updated ROM based on new data since the 2017 edition; clinical management algorithms for each category have also been revised
• Subdivision of AUS category into AUS - nuclear atypia and AUS - other, with different implied ROM (higher in AUS - nuclear) and molecular profile
• New discussions on pediatric thyroid disease have been included, supplied with pediatric ROMs and management algorithms
• Revised terminology in accordance with the 2022 WHO classification of thyroid tumors
• Reference: Thyroid 2023;33:1039

Contributed by Ayana Suzuki, Ph.D.

Unsatisfactory


Benign



Suspicious for malignancy


Malignant

• Papanicolau Society of Cytopathology: Image Atlas [Accessed 9 May 2025]

What is most likely The Bethesda System for Reporting Thyroid Cytopathology category of this thyroid aspirate?

1. Atypia of undetermined significance
2. Benign
3. Follicular neoplasm
4. Malignant
5. Nondiagnostic

E. Nondiagnostic. When the aspirated material contains only foamy histiocytes and no follicular epithelium or colloid, it is qualified as nondiagnostic; however, in some local reporting systems (e.g., Japanese), these cases are reported as adequate, cyst fluid only, because their malignancy risk is almost the same as that of the benign category and lower than that in the nondiagnostic category. Answer B is incorrect because there are no cells in the photo that meet the adequate and benign criteria. Answers A, C and D are incorrect because there are no atypical cells in the photo.

Comment Here

Reference: Bethesda system diagnostic categories

Which is the most recommended management for the benign category of The Bethesda System for Reporting Thyroid Cytopathology?

1. Follow up
2. Molecular testing
3. Reaspiration
4. Thyroid lobectomy
5. Total thyroidectomy

A. Follow up. The American Thyroid Association recommends follow up based on the ultrasound pattern for a benign lesion. Answer B is incorrect because molecular testing is typically reserved for indeterminate categories, such as atypia of undetermined significance (AUS) or follicular neoplasm (FN) / suspicious for a follicular neoplasm (SFN), where the risk of malignancy is higher and the cytological findings are ambiguous. Answer C is incorrect because reaspiration is not routinely recommended for benign cytology unless there are new clinical concerns or suspicious ultrasound features. Answer D is incorrect because lobectomy is typically considered for categories with a higher risk of malignancy, not benign lesions. Answer E is incorrect because total thyroidectomy is usually reserved for confirmed malignant cases or when there are other indications, such as large goiters causing compressive symptoms.

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Reference: Bethesda system diagnostic categories