Bethesda System: Thyroid Biopsy Result
The Bethesda System is an international classification that reports thyroid fine-needle aspiration biopsy in six groups. Each group suggests a different cancer risk and clinical approach. This page explains what Bethesda I–VI categories mean and how follow-up or surgery is decided.
Bethesda groups biopsy results as I (non-diagnostic), II (benign), III (atypia of undetermined significance), IV (follicular neoplasm suspicion), V (suspicious for malignancy) and VI (malignant). Bethesda II is usually followed; V and VI bring surgery to the forefront. III and IV are individualized using ultrasound, nodule size, clinical features and, when needed, repeat biopsy.
What Is the Bethesda System?
The Bethesda System for Reporting Thyroid Cytopathology standardizes biopsy results so they translate cleanly into clinical decisions.
Each category carries an estimated malignancy risk and a recommended approach. Bethesda is interpreted together with ultrasound, TIRADS and patient-specific risk factors — never in isolation.
Meaning of Bethesda I–VI
- Bethesda I — Non-diagnostic: insufficient cells; ultrasound-guided repeat biopsy is usually considered.
- Bethesda II — Benign: typically followed with ultrasound if features remain reassuring.
- Bethesda III — Atypia of undetermined significance (AUS/FLUS): repeat biopsy, molecular testing or diagnostic lobectomy may be considered.
- Bethesda IV — Suspicious for follicular neoplasm: most patients are offered diagnostic lobectomy.
- Bethesda V — Suspicious for malignancy: surgical evaluation is recommended.
- Bethesda VI — Malignant: surgical treatment is planned according to cancer type and extent.
Clinical Approach by Category
Bethesda II usually means regular ultrasound follow-up.
Bethesda III and IV are individualized using ultrasound, nodule size, age, comorbidities and family history; molecular tests may help in selected centres.
Bethesda V and VI bring surgery forward. The extent of surgery depends on tumour type, size, spread and lymph node status.
When Is Surgery Considered?
- Bethesda V (suspicious for malignancy)
- Bethesda VI (malignant)
- Bethesda IV with concerning ultrasound/clinical features
- Selected patients with repeated Bethesda III results
- Large, compressive or rapidly growing nodules
When to Seek Urgent Care
- • Rapidly growing neck mass
- • New or progressive hoarseness
- • Shortness of breath or difficulty swallowing
Frequently Asked Questions
Related Pages
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This information is provided for general patient education only and does not replace diagnosis, treatment or a surgical decision. Personal evaluation requires a clinical examination and the necessary tests.
Last updated: 27 June 2026 · Medical content review: Op.Dr.Gökhan ATEŞ
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