If you have had a thyroid nodule biopsy (FNAC), your result is reported on the Bethesda classification — a standardized system from Category I to VI. Here is what each means.
The Bethesda categories
| Category | Result | Meaning | Typical next step |
|---|---|---|---|
| I | Non-diagnostic | Insufficient cells obtained | Repeat biopsy, often with on-site adequacy check |
| II | Benign | Benign follicular nodule | Monitoring with ultrasound |
| III | AUS/FLUS | Atypia of undetermined significance | Repeat FNAC, molecular testing, or diagnostic surgery |
| IV | Follicular neoplasm | Follicular lesion — cannot exclude cancer | Diagnostic lobectomy usually needed |
| V | Suspicious for malignancy | Likely cancer | Surgical removal |
| VI | Malignant | Cancer confirmed | Staging and treatment planning |
The tricky categories: III and IV
Bethesda III (AUS/FLUS) is the most uncertain — cells show some atypia but not enough for a definitive call. Options include repeat biopsy (often with on-site adequacy assessment), molecular testing (where available), or diagnostic surgery for persistent uncertainty.
Bethesda IV (follicular neoplasm) cannot be definitively diagnosed by needle biopsy — follicular adenoma and carcinoma look identical under the microscope. The distinction requires examining the surgical specimen for capsular or vascular invasion. This is why diagnostic lobectomy is often recommended.
What this means for you
- Category II — the most common result; most nodules are benign and need only monitoring
- Category V/VI — cancer is likely; surgical planning follows
- Category III/IV — genuinely uncertain; decisions are individualized
Important caveats
- FNAC has a small false-negative rate — suspicious nodules with benign results may still need monitoring
- Ultrasound findings and clinical context matter alongside cytology
- A specialist interprets results in the full picture — biopsy alone doesn’t determine treatment
The bottom line
Bethesda categories standardize biopsy reporting so patients and doctors speak the same language. Most results are benign; the uncertain categories (III–IV) are where specialist input is most valuable for deciding between monitoring, repeat biopsy, molecular testing, or surgery.