# Thyroid Nodule Biopsy Results: Understanding Bethesda Categories

URL: https://thethyroidologist.com/blog/thyroid-biopsy-bethesda/
Published: 2025-11-19
Author: The Thyroidologist Editorial Team
Medical reviewer: Thyroid Specialist

> FNAC biopsy results are reported on the Bethesda scale — from non-diagnostic to malignant. What each category means and what happens next.

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.

## References & further reading

- [Thyroid cancer diagnosis — National Cancer Institute](https://www.cancer.gov/types/thyroid)
- [Thyroid biopsy — American Thyroid Association](https://www.thyroid.org/thyroid-disease/)

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