# Thyroid cytology Bethesda category III (AUS/FLUS): surgical follow-up and histologic outcomes in a tertiary-care setting

## TL;DR

This study examines the surgical outcomes of thyroid cases classified as indeterminate, finding a significant malignancy rate and highlighting the importance of follow-up strategies.

## Contribution

The study provides real-world malignancy rate estimates for Bethesda category III thyroid cases in a tertiary-care setting without routine molecular testing.

## Key findings

- A malignancy rate of 41.9% was observed in definitively linked cytology–histology cases.
- A more conservative malignancy estimate of 30.6% was found when considering all operated cases.
- AUS/FLUS cases often led to surgical intervention, emphasizing the need for clear counseling and ultrasound-informed follow-up.

## Abstract

Indeterminate thyroid cytology, specifically category III atypia/follicular lesion of undetermined significance (AUS/FLUS), poses a diagnostic challenge and affects patient management.

A retrospective review of cases reported as AUS/FLUS was conducted using the institutional thyroid fine needle aspiration (FNA) registry (2013–2022). Proportions were reported with 95% confidence intervals (CIs). Direct registry correlation and a larger all-operated sensitivity denominator were examined to capture real-world outcomes.

AUS/FLUS comprised 243 out of 1,872 FNAs (13.0%; 95% CI: 11.5–14.6). Of these, 186 patients (76.5%; 95% CI: 70.8–81.4) underwent surgery. Histologic follow-up identified 57 malignancies, resulting in a malignancy rate of 41.9% (95% CI: 34.0–50.3) among definitively linked cytology–histology cases (n = 136), and a more conservative estimate of 30.6% (95% CI: 24.5–37.6) when considering all operated cases (n = 186).

In a tertiary-care setting without routine molecular testing, AUS/FLUS was prevalent and often led to surgical intervention. The malignancy reported rate was clinically significant but varied according to the denominator applied, highlighting the impact of verification and linkage effects and the necessity for clear counseling and ultrasound-informed follow-up strategies.