What is Thy3a? A study of 336 Thy3a (AUS/FLUS) thyroid FNAs with histology compares UK RCPath with other reporting systems and shows how Thy3a subclassification can improve risk stratification and help address overuse of this category.

Introduction: Thy3a (AUS/FLUS) is an indeterminate and heterogeneous category in thyroid cytology. Thy3a reporting rates vary widely, with many laboratories documenting overuse. Subclassification of Thy3a helps with risk stratification. We aimed to investigate whether subclassification can also help address Thy3a overuse. We compare the UK reporting system with other terminologies.

Methods: An audit of thyroid fine needle aspirations (FNAs) reported at our institution between 2012 and 2017 was performed. Thy3a FNAs followed by histology were reviewed and subcategorised into four subgroups: Scanty Atypia (SA), Scanty Microfollicular (SMF), Favour Benign (FB) and Thyroiditis versus Neoplasm (TVN). Review and subclassification were blinded to histology outcomes. FNAs were correlated with histology and statistical analysis was performed.

Results: Our Thy3a rate was high (24% of all thyroid FNAs). For 336 Thy3a FNAs with histology, the malignancy rates of the four subgroups were: SA 68%, SMF 20%, FB 4%, TVN 31%. There were significant associations between subgroup and malignancy risk, and between subgroup and tumour risk. On histology, SA had more malignancies than expected and FB had fewer. SA and SMF had more tumours than expected and FB had fewer. SMF and Thy3f FNAs were similar in terms of tumour and malignancy outcomes.

Conclusions: Subclassification of Thy3a FNAs into these four subgroups is recommended. This can improve risk stratification and help address overuse of Thy3a. We propose that some FB and SMF cases could be safely diverted to Thy2 and Thy3f respectively. We compare various reporting terminologies and question how indeterminate FNAs should be classified.