Unraveling the significance of TSHR mutations in indeterminate thyroid cytology specimens

Abstract Objectives We investigated the clinical significance of thyroid‐stimulating hormone receptor ( TSHR ) mutations detected in thyroid fine needle aspiration (FNA) specimens. Methods The pathology archives at our institution were reviewed between 2018 and 2021 for indeterminate (Bethesda category III and IV) specimens with Thyroseq® analysis showing TSHR mutations. Results A total of 2184 cases diagnosed as atypia/follicular lesion of undetermined significance (AUS/FLUS), and 2625 diagnosed as follicular neoplasm/suspicious for follicular neoplasm (FN/SFN) were identified. A total of 1735 AUS/FLUS and 2339 SFN/FN underwent Thyroseq® analysis; 69 showed TSHR mutations (1.6%, 59 female, 10 male, average age: 55 years). Ten cases showed oncocytic features. Twelve patients underwent radionuclide scans within 1 year of FNA:11 were hyperfunctioning. Nodule size and TSH levels were weakly correlated. Twenty‐two different TSHR mutations were identified (most common: M453T). A second mutation was found in five cases ( EZH1 n = 2, and EIF1AX n = 3). The expression of sodium‐iodide transporter ( NIS ) mRNA was in the range of 0.01%–62.43% out of all sequencing reads, and was elevated in 49 (71%) cases. Surgical pathology follow‐up was available in five cases (all benign except one). On follow‐up available for 38 cases (mean: 24 months; range: 7–47 months), 34 (89.5%) nodules remained stable and 3 (8%) increased in size. Conclusions TSHR mutations in thyroid FNA samples classified as indeterminate are rare, generally benign, and commonly associated with autonomy on scan if performed.