Unraveling the significance of TSHR mutations in indeterminate thyroid cytology specimens.

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.
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