BACKGROUND: Medullary thyroid carcinoma (MTC) is an aggressive malignancy originating from the parafollicular C cells. Preoperatively, thyroid nodule fine-needle aspiration cytology (FNAC) and pathogenic gene mutations are definitive in approximately one-half of cases. MicroRNAs (miRNAs) are endogenous, noncoding, single-stranded RNAs that regulate gene expression, a characteristic that confers the potential for identifying malignancy. In the current study, the authors hypothesized that differential pairwise (diff-pair) analysis of miRNA expression levels would reliably identify MTC in FNA samples. METHODS: The relative abundance of 10 different miRNAs in total nucleic acids was obtained from ThyraMIR test results. Diff-pair analysis was performed by subtracting the critical threshold value of one miRNA from the critical threshold values of other miRNAs. Next-generation sequencing with the ThyGeNEXT panel identified oncogenic gene alterations. The discovery cohort consisted of 30 formalin-fixed, paraffin-embedded benign and malignant thyroid neoplasms, including 4 cases of MTC. After analytical validation, clinical validation was performed using 3 distinct cohorts (total of 7557 specimens). RESULTS: In the discovery cohort, 9 diff-pairs were identified as having significant power using the Kruskal-Wallis test (P < .0001) to distinguish MTC samples from non-MTC samples. The assay correctly classified all MTC and non-MTC samples in the analytical validation study and in the 3 clinical validation cohorts. The overall test accuracy was 100% (95% confidence interval, 99%-100%). In indeterminate FNAC samples, the sensitivity of the diff-pair analysis was greater than that of the MTC-specific mutation analysis (100% vs 25%; P = .03). CONCLUSIONS: Pairwise miRNA expression analysis of ThyraMIR results were found to accurately predict MTC in thyroid FNA samples, including those with indeterminate FNAC findings.