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Purpose Anaplastic thyroid carcinoma (ATC) exhibits limited responsiveness to immunotherapy. The current study characterises the immune cell repertoire in ATC compared with differentiated thyroid carcinoma (DTC) and thyroid follicular nodular disease (TFND) and evaluates IFN-γ and TGF-β mRNA expression.Methods Cytotoxic T lymphocytes (CTLs), regulatory T cells (TREGs), natural killer cells (NK), B lymphocytes and myeloid-derived suppressor cells (MDSCs) were analysed by multicolour flow cytometry in prospectively collected ATC, DTC and TFND tissues. IFN-γ and TGF-β mRNA expression was quantified using RT-PCR.Results ATC demonstrated significantly higher MDSC infiltration than DTC or TFND. CTLs were elevated in ATC relative to DTC, while TREGs, NK and B-cells were lower. IFN-γ and TGF-β expression did not differ significantly although IFN-γ expression was higher in DTC and ATC than in TFND.Conclusion ATC microenvironment is rich in immunosuppressive MDSCs. These findings support MDSC-focused immunomodulation as a promising adjunct therapy in ATC.