Abstract
Papillary thyroid carcinoma is the most common type of thyroid cancer, and its development has been increasingly associated with the tumor immune microenvironment. The objective of this study was to evaluate the expression of FOXP3+ regulatory T cells, CD8+ cytotoxic T lymphocytes, and CD138+ plasma cells in papillary thyroid carcinoma, and their correlation with clinicopathological parameters and their relation to Hashimoto's thyroiditis. 44 of papillary thyroid carcinoma cases were retrieved from archives; 25 patients had multinodular goitre, and 19 patients had Hashimoto's thyroiditis. Clinical and pathological data were collected from patient files, archival data, and relevant histopathological reports. These were studied by immunohistochemistry, and the H-score was used to semi-quantitatively evaluate marker expression. Concomitant Hashimoto's thyroiditis significantly affected the tumor immune microenvironment, with increased CD8+ cytotoxic T lymphocyte density (Scores 2–3) and increased CD138+ plasma cell infiltration compared to the MNG background. Conversely, FOXP3+ regulatory T cell levels were consistently low in both groups except for a significant association between low-score (Score 1) FOXP3+ regulatory T cell expression and younger age (p = 0.004). No direct statistical correlations were observed between marker densities and features such as size, focal distribution, or lymph node metastasis. Co-occurrence of Hashimoto's thyroiditis creates a highly specialized immune landscape in PTC, primed for inflammation, with robust recruitment of cytotoxic T cells and plasma cells, but Treg infiltration is structurally suppressed.