Scary but innocent "blast-like" cells-potentially helpful diagnostic feature for chronic lymphocytic thyroiditis

Chronic lymphocytic thyroiditis (CLT), or Hashimoto thyroiditis, is the second most common thyroid disorder diagnosed on fine needle aspiration cytology (FNAC). Accurate diagnosis is important as patients may subsequently become hypothyroid and require lifelong thyroxin supplement. Cellular lymphoplasmacytes infiltrating follicular and oncocytic cells are diagnostic features of CLT on FNAC. "Blast-like" large lymphocytes can be appreciated on DQ smears in some CLT cases and are believed to represent cells from reactive lymphoid follicles.

The FNAC slides of CLT diagnosed between January 2022 and March 2024 were reviewed to evaluate the significance of "blast-like" cells in CLT. Correlation of their presence with clinical presentation of hypothyroidism was performed.

Fourteen of 35 CLT cases were found to have "blast-like" cells present on DQ smears, ranging from "rare" (≤5/3 HPF) in 4 cases to "nonrare" (>5/3 HPF) in 10 cases in lymphoid-rich areas. Eleven of these 14 patients were confirmed to have clinical hypothyroidism due to CLT, with a specificity of 82%, in contrast to only 7 of the 21 patients without these cells. The specificity increases to 94% if only cases with "nonrare" large "blast-like" lymphocytes present are counted.

"Blast-like" cells admixed with polymorphic lymphoplasmacytic infiltrates are more frequently identified in full-blown CLT cases, and their presence shows high correlation with clinical hypothyroidism due to CLT and enhances the diagnosis. The diagnostic value of these "blast-like" cells in cases with insufficient diagnostic features for CLT requires more evaluation.