Background: Differentiating benign from malignant thyroid lesions using histology alone can be difficult, particularly in follicular-patterned neoplasms. Cytokeratin 19 (CK19), a low molecular weight cytokeratin, is emerging as a useful immunohistochemical (IHC) marker in the diagnosis of papillary thyroid carcinoma (PTC).

Objective: To evaluate the utility of CK19 in differentiating benign from malignant thyroid lesions.

Materials and Methods: This prospective study was conducted from June 2015 to June 2017 at a tertiary care hospital in Nandyal. A total of 108 thyroidectomy specimens were processed and subjected to hematoxylin and eosin (H&E) staining. CK19 IHC was performed where necessary. CK19 positivity was defined as membranous with or without cytoplasmic staining in ≥10% of cells.

Results: Of 108 thyroid specimens, 85 were benign and 23 were malignant. CK19 was positive in 17/18 (94%) PTCs and 1/2 follicular variant PTCs. All cases of medullary carcinoma and most benign lesions were negative. CK19 had a sensitivity of 94.4%, specificity of 84.2%, and a statistically significant association with malignancy (p<0.001).

Conclusion: CK19 is a sensitive and specific marker for PTC and serves as a valuable diagnostic tool to differentiate it from benign mimickers.