Background. Morphological diagnostics of thyroid gland tumours faces certain differential diagnostic problems. Extensive histological examination of the entire tumour is required for the final diagnosis of follicular and oncocytic tumours. Thus, assessment of reliable definitive cytological and/or intraoperative histological diagnosis is not possible. No marker of malignancy has been so far generally accepted in the thyroid tumour diagnosis. The aim of the study was to evaluate membrane protease dipeptidyl peptidase IV(DPP IV) in the differential diagnosis of thyroid tumours. Methods and Results. DPP IV was assessed cytochemically in 254 smears, histochemically in 314 cryostat sections, and immunohistochemically in 309 paraffin-embedded sections obtained from the group of 336 patients. There were 283 females and 53 males with themean age of 48 years (range 15–80 years) in this series. Sensitivity of cytochemical detection was 71 %, specificity was 96 %, and diagnostic accuracy was 93% using the 50 % threshold. Histochemically, sensitivity was 71 %, specificity was 93 %, and diagnostic accuracy was 90 % using the 5% threshold. Using the immunohistochemical assessment, sensitivity was 68 %, specificity was 94 %, and diagnostic accuracy was 91% using the 5% threshold. Conclusions. According to our results, DPP IV can be used as a marker of malignancy in well-differentiated carcinomas of follicular cell origin, namely in papillary carcinoma. However, it is less reliable in follicular and oncocytic carcinomas.

        Key words: dipeptidyl peptidase IV, thyroid gland, thyroid tumours, thyroid diagnosis, cytochemistry, histochemistry, immunohistochemistry.