Transforming growth factor beta (TGF-β) plays an important role in many pathophysiological conditions, including cancer. The level of TGF-β in patients with differentiated thyroid cancer (DTC) has not been examined so far. The aim of this study was to measure TGF-β concentration in serum samples and in PHA -stimulated whole blood culture in vitro and to analyze possible associations of TGF-β1 levels with leukocyte, lymphocyte and platelets counts, the histological type of thyroid cancer, and stage of disease. TGF-β1 was measured in 22 DTC patients and 20 healthy controls using the duoSet ELISA Development kit for human TGF-β1. The concentration of TGF-β1 in serum samples from both groups correlated positively with the platelet counts. There was no statistically significant difference in the serum concentrations of TGF-β1 between DTC patients and control subjects, but PHA stimulated whole blood cultures of DTC patients produced less TGF-β1 than those from controls. Additional studies are needed to determine the significance of these in vitro findings.