TPO and neoplasm: Although TPO was significantly reduced or negative in all nonfollicular neoplasms, it was not of sufficient discriminatory value in the diagnosis of follicular adenoma vs. carcinoma,30,34 in particular of the oxyphilic type.34 These results have been confirmed by observations on TPO mRNA analyses on 20 thyroid carcinomas,28 and it has further been demonstrated that the reduced TPO antigenicity is due to a quantitative rather than a qualitative loss.29