Clinical Research Thyroid PeroxidaSe Expression in Diseased Human Thyroid Glands Marcus A. Lima, 1 Val ria A. Gontijo, 1 Marlene C. Santos, 1 and Fernando C. L. Schmitt 2 Abstract Histoenzymologic studies of the human thyroid have been restricted to attempts to dif- ferentiate benign from malignant tumors and, to our knowledge, there is no systematic study on the distribution and behavior of enzymes for several types of lesions of the thyroid gland. The aim of the present study was to investigate thyroid peroxidase (TPO) expression in diseased thyroids, as well as to study the role of this enzyme as a parameter to define the biological behavior of thyroid lesions. A total of 117 neoplastic and nonneoplastic lesions of the thyroid were evaluated. The expression of TPO was determined by the analysis of at least 200 cells/specimen. A rate of 80% of positive cells was considered a threshold for TPO positivity. TPO expression was detected in all nonneoplastic lesions of thyroid as well as in 78.8% of the adenomas. We also observed positivity for TPO in 20% of malignant lesions.There- fore, these findings do not allow separation of benign and malignant lesions of the thy- roid based in the expression of TPO. Key Words: Thyroid peroxidase; immunohistochemistry; thyroid; neoplasia; hyperplasia; thyroiditis. iDiscipline of Endocrinology, Tri~ngulo Mineiro Medical School, Uberaba, MG, Brazil; 2Unit of Molecular Pathology, Institute of Pathology and Molecular Immunology of Porto University(IPATIMUP), Portugal. Address correspondence to Dr. Marcus Aurelho de Lima, Endocrinologia, Hospital Escola, Faculdade de Medicina do Tri~mguloMineiro, Rua Getfilio Guarit4 130, 38.025-440, Abadia,Uberaba, MG,Brazil. E-mail: lima@mednet.com.br. Presented at the XXI International Congress of the International Academy of Pathology and 12th World Congress of Academic and Environmental Pathology, Budapest, Hungary, 20-25 October 1996. Endocrine Patholog)~ vol. 10, no. 3, 223-228, Fall 1999 9 Copyright 1999 by Humana Press Inc. All rights of any nature whatsoever reserved. 1046-3976/99/10:223-228/ $11.50 Introduction Histoenzymologic studies of the human thyroid have been restricted to attempts to differentiate benign from malignant tumors [1-6] and, to our knowledge, there is no systematic study on the distribution and behavior of enzymes for several types of lesions of the thyroid gland. Among the enzymes investigated [1-8], thyroid peroxidase (TPO) has the advantage of consistent expression, as well as easy manipulation, reproducibility, and spe- cificity [9-12]. TPO plays an essential role in the bio- synthesis of thyroid hormones [9,12]. Abnormalities of hormone biosynthesis in thyroid lesions are related to quantita- tive and qualitative changes in TPO activ- ity, which is increased in Graves' disease [13-16] and reduced in hypothyroidism [17,18] and carcinoma [13-15]. Immuno- histochemistry [9-11] shows its pervasive presence in benign thyroid disorders as well as in normal thyroids. It is absent or faintly positive in malignant thyroid neoplasms. Several factors may influence TPO expression. Many are possibly related to the degree of morphologic and functional differentiation of the thyroid cell. Mor- phologic and functional differentiation may be determined by histological study of the gland and its lesions. The principal aim of the present study was to investigate TPO expression in dis- eased thyroids. We have also evaluated whether the peculiar monodonal anti- TPO antibody, previously used by De Micco et al. [10], could be used to differentiate benign from malignant thyroid nodules. 223