Thyroid malignancy has been induced by long-term endogenous thyrotropin (TSH) stimulation in experimental animals, leading to local and distant metastasis. It has been postulated that constant and prolonged endogenous TSH stimulation in dyshormonogenetic thyroid tissues could result in thyroid neoplasia. The possible role of growth factors and oncogenes in goitrogenesis and favoring neoplasia has also been mentioned. Overexpression of certain growth factors and/or their receptors, and of oncogenes implicated in growth promotion may play a significant role in the relatively frequent finding of thyroid malignancy in congenital goiters. In this study the expression of epidermal growth factor (EGF), epidermal growth factor receptor (EGF-R), transforming growth factor-beta (TGF-beta), c-myc, and p53 mRNAs was determined in 14 thyroid tissue samples: 6 from patients with thyroid peroxidase (TPO) gene mutations, 4 with thyroglobulin (Tg) gene defects and 4 normal thyroid tissues. EGF mRNA overexpression was seen in 7 of 10 dyshormonogenetic tissues (3.5 to 12.0 arbitrary optical densitometry units [AODU]) and considered significantly higher (p < 0.01) when compared to normal thyroid tissues (0.25 to 0.32 AODU). Moreover, overexpression of EGF-R mRNA was present in 6 of 10 dyshormonogenetic tissues (2.23 to 13.03 AODU) and considered significantly higher (p < 0.01) when compared to normal thyroid tissues (0.42 to 0.65 AODU). There was no difference in c-myc, p53, and TGF-beta mRNAs expression between dyshormonogenetic and normal tissues. The overexpression of EGF and EGF-R mRNAs found in dyshormonogenetic tissues may suggest that this growth factor may play a role in cellular proliferation and contribute to goiter formation.