The lipid phosphatase, phosphatase and tensin homolog (PTEN), is a key element in controlling cell growth and survival and has a well established role as tumor suppressor protein in many neoplasia. Several data indicate that silencing of PTEN gene expression may be relevant in follicular thyroid cell transformation. Thus, in the present study regulation of PTEN gene expression in thyroid cells was investigated. Cotransfection experiments indicated that in normal FRTL-5 rat thyroid cells, PTEN promoter activity was increased by overexpression of the transcription factor early growth response protein-1 (Egr-1). Moreover, Western blot experiments indicated that when Egr-1 expression was up-regulated by treating FRTL-5 cells with H2O2, an increase in PTEN expression was also observed. TSH induced opposite modifications on PTEN and Egr-1 protein levels. Moreover, acute or chronic TSH stimulation determined distinct effects. In fact, acute TSH stimulation (30 and 60 min) induced a decrease in PTEN, but an increase in Egr-1 protein levels. These effects were cAMP dependent; in fact, they were mimicked by forskolin. A chronic TSH treatment (5 d) stimulated PTEN protein expression, whereas Egr-1 protein was down-regulated. In contrast to normal thyroid cells, when the thyroid tumor cell lines ARO and BCPAP were exposed to H2O2, neither Egr-1 nor PTEN protein levels were increased. Acute stimulation of ARO and BCPAP cells with forskolin increased Egr-1, but not PTEN, protein levels. Therefore, thyroid tumor cell lines show alteration of PTEN gene expression regulation. RT-PCR experiments performed on human thyroid tumors showed that the absence of Egr-1 mRNA is always paralleled by the absence of PTEN mRNA. Thus, modification of the Egr-1-dependent mechanisms may play a role in the silencing of PTEN gene expression occurring during thyroid cell transformation.