Several findings suggest that the patient's hormonal context plays a crucial role in determining cancer outcome. The exact nature of thyroid hormone action on tumour growth has not been established yet, in fact contrasting data show thyroid hormones have a promotory or an inhibitory action on cancer cell proliferation depending on the case. We hypothesized that not only tissue specificity, but also specific mutations occurring during tumoral development in different thyroid hormone cellular targets are responsible for this dual effect. To test our hypothesis we analysed, by time-course and bromodeoxyuridine assay, thyroid hormone effects on the proliferation of six cancer cell lines originating from the same tissue or organ but carrying different mutations (in phospho-inositide 3 kinase or β-catenin genes). The data obtained in this study show how mutations that affect the balance between degradation and stabilization of β-catenin assume a remarkable importance in determining the cell-specific thyroid hormone effect on cell growth.