Smad4 is tumor suppressor gene and is crucial for thyroid development and function. It is key component of the transforming growth factor beta (TGFB) pathway which regulates the expression of thyroid-specific genes. The role of SMAD4 in thyroid tumorigenesis remains unclear. However, mutations affecting the coding region of the SMAD4 gene have been detected in some sporadic thyroid neoplasms. Studies that analyzed expression of SMAD4 in thyroid tumors gave contradictory results. Some studies showed that the expression of SMAD4 was reduced in thyroid cancer cells. Other studies indicated that it was expressed and present in the nucleus in all thyroid cell lines and controls analyzed, indicating propagation of TGFB signaling in thyroid tumors. Of note, a SMAD4 mutation C324Y, recently isolated from nodal metastases of papillary thyroid carcinoma (PTC), was confirmed to cause an increase in TGFB signaling and lead to the acquisition of transformed phenotype and invasive behavior in thyroid cells, enabling them to proliferate independently from thyroid-stimulating hormone.