SLC5A5 and metastatic neoplasm: Importantly, in about 30–40% of patients with metastatic disease this treatment strategy is not curative because of tumor cell resistance to RAI caused by a process of dedifferentiation and concomitant loss of thyroid-specific gene expression including the gene encoding the human sodium iodide symporter (SLC5A5), leaving patients at high risk of recurrent or persistent disease.3,4