Apart from iodine contamination, radioiodine scan can be negative due to the dedifferentiation of tumour which still can produce Tg, but lost its ability to accumulate iodine, microscopic metastases which are too small to be visualized, and mutation of NIS (potassium/iodine symporter) in thyroid/ tumour cells.19 All these facts might be responsible for 131I negative scans in our patients, in spite of relatively effective LID. Here, SLC5A5 is linked to neoplasm.