Sodium iodide symporter expression and radioiodine distribution in extrathyroidal tissues

The functional role of the sodium iodide symporter (NIS) in extrathyroidal tissues was investigated by examining its mRNA and protein expression, together with the evidence of radioiodine (¹³¹I) uptake in 302 patients who underwent ¹³¹I total body scanning, following the administration of high doses of ¹³¹I for a papillary or follicular thyroid carcinoma. By using a real-time kinetic quantitative RT-PCR and immunohistochemistry, the expression of NIS protein was detected mainly in secretory tissues. In parallel, ¹³¹I uptake was evidenced in the majority of patients in the salivary glands (in 39%) and stomach (in 78%), but was found in breast in only 4 young female patients. These data demonstrate a strong correlation between the organ radioactivity distribution, as observed in vivo, and NIS protein expression. Interestingly, ¹³¹I is rarely concentrated by mammary glands, even when large doses are administered. Moreover, a ¹³¹I transfer in secretion fluids may represent a potential source of contamination responsible for false positive images and diagnostic pitfalls.