Isotopes of Iodine - Non-radioactive Iodide (I-127) As Protection From Unwanted Radioiodine Uptake By The Thyroid

Non-radioactive Iodide (I-127) As Protection From Unwanted Radioiodine Uptake By The Thyroid

Thyroid iodine uptake blockade with potassium iodide is used in nuclear medicine scintigraphy and therapy with some radioiodinated compounds that are not targeted to the thyroid, such as iobenguane (MIBG), which used to image or treat neural tissue tumors, or iodinated fibrinognen, which is used in fibrinogen scans to investigate clotting. These compounds contain iodine, but not in the iodide form. However, since they may be ultimately metabolized or break down to radioactive iodide, it is common to administer non-radioactive potassium iodide to insure that iodide from these radiopharmaceuticals is not sequestered by the normal affinity of the thryoid for iodide.

Potassium iodide has been distributed to populations exposed to nuclear fission accidents such as the Chernobyl disaster. The iodide solution SSKI, a saturated solution of potassium (K) iodide in water, has been used to block absorption of the radioiodine (it has no effect on other radioisotopes from fission). Tablets containing potassium iodide are now also manufactured and stocked in central disaster sites by some governments for this purpose. In theory, many harmful late-cancer effects of nuclear fallout might be prevented in this way, since an excess of thyroid cancers, presumably due to radioiodine uptake, is the only proven radioisotope contamination effect after a fission accident, or from contamination by fallout from an atomic bomb (prompt radiation from the bomb also cases other cancers, such as leukemias, directly). Taking large amounts of iodide saturates thyroid receptors and prevents uptake of most radioactive iodine-131 that may be present from fission product exposure (although it does not protect from other radioisotopes, nor from any other form of direct radiation). The protective effect of KI lasts approximately 24 hours, so must be dosed daily until a risk of significant exposure to radioiodines from fission products no longer exists. Iodine-131 (the most common radioiodine contaminant in fallout) also decays relatively rapidly with a half-life of eight days, so that 99.95% of the original radioiodine has vanished after three months.