When large amounts of radioactive iodine are released into the environment, as happened in both Chernobyl and Fukushima Japan, the thyroid will easily take it up. The thyroid cannot tell the difference between radioactive iodine and nonradioactive iodine, so if it is present in the environment, the thyroid will take it in.
Nonetheless, the risk of radiation-induced thyroid cancer after Japan's crisis is expected to be fairly minimal. First, although large amounts of radioactive iodine were released into the environment, the Japanese people were evacuated from the area quickly, and afterward, they were instructed to restrict their intake of green leafy vegetables, milk and water from potentially contaminated sources. They also took potassium iodide, a safe form of iodine, to saturate the thyroid and block it from absorbing radioactive iodine. That did not happen at Chernobyl in 1986.
Second, the types of radiation exposure differed in the two incidents. In 1986, people were exposed to high concentrations of radioactive cesium, strontium and iodine. Cesium and strontium have long half-lives, meaning that radioactive contamination will remain for hundreds of years. Potassium iodine, a safe form of iodine, which can be used to saturate the thyroid and block it from absorbing radioactive iodine does not protect the thyroid from these two radionuclides.
In Japan, cesium and radioactive iodine were released. Although cesium increases the risk for cancer in general, it poses no specific risk to the thyroid since it is not concentrated within the thyroid gland. Radioactive iodine, with its half-life of eight days, was virtually gone in about two months. Since the Japanese largely evacuated the area around the nuclear power plant, their overall exposure was much lower than those living near Chernobyl 25 years before. Residual radioactive iodine that might have spread to surrounding areas still would have degraded and disappeared relatively quickly, making it very unlikely that people in distant areas would have been impacted.