Thyroid 131I activities were determined for five human subjects from a disaster medical assistance team of Fukui Prefectural Hospital. The team was dispatched to the Tamura City Sports Park, 40 km from the Fukushima Daiichi nuclear power plant. They were exposed to a radioactive plume on 15 March 2011. In vivo measurements at Fukui Prefectural Hospital were conducted around 17 h after the team left the park. A thyroid counter equipped with a 51-mm-diameter × 51-mm-thick NaI(Tl) detector with a 20-mm-thick lead collimator was used. Mock iodine (133Ba and 137Cs) with a thyroid uptake neck phantom was used for calibration. On 16 March 2011, at 11:30, thyroid activity of a member of the team age 53 y, who was never administered stable iodine, was 268 ± 38 Bq. The remaining four men, aged 49, 35, 34, and 27 y, ingested two stable iodine pills (a total of 100 mg of potassium iodide) approximately 36 h before being exposed to the plume. Their thyroid activity values were 249 ± 86 Bq, 676 ± 107 Bq, 569 ± 96 Bq, and 1,082 ± 119 Bq, respectively. An inverse relationship between age and thyroid activity was observed among those who ingested potassium iodide before exposure, indicating that stable iodine administration may have a protective effect. Thyroid 131I activity was reduced by approximately 70% in the oldest person. This can be explained by the iodine metabolism in the thyroid of younger individuals being significantly faster than that of older individuals.