Inhibitory effect of low-dose inorganic iodine on thyroidal radioactive iodine uptake in healthy Japanese adults.

In the event of a nuclear power plant accident, prophylactic administration of potassium iodide (KI) is recommended to prevent thyroid damage due to uptake of radioiodine. To assess the inhibitory effect of low-dose inorganic iodine on thyroidal radioactive iodine uptake (RAIU) in healthy adults without dietary iodine restriction, single or repeated doses of 10 mg inorganic iodine solution were given to 22 Japanese volunteers, 18 men and 4 women with the mean age of 35.7 years, between 2011 and 2013. Changes in urinary iodine excretion, thyroid function and 24-hour RAIU were also evaluated. The median 24-hour RAIU without iodine restriction was 13% (range, 5-26%). A single-dose of 10 mg inorganic iodine suppressed the median 24-hour RAIU measured one hour after iodine administration to 3% (range, 1-7 %) and, in 90.9% of 22 participants their 24-hour RAIU was < 5%. For seven participants given 10 mg of inorganic iodine daily for 14 days, the median 24-hour RAIU measured at 24 hours after the last administration of iodine was 6% (range, 2-12%), although the inhibitory effect was diminished in two participants. Serum thyroid stimulating hormone concentration was slightly elevated in three participants without decreased serum FT3 and FT4 levels. We conclude that a single-dose of 10 mg inorganic iodine is sufficient to inhibit RAIU in adults, although the inhibitory effect of repeated-dose on RAIU is diminished when KI is given once daily. The dose, duration or interval of iodine administration should be evaluated in iodine-sufficient regions in a future.