Simulations of radioiodine exposure and protective thyroid blocking in a new biokinetic model of the mother-fetus unit at different pregnancy ages

In the case of nuclear incidents, radioiodine may be released. After incorporation, it accumulates in the thyroid and enhances the risk of thyroidal dysfunctions and cancer occurrence by internal irradiation. Pregnant women and children are particularly vulnerable. Therefore, thyroidal protection by administering a large dose of stable (non-radioactive) iodine, blocking radioiodide uptake into the gland, is essential in these subpopulations. However, a quantitative estimation of the protection conferred to the maternal and fetal thyroids in the different stages of pregnancy is difficult. We departed from an established biokinetic model for radioiodine in pregnancy using first-order kinetics. As the uptake of iodide into the thyroid and several other tissues is mediated by a saturable active transport, we integrated an uptake mechanism described by a Michaelis–Menten kinetic. This permits simulating the competition between stable and radioactive iodide at the membrane carrier site, one of the protective mechanisms. The Wollf–Chaikoff effect, as the other protective mechanism, was simulated by adding a total net uptake block for iodide into the thyroid, becoming active when the gland is saturated with iodine. The model’s validity was confirmed by comparing predicted values with results from other models and sparse empirical data. According to our model, in the case of radioiodine exposure without thyroid blocking, the thyroid equivalent dose in the maternal gland increases about 45% within the first weeks of pregnancy to remain in the same range until term. Beginning in the 12th pregnancy week, the equivalent dose in the fetal thyroid disproportionately increases over time and amounts to three times the dose of the maternal gland at term. The maternal and fetal glands’ protection increases concomitantly with the amount of stable iodine administered to the mother simultaneously with acute radioiodine exposure. The dose–effect curves reflecting the combined thyroidal protection by the competition at the membrane carrier site and the Wolff–Chaikoff effect in the mother are characterized by a mean effective dose (ED₅₀) of roughly 1.5 mg all over pregnancy. In the case of the fetal thyroid, the mean effective doses for thyroid blocking, taking into account only the competition at the carrier site are numerically lower than in the mother. Taking into account additionally the Wolff–Chaikoff effect, the dose–effect curves for thyroidal protection in the fetus show a shift to the left over time, with a mean effective dose of 12.9 mg in the 12th week of pregnancy decreasing to 0.5 mg at term. In any case, according to our model, the usually recommended dose of 100 mg stable iodine given at the time of acute radioiodine exposure confers a very high level of thyroidal protection to the maternal and fetal glands over pregnancy. For ethical reasons, the possibilities of experimental studies on thyroid blocking in pregnant women are extremely limited. Furthermore, results from animal studies are associated with the uncertainties related to the translation of the data to humans. Thus model-based simulations may be a valuable tool for better insight into the efficacy of thyroidal protection and improve preparedness planning for uncommon nuclear or radiological emergencies.

A comparison of thyroidal protection by iodine and perchlorate against radioiodine exposure in Caucasians and Japanese

Radioactive iodine released in nuclear accidents may accumulate in the thyroid and by irradiation enhances the risk of cancer. Radioiodine uptake into the gland can be inhibited by large doses of stable iodine or perchlorate. Nutritional iodine daily intake may impact thyroid physiology, so that radiological doses absorbed by the thyroid as well as thyroid blocking efficacy may differ in Japanese with a very rich iodine diet compared to Caucasians. Based on established biokinetic-dosimetric models for the thyroid, we derived the parameters for Caucasians and Japanese to quantitatively compare the effects of radioiodine exposure and the protective efficacy of thyroid blocking by stable iodine at the officially recommended dosages (100 mg in Germany, 76 mg in Japan) or perchlorate. The maximum transport capacity for iodine uptake into the thyroid is lower in Japanese compared to Caucasians. For the same radioiodine exposure pattern, the radiological equivalent thyroid dose is substantially lower in Japanese in the absence of thyroid blocking treatments. In the case of acute radioiodine exposure, stable iodine is less potent in Japanese (ED50 = 41.6 mg) than in Caucasians (ED50 = 2.7 mg) and confers less thyroid protection at the recommended dosages because of a delayed responsiveness to iodine saturation of the gland (Wolff-Chaikoff effect). Perchlorate (ED50 = 10 mg in Caucasians) at a dose of 1000 mg has roughly the same thyroid blocking effect as 100 mg iodine in Caucasians, whereas it confers a much better protection than 76 mg iodine in Japanese. For prolonged exposures, a single dose of iodine offer substantially lower protection than after acute radioiodine exposure in both groups. Repetitive daily iodine administrations improve efficacy without reaching levels after acute radioiodine exposure and achieve only slightly better protection in Japanese than in Caucasians. However, in the case of continuous radioiodine exposure, daily doses of 1000 mg perchlorate achieve a high protective efficacy in Caucasians as well as Japanese (> 0.98). In Caucasians, iodine (100 mg) and perchlorate (1000 mg) at the recommended dosages seem alternatives in case of acute radioiodine exposure, whereas perchlorate has a higher protective efficacy in the case of longer lasting radioiodine exposures. In Japanese, considering protective efficacy, preference should be given to perchlorate in acute as well as prolonged radioiodine exposure scenarios.

A comparison of thyroidal protection by stable iodine or perchlorate in the case of acute or prolonged radioiodine exposure

In the case of a nuclear power plant accident, repetitive/prolonged radioiodine release may occur. Radioiodine accumulates in the thyroid and by irradiation enhances the risk of cancer. Large doses of non-radioactive iodine may protect the thyroid by inhibiting radioiodine uptake into the gland (iodine blockade). Protection is based on a competition at the active carrier site in the cellular membrane and the Wolff-Chaikoff effect, the latter being, however, only transient (24-48 h). Perchlorate may alternatively provide protection by a carrier competition mechanism only. Perchlorate has, however, a stronger affinity to the carrier than iodide. Based on an established biokinetic-dosimetric model developed to study iodine blockade, and after its extension to describe perchlorate pharmacokinetics and the inhibition of iodine transport through the carrier, we computed the protective efficacies that can be achieved by stable iodine or perchlorate in the case of an acute or prolonged radioiodine exposure. In the case of acute radioiodine exposure, perchlorate is less potent than stable iodine considering its ED50. A dose of 100 mg stable iodine has roughly the same protective efficacy as 1000 mg perchlorate. For prolonged exposures, single doses of protective agents, whether stable iodine or perchlorate, offer substantially lower protection than after acute radioiodine exposure, and thus repetitive administrations seem necessary. In case of prolonged exposure, the higher affinity of perchlorate for the carrier in combination with the fading Wolff-Chaikoff effect of iodine confers perchlorate a higher protective efficacy compared to stable iodine. Taking into account the frequency and seriousness of adverse effects, iodine and perchlorate at equieffective dosages seem to be alternatives in case of short-term acute radioiodine exposure, whereas preference should be given to perchlorate in view of its higher protective efficacy in the case of longer lasting radioiodine exposures.