LIMITS OF FETAL THYROID RISK FROM RADIOIODINE EXPOSURE

An incident in which a young woman became pregnant soon after being tr eated with 444 MBq I-131 for Graves disease prompted us to search loca l records for the occurrence of thyroid abnormalities among people exp osed in utero to fallout radioiodine, The data base from the Utah Fall out Study (Kerber et al. 1993) indicated that there had been 480 cohor t subjects for whom dose to thyroid from fallout radioiodine had been calculated and who could have received any thyroid dose before birth ( 2473 subjects had been re-examined in 1985-86 of the 4818 examined in 1965-70). Of these 480 subjects in this category, 403 of them could be located in the 1980's and were examined for abnormalities, Although n odules, thyroiditis, hypothyroidism and goiter were seen among the 375 persons with in utero thyroid doses from fallout radioiodine below 0. 42 Gy, no thyroid abnormalities of any kind occurred in the 4 persons with in utero thyroid doses of 0.5 to 2.6 Gy. In addition, no neoplasi a was found in any of the 403 subjects examined about 3 decades after in utero fallout exposure, These limited data do not indicate that the fetal thyroid is more sensitive than the postnatal thyroid by more th an about a factor of about 4 when thyroid dose is considered and by no t much more than unity when the comparison is based on dose equivalent (x-ray vs, radioiodine).