In contrast to external X and γ radiation, radiation from internally deposited131 I has not been clearly shown to cause thyroid tumors or nodules. Given the wide use of131 I in medicine and concern about health risks associated with the release of radioiodines from nuclear reactors, more information is needed. The purpose of this study was to evaluate the risk of thyroid nodularity among women previously exposed to radioiodine (131 I) for diagnostic reasons. A clinical examination survey, including thyroid palpation, was conducted for a sample of women drawn from a cohort of patients exposed to131 I and a comparison group of women attending a mammography screening clinic. The study was conducted during 1991-1993 at Radiumhemmet, Karolinska Hospital, Stockholm, Sweden. A total of 1,005 women referred for a thyroid scintigraph or tracer test during the period 1952-1977 were included in the study. For comparison, 248 nonexposed women attending a mammography screening clinic were also enrolled. The primary outcome measure was the presence or absence of palpable thyroid nodules at the time of the clinical examination. Odds ratios were used as estimates of relative risk (RR). The mean dose to the thyroid from131 I was 0.54 Gy, and the average age at131 I administration was 26 years. The prevalence of thyroid nodularity was 10.6% among women who had been exposed to131 I and 11.7% among the nonexposed women [RR = 0.9; 95% confidence interval (CI) 0.6-1.4]. When analysis was restricted to women exposed to131 I, prevalence was associated positively with thyroid dose (excess RR = 0.9 per Gy; 95% CI 0.2-1.9). However, the excess RR was similar for women exposed before age 20 years and those exposed after age 20, which is contrary to findings for populations exposed to external radiation. While results demonstrated a positive association between diagnostic administration of131 I and the subsequent occurrence of thyroid nodules, it is unclear whether the association is causal, as the possibility of confounding by indication for131 I administration cannot be ruled out. No tumors that were subsequently diagnosed as cancer were found during the thyroid examinations, which supports the view that exposure to131 I administered for diagnostic reasons during adulthood rarely causes thyroid cancer.

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