Thyroid effects

Risk coefficients for thyroid disorders have been developed for both 131I and external x or gamma low-LET radiation. A linear, no-threshold model has been used for thyroid neoplasms. A linear, threshold model has been used for other thyroid disorders. Improvements since the Reactor Safety Study were made possible by relevant new animal and human data. Major changes are as follows. Animal data are used to supplement the human experience where necessary. A specific risk estimate model is used for thyroid neoplasms, which accounts for observed effects of gender and age at exposure on risk. For thyroid cancer, the basis of the risk coefficients is the experience of North Americans following x-irradiation for benign disease in childhood. This recognizes possible differences in susceptibility in people of different heritage. A minimum induction period for thyroid neoplasms following irradiation is used to define periods at risk. An upper bound risk coefficient for cancer induction following exposure to 131I is based on human experience at relatively low dose exposures. While the overall lifetime risks of death due to thyroid cancer are consistent with projections by the ICRP, BEIR III, and UNSCEAR Reports, the current model permits greater flexibility in determining risk for population subgroups. 88 references, 8 tables