The Hanford Thyroid Disease Study (HTDS) is one of the largest and most complex epidemiologic studies of the relation between environmental exposures to I-131 and thyroid disease. The study detected no dose-response relation using a 0.05 level for statistical significance. The results for thyroid cancer appear inconsistent with those from other studies of populations with similar exposures. The absence of statistically significant dose-response relationships and the low estimates of excess risk reported for the HTDS can be explained by the high uncertainty associated with the use of mathematical models to reconstruct iodine 131 releases, environmental transport, and thyroid doses for individual cohort members. This uncertainty arises from unknown degrees of systematic overestimation of dose, as well as random measurement errors. Incomplete accounting for the full effect of these problems would result in an overestimation of statistical power and inappropriately narrow interval estimates for the excess risk of disease.
A paper published in Health Physics explores evidence that the HTDS statistical power was inadequate due to complex uncertainties associated with the mathematical models and assumptions used to reconstruct individual doses and concludes that, at the very least, the confidence intervals reported by the HTDS for thyroid cancer and other thyroid diseases are too narrow because they fail to reflect key uncertainties in the measurement-error structure.
A letter to the editor of the Journal of the American Medical Association explores the same topic: JAMA, August 2, 2006 - Volume 296, No. 5