<qa>  What is the null hypothesis that the difference is no greater than 5?Using the measured concentrations of radioiodine in the blood samples, the radiation dose to the bone marrow of each participant will be calculated following the methods described in European guidance Luster et al, Eur J Nucl Med Mol Imaging 3510 1941 2008. This calculation will be performed for each of the two blood sampling methods, creating paired measurements of the same variable for each participant. The statistical significance of the percentage difference in bone marrow dose calculated from each blood sampling method will then be assessed using a Wilcoxon matched pairs test with the null hypothesis that there is no difference in the calculated bone marrow doses. This study has 80 power to demonstrate at the 5 level of statistical significance a difference of no less than 5 between the mean values for the two bone marrow doses. Therefore statistical analysis of the differences has 80 power to show that the null hypothesis that the difference is no greater than 5 is tenable, if indeed this null hypothesis holds true in the population. 