 Each sample will be labelled with the patient identification number, the date, time and radioisotope I-131. To minimise the risk of spills, the samples will be transferred to a tray and moved to the Nuclear Medicine Department using a trolley. Sample manipulations will be carried out in the low level counting laboratory in the Nuclear Medicine Department. Three 1ml aliquots of whole blood will be removed from each vacutainer and transferred to appropriately labelled sample containers using an Eppendorf pipette. The volume of the samples collected by pipette will be determined by measuring the weight of the sample and subtracting the previously determined combined weight of the empty pipette and sample tube. Once all the blood samples have been collected and processed, measurements will be taken in an automated gamma counter to determine the concentration of radioiodine. The counting time for the pipette samples will be increased compared to the intravenous samples, in order to ensure the final dosimetry calculations are not influenced by the statistical uncertainty in the measured counts from the smaller volume samples. The resulting data will then be used in the detailed dosimetry calculations, in accordance with the published European guidelines. 