The approximations in the equations used for the calculation of the radioiodine therapy are reviewed and compared with a reference formula, based on ICRP Publication 30. Applying these equations to two clinical cases, we find pronounced differences compared to the reference method. Investigations of a collective of 314 patients yield in a relatively high fraction delayed maxima in the uptake curves. Simulations prove that serious errors occur when the ascending slope of the uptake curve is neglected. An equation taking care of this effect is presented. Pronounced individual differences are found between uptake curves recorded before and after therapy. The effects on the difference between planned and actually applied dose to the thyroid are shown. The difficulties of the determination of the thyroid mass are discussed and a combined method using scinti- and sonographic data presented.