In 131-J treatment as in other forms of internal radiotherapy using biologically distributed radionuclides it is important to differentiate between projected and obtained internal radiation doses which may differ considerably because of a change in the kinetics during and because of the irradiation by radioiodine. The clinical result (remission, recurrence, ablation, hypothyroidism) as well as possibly necessary repetition of the treatment depend upon the obtained rather than the projected internal radiation dose. Dosimetry of the internal radiation dose to the thyroid considers the emitted energy of the radionuclide, the absorbed fractions, the mass of the thyroid tissue, maximum thyroid uptake, and the mean residence time in the target tissue (derived from the effective elimination half time). A monoexponential time course is observed in only about one half of the patients, whereas the other one half displays biexponential or nonlinear curves which can not be established reliably within 48 hours. While sonographic volumetry and thyroid uptake can be performed prior to iodine therapy, the true effective elimination halftime of the radioiodine can be measured only following its administration. About 97% of the internal radiation dose to the normal 20-g-thyroid gland is caused by the nonpenetrating emission. In goitres the contribution by the penetrating emissions increases proportional to the cubic root of the square of the mass. A dose of 5 mCi will cause in the 80-g-thyroid with 80% uptake internal radiation doses differing by a factor of 2 or more (i. e. approx. 1500 rad or less in one case or about 3000 rad or more in another case) depending upon the mean intrathyroidal residence time. The internal radiation doses to the gonads and the total body are dimensionally smaller than the thyroid dose by about one thousand. Radioiodine therapy using 125-I has been abandoned.