In this study the microscopic energy absorption from Auger electrons of iodine-125 in colloid of two different thyroid models, which represent the normal and thyrotoxic thyroid glands, was theoretically calculated. These calculations were based on the method recently given by Unak (1987; Nucl. Intrum. Meth. Phys. Res. A255, 274) and the Auger electron spectrum given by Charlton and Booz (1981; Radiat. Res. 87, 10). Subsequently the range of highest energy Auger electrons of iodine-125 in thyroidal colloid was found to be about 72-mu-m. This range is considerably shorter than the radius of a normal thyroid follicle, about 150-mu-m, but is considerably longer than the 25-mu-m radius of a thyrotoxic thyroid follicle. On the other hand, the comparison of dose distributions in two different thyroid models showed that the similar activity of iodine-125 per gram of thyroid tissue is able to produce more radiotoxic effects on the thyrotoxic follicular cells.