When 131 iodine is taken up by the thyroid gland, almost all of the ray energy is absorbed by the thyroid tissue, but the energy absorbed by the follicular epithelial cells in the center of the gland is greater than that absorbed by the 2-mm-thick layer of cells around the gland, so that the destruction of the center is greater than that of the peripheral parts of the gland. This is because part of the energy of the rays emitted from the peripheral cells of the thyroid gland is lost outside the thyroid tissue. It has been observed that when a person takes 131 iodine not more than 100mmCi, it does not cause pathological changes in the peripheral thyroid organs and its other endocrine glands. One week after the administration of 131 iodine, the thyroid gland is usually free of any histological changes. 2-4 weeks, there are already marked destructive changes in the central part of the thyroid gland, such as stromal edema, degeneration, acute thrombotic and hemorrhagic chorioretinitis, epithelial swelling with the formation of vacuoles, and follicular destruction with the infiltration of polymorphonuclear cells. In the periglandular area, the damage is much less severe. 2-3 months, the gland begins to have fibrous tissue hyperplasia, lymphocyte infiltration, thickening and hyalinization of small arterioles, and follicular epithelial detachment to the point of gradual death. 1-3 years, most of the normal tissues of the thyroid gland have been replaced by dense fibrous tissues.