[Exposure to radioactive iodine in pregnancy: significance for mother and child].

The embryonal stage in mammalians is characterized by a quick proliferation and differentiation of cells. The special features of this stage of development in all living beings is therefore an increased sensitivity for the exposure with ionizing radiation. Radiation exposure during the prenatal development can therefore lead to various impairments, which can be short-termed or long-termed, showing effects even in the postnatal period. The pattern of radiation induced effects is dependent upon the radiation dose on the one hand and upon the stage of fetal development when radiation exposure occurs on the other hand. Radiation induced effects can be growth retardation, malformations, functional impairments or death as well as increased occurrence of cancer and leukemia during childhood. The main effects of a radiation exposure in the fetal period are: 1) lethal effects for the embryo, 2) malformations and changes in growth or other functional changes, 3) mental retardation, 4) induction of malignomas including leukemia. Lethal effects can be induced experimentally in animals by relatively low radiation doses of 10 cGy, administered before or immediately after the implantation of the embryo. Malformations can be induced if the exposure occurs during the period of organogenesis especially if the radiation exposure occurs during the active stage of increased cell formation and cell differentiation of a specific organ. For many types of effects of ionizing radiation especially for the death of the embryo or fetus and for macroscopic anatomical malformation a dose-effect relationship with certain threshold doses can be supposed. This threshold dose is not smaller than 5 cGy if the exposure results from a low level radiation with low LET> Radiation exposure at the end of the organogenesis and during the following fetal period can induce growth retardation and functional disturbances, which are characterized by abnormalities in the postnatal period. Of special importance are the abnormalities of the CNS, like mental retardation particularly if the radiation exposure occurred during the interval between the 8th and 15th week of pregnancy. During that time period cell formation for the development of the frontal brain occurs. The induction of this type of abnormalities as well as of other malformations is due to non stochastic effects. A threshold dose of 5 cGy ist discussed. The induction of malignancies and leukemia as a consequence of a radiation exposure in the prenatal period is to be seen as a deterministic (non stochastic) radiation effect. The sensitivity of the fetus for these effects ist 2 to 3 times higher than that of adults.