The radiotoxicology of iodine

Thirty radioisotopes of iodine are known but only those with mass numbers from 123 to 135 are of major radiotoxicological interest. Exposure of animals or man to inorganic¹³¹I or¹²⁵ I may result in the induction of benign or malignant thyroid tumors or depression of thyroid function; Bq for Bq¹²⁵I is less toxic than¹³¹I. However, the shorter lived radioiodines¹³²I,¹³³I, and¹³⁵I appear to be 10 to 100 X more toxic than¹³¹I alone. Adrenal, pituitary and ovarian tumours, as well as parathyroid hypofunction and other biochemical disturbances, have been reported in animals but not, so far, in man. Gonad doses from¹³¹I up to at least 800 mGy do not appear to cause in man an increased incidence of congenital abnormalities or spontaneous abortions. Little information is available about the toxicity of radioiodine containing organic compounds. The DNA precursor, iododeoxyuridine when labelled with¹²⁵I becomes incorporated into the cell nucleus and produces severe and often irreparable damage due to the emission of Auger electrons. The risk estimate for the induction of thyroid carcinoma or adenoma by inorganic¹³¹I is considered to be 10 to 20·10⁻⁶ persons Gy⁻¹ y⁻¹, but may be up to 100 times larger for persons exposed to mixtures of short-lived radioiodines.