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.