The Uptake Of Radioactive Iodine, 1 131, By Various Organs In Euthyroid And Hypothyroid Rats

More than 375 radioactive isotopes of the common chemical elements have been produced by nuclear bombardment. These include isotopes of every known stable chemical element. Many have proved exceptionally useful in biological and medical experimentation (7). The use of isotopes as tracers in the study of biological and other systems has been widely applied. The utilization of isotopes constitutes an extremely powerful method of studying the complex systems encounted in living organisms under equilibrium conditions without interacting, to any appreciable extent, with the system (16). Chief among the advantages of radioactive isotopes in medical investigative work is the ability to· discriminate between these tracers and their nonradioactive counterpart already present in the organism (11). The existence of elements practically identical in chemical properties, yet differing in mass, became known through study of radioactive decay. An example is one of the decay products of radium D, which occupies the same place in the periodic table as lead, is practically identical in chemical properties with ordinary lead and is apparently inseparable from it but at the same time differs in mass. To such elements of differing masses occupying the same place in the periodic table, were given the name isotopes (Greek, equal place). In time it became quite clear that the atomic number and not the atomic weight determined the place of an element in the periodic table (16). Aston (3) demonstrated that many lights, non-radioactive elements were mixtures of atoms of more than one mass species occupying the same place in the periodic table, that is, they were mixtures of isotopes. Aston\u27s (3) work not only clearly demonstrated the existence of stable isotopes, but furnishes the means, and the mass spectrograph of measuring accurately the mass and the relative abundance of an element