Principles of Radioiodination and Iodine-Labeled Tracers in Biomedical Investigation †

3In 1938, Hertz, Hamilton, and co-workers ^1,2 pioneered the use of ¹²⁸I radionuclide (T_1/2 = 25 min) in the investigation of thyroid physiology. ¹²⁸I radionuclide was prepared by the Szilard-Chalmers reaction through the interaction of neutrons from a plutonium-beryllium source, with ethyl iodide as target, by the ¹²⁷I (n, γ) ¹²⁸I nuclear reaction. Because of the importance of the physical properties of y-ray energy and the physical half-life in tracer techniques, other radioisotopes were explored, and the radionuclides of ¹²⁶I, ¹³⁰I, and ¹³¹I were discovered in 1938. Iodine has about two dozen radionuclides. The physical characteristics of several radionuclides of iodine used in biomedical investigation are listed in Table 1.