Definition of iodine 131
: a heavy radioactive isotope of iodine having the mass number 131 and a half-life of 8 days that is produced in the fission of uranium or by bombardment of tellurium with neutrons and that is used especially in the form of sodium radioiodide in the diagnosis of thyroid disease and the treatment of goiter —symbol I131 or 131I —called also radioiodine
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