iodine-125
noun
\ -ˌwən-ˌtwen(t)-ē-ˈfīv
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Medical Definition of iodine-125
: a light radioactive isotope of iodine that has a mass number of 125 and a half-life of 60 days, gives off soft gamma rays, and is used as a tracer in thyroid studies and as therapy in hyperthyroidism