The purity of radioiodide-I-131 assessed by in vivo and in vitro methods.

Between 41 and 94% of the radioactivity of 24 preparations of I(131) supplied without cysteine preservative was non-iodide on chromatographic analysis. Extraneous radio-activity was essentially absent from I(131) supplied with cysteine. It was converted to iodide-I(131) by 10(-3) M cysteine or iodide but not by incubation at pH 2. The average thyroid uptake of I(131) containing extraneous radioactivity was significantly lower than the uptake of I(131) free from non-iodide impurity in 16 human subjects measured under controlled conditions and in a random group of 669 patients. Incubation of samples of I(131) containing non-iodide radioactivity with tyrosine and cupric chloride resulted in the non-enzymatic formation of monoiodotyrosine-I(131) either in the presence or absence of thyroid homogenate. Enzymatic formation of monoiodotyrosine-I(131) by thyroid homogenates could be demonstrated only when I(131) free from extraneous activity was used.