526 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 27 



species have found biological application: I 128 (24.99-min half-life) produced 

 in the cyclotron by the reactions I 127 (d, p)I 128 and I 127 (n, 7)I 128 ; I 130 (12.6-hr 

 half-life) and I 131 (8.0-day half-life), produced together in the cyclotron by 

 the reactions Te 130 (d, 2n)I 130 and Te 130 (d, n)I 131 , respectively. I 130 and I 131 

 also occur as fission products, the latter in much greater quantities; I 130 can 

 be prepared by itself in the cyclotron by the reaction Te 130 (p, n)I 130 ; and I 131 

 can be recovered in carrier-free form from the nuclear pile and subsequent 

 decay reaction Te 130 (n, 7)Te 131 — > I 131 -f- (3~. In addition there are several 

 other as yet unapplied species of varying half-lives: I 124 (4-day), I 125 (56-day), 

 I 126 (13.0-day), I 133 (22-hr), and I 135 (6.6-hr). Some of these may find biolog- 

 ical application. 



Early work with iodine was done with I 128 (in some cases produced by 

 bombarding I 127 with neutrons from a radium-beryllium mixture). The 

 concentration of trace amounts of iodine in the thyroid gland was demon- 

 strated strikingly by this early work [143,70,72, etc.]. 



Most recent tracer work has been done with the two isotopes I 130 and I 131 , 

 which are produced as a mixture in the cyclotron by deuteron bombardment 

 of tellurium. Currently I 131 is used almost exclusively. The primary 

 application of the iodine label has, of course, been in the study of thyroid 

 physiology. This has recently been reviewed by Leblond [163]. 



Before radioactive iodine became available, the entry of iodine into the 

 thyroid gland could not be studied under physiological conditions. It 

 has become possible with the radioactive species to study not only entry 

 but also the incorporation of iodine into the thyroid hormone. When 

 minute doses of iodine are administered to the mammal, a high proportion 

 becomes incorporated in the gland — about 60 per cent in the rat by 24 hr 

 [193] and about 50 per cent in man [19,41,69]. However, when these small 

 doses are given after pretreatment with pharmacological amounts, the 

 uptake of the labeled material is much reduced [144,72]. Initially the 

 inorganic iodide fraction of the thyroid iodine has a higher specific activity 

 than the organically bound. This becomes incorporated into diiodotyrosine 

 and thyroxine. Diiodotyrosine turnover in man is thought to take about 

 24 hr [169]. For several days the specific activity of the thyroxine iodine 

 is less than that of the inorganic or diiodotyrosine iodine, but, while the 

 diiodotyrosine fraction remains more or less constant, the thyroxine activity 

 gradually rises. This is satisfactorily explained by assuming the conversion 

 of diiodotyrosine into thyroxine rather than the direct iodination of thyronine, 

 the non-iodine-containing analogue of thyroxine [181]. 



With radioactive iodine it has been possible to study precisely the effects 

 of various extrinsic factors on iodine metabolism in the thyroid, e.g., pituitary 

 [190,91, etc.], thiourea [14], sulfonamides [114,123], goitrogenic substances 

 [115,123], etc. 



