566 Tracer Techniques /30 : 5 



beta particles of 0.6 and 0.3 Mev maximum energy, as well as gamma 

 rays of 0.08, 0.28, 0.37, and 0.64 Mev. The last two pass readily 

 through tissue and through thin aluminum sheets; they can be detected 

 very easily. By surrounding the detector with a metal shield, it is 

 possible to reduce the background noise without excessive loss of sensi- 

 tivity to the gamma rays. 



The isotope I 131 can be made by bombarding I 130 with neutrons. A 

 more satisfactory method is to bombard Te 130 with neutrons. In this 

 way, I 131 is formed which can be separated in a highly purified form 

 from all other elements. Most I 131 production is by separation of 

 fission products. For biological studies, I 131 is usually converted to 

 iodide. 



Iodine is an important metabolite for vertebrates because it forms 

 part of the thyroid hormones. The concentration of iodine in the 

 thyroid is 10,000 times greater than in any other body organ. Only 

 very small amounts of iodine are needed daily by humans, approxi- 

 mately 100 micrograms per day. If less is included in the diet, the 

 person develops various thyroid difficulties. 



The tracer isotope I 131 has been used to follow the course of iodine 

 from its ingestion through its concentration in the thyroid, distribution 

 throughout the body in thyroid hormones, and its final excretion. A 

 specific example is the uptake of intravenously injected iodide by the 

 thyroid. Before the advent of tracers, it was impossible to demonstrate 

 this process in normal individuals injected with physiological amounts 

 of iodide. 



Experiments have been conducted in which varying amounts of I 131 

 were injected into guinea pigs, rats, dogs, humans, and so on. In every 

 case, at low iodide injections, a large part or perhaps all of the injected 

 iodide was concentrated within the thyroid within 24 hours. With high 

 "pharmacological" doses of 5 mg of I 131 per kilogram of body weight, 

 the thyroid concentrated a small fraction of the injected iodide in 5 

 minutes and thereafter became saturated. Keeping subjects on an 

 extra-high iodide diet also saturated the thyroid. Experiments with 

 inhibitors showed that the thyroid concentrates iodide per se, even if it 

 is inhibited from forming di-iodo-tyrosine and thyroxin. These experi- 

 ments emphasize that the thyroid of a person receiving a normal human 

 diet is not saturated with iodine but is in a position to absorb it against 

 tremendous concentration gradients (at least 500 : 1 for blood/thyroid 

 iodide ratio). 



32 



5. P 



Phosphorus is an important constituent of all living matter. It is an 

 essential part of the nucleic acids which transmit genetic information 



