60 



RADIOISOTOPES IN BIOLOGY AND AGRICHLTTJHE 



be an important consideration. The mass that must be employed 

 depends upon the specific activity of the preparation available, the degree 

 of accumulation in the samples to be measured, and the sensitivity of the 

 measurement. These factors may frequently be varied to fit the experi- 

 mental requirements. The following terminology is often used: Tracer 

 dose — the amount of the administered element is small compared with the 

 normal intake or that normally present in the system. Physiological dose 

 — the amount of the administered element is of the same order of magni- 



100 



10 



■a 

 'o 



J3 



■o 



o 



o 

 _o 



'■o 



o 



3 0.1 



001 



10 



100 



000001 0.0001 0,001 0.01 0.1 1 



Iodine, mg/kg body weight 



Fig. 2-1. Effect of the mass of iodine administered on the percentage of thyroidal 

 uptake of P^'. (From J. G. Hamilton, Summary of Iodine Physiology and Metabolic 

 Studies Using Radioactive Isotopes of Iodine, MDDC-1060, 1944.) 



tude as the normal intake. Massive dose — the amount of the adminis- 

 tered element far exceeds the normal intake. The tracer and physiolog- 

 ical doses will usually follow the normal path, since the amount of the 

 element introduced into the system will not be large enough to disturb 

 equilibrium or steady-state conditions. However, a massive dose may 

 well flood the tissues and present an abnormal picture. 



The classic example of flooding is the effect of the mass of iodine admin- 

 istered upon the percentage of uptake by the thyroid, as summarized by 



