Reactions to Standard Stimuli and Ageing 



53 



one activity of the thyroid, namely, iodine uptake, as a func- 

 tion of age. This method has the advantages that it is inde- 

 pendent of the histological picture of the gland, and that the 

 test itself does not disturb the subject's physiological condi- 

 tion. The disadvantage is that to get an absolute measure of 

 uptake the animals must be sacrificed. The method has, 

 however, been based on measurements in 371 rats, aged from 



200 



PERCENTAGE 



UPTAKE PER 



IOOmgTHYROID 



TISSUE 



150 



100 



WEANING 

 TIME 



5 6 7 8 



AGE IN MONTHS 



Fig. 4. Graph showing 24-hour percentage uptake of tracer dose of radio- 

 iodine per 100 mg. thyroid tissue. Subjects : 371 Wistar rats. 



1 day to over 2 to 3 years old, and with the normal percentage 

 uptake established this value can be used as a standard for 

 survival experiments in which surface methods of counting, 

 as with a collimated Geiger counter, can be used. The series 

 falls into two groups, as the percentage uptake of radioiodine 

 is directly proportional to the iodine intake in the diet. Thus, 

 the first group are the foetal and neonatal animals, fed on milk 

 only, and the second group the weaned animals, fed on a diet 

 of fixed iodine content. 



