54 



K. J. Franklin 



The results themselves show the immense activity of the 

 thyroid gland in the neonatal period, compared with its 

 activity in the adult and aged stages. The weight of the 

 thyroid tissue varied from 1 mg. in the neonatal animals to 

 25 mg. in the adults. The average uptake was 20-22 per cent 

 in the latter, and ca. 10-12 per cent in the former. However, 

 expressed as a function per 100 mg. thyroid tissue (Fig. 4), the 

 younger gland shows the higher activity and thereafter there 

 is a gradual fall-off with increase in age. 



PERCENTAGE 

 UPTAKE OF 

 0-5fic TRACER 



10 



15 



20 



25 



12 3 4 5 6 7 8 



AGE IN MONTHS 



Fig. 5. Graph showing 24-hour percentage uptake of tracer dose of 



radioiodine by thyroid gland at different ages. Subjects : 371 



Wistar rats. 



Fig. 5 shows the relative constancy of the percentage uptake 

 after the rats are one month old. This can be used to determine 

 the relative sensitivity of the thyroid gland to internal irradi- 

 ation at different ages. In an attempt to study foetal thyroid 

 uptakes, full-term mother rats have been injected with a known 

 dose of radioiodine, and the foetuses have been sacrificed 

 after a further 24 hours, following either spontaneous birth or 

 delivery by Caesarean section. They have been found to take 

 up ca. 2-5 per cent of the dose given to the mother. Now that 

 this level of uptake is established, it should be possible to 



