Part II. 
ON THE NATURE OF THE RELATION BETWEEN THE IODINE PER- 
CENTAGE AND THE PHYSIOLOGICAL ACTIVITY OF THYROID. 
It has been shown above that there is almost invariably a paral- 
lelism between the iodine content of thyroid and its physiological 
activity. The nature of this relation may now be discussed. Some of 
the writers who recognize that there is such a parallelism hold that 
the larger amount of iodine in the more active thyroid is a result and 
not the cause of the greater activity. In other Avords, according to 
this view, the more active the gland the larger the amount of iodine 
with which its substance can combine.® 
The arguments advanced for this theory are in part the same as 
those which have been urged against the view that the iodine is an 
important constituent of the thyroid and which have been discussed 
above. Such arguments are based largely upon what may be called 
statistical studies, and, as we have already seen, conclusions drawn 
from such studies do not necessarily apply to the thyroid when used 
as a drug. Moreover, were this view correct, we should expect to find 
at times thyroid poor in iodine but with a high degree of physiological 
activity, for the amount of iodine present in the thyroid is to a con- 
siderable extent accidental, being dependent upon the amount of this 
element available in the food. We have, however, never found a 
specimen of thyroid with a low percentage of iodine and with a high 
degree of physiological activity. Whether from the same or from 
widely separated species, the activity was in general parallel with the 
iodine content. 
A stronger argument against the view that the activity of thyroid 
is independent of the percentage of iodine is found in a metabolism 
experiment of Roos, 5 in which the activity of normal thyroid was 
compared with that of thyroid which had combined with a larger 
amount of iodine in vivo. Roos found the iodine-free thyroid of a 
dog to be without appreciable effect upon nitrogen excretion, whereas 
an equal amount of thyroid from a dog which had received 20 grams 
of potassium iodide in eighteen days and which contained 0.35 per 
cent iodine had a very distinct effect. Only one experiment was per- 
formed, and the result, although distinct, was not very marked. 0 
a cf. S. J. Meltzer, N. Yorker med. Monatschr., 1907, 19, p. 223. 
&E . Roos, Ztschr. f. physiol. Chem., Strassb., 1899, 28, p. 47. 
C cf. F. Blum, Verhandl. d. Kong. f. inn. Med., 1906, 23, p. 196. 
( 93 ) 
