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THE PARALLELISM BETWEEN THE PHYSIOLOGICAL ACTIVITY 
AND THE IODINE CONTENTS OF THYROID. 
A. HISTORICAL. 
The views as to the relation of the iodine to the physiological activ- 
ity of the thyroid may be classified under three heads: (1) Some 
investigators hold that the activity of thyroid is directly proportional 
to and dependent upon its iodine content and that thyroid free of 
iodine has no physiological activity. (2) There is another group of 
writers who take the view that there is no relation between the physio- 
logical activity of thyroid and its iodine content and that no impor- 
tance should be attached to the iodine which is usually present in the 
thyroid. (3) Other writers admit that there is a parallelism between 
the physiological activity and the iodine content, but deny that the 
former is dependent upon the latter. Some of those holding this 
view apparently consider that the iodine is simply associated with 
the active part’ of the gland in some unknown, perhaps accidental, 
way. Others hold that the larger amount of iodine in active thyroid 
results from the activity of the gland itself; they consider that the 
more active the gland the larger the amount of iodine it is able to 
hold in combination. This latter view is closely connected with the 
theory that one of the functions of the thyroid is to take up injurious 
substances (including iodine) and to render them harmless. 
Some of these theories have been combined. Thus von Cyon 
supposes the thyroid to take up iodine, by which process the latter 
is rendered harmless and at the same time a new compound, useful 
as an internal secretion, is formed. 
TYe believe that these conflicting views have resulted partly from 
the failure of previous writers to recognize that thyroid free of 
iodine has a certain degree of physiological activity and partly from 
the fact that there has been a confusion of several distinct problems 
and that authors have often looked upon the subject from but a 
single point of view. 
Baumann and some of the other earlier workers in this field appar- 
ently entertained no doubts as to the iodine being an important part 
of the “ active principle” of the thyroid. They based this view upon 
the physiological action of iodothyrine in health and disease a and 
a E. Baumann and E. Roos, Ztschr. f. physiol. Chem., Strassb., 1895-6, 21, p. 487. 
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