49 
bear. The experiments with the cat (D) and deer (F) thyroids show 
that there is a direct parallelism between the iodine content and 
the physiological activity of thyroid. 
SUMMARY AND GENERAL CONCLUSIONS. 
The above experiments show that iodine free thyroid, or what 
is usually called “iodine free” thyroid, has, when tested by our 
method, a low degree of physiological activity; this result is contrary 
to that reported by others. The experiments also show that the 
activity of such thyroid is much less than that of thyroid containing 
more than distinct traces (0.01 per cent, for example) of iodine. It 
is also evident that there are differences in the activity of different 
“iodine free” thyroids; those of the Alaskan bear and of children 
were more active than those of the Maltese kid or of the aoudad. 
This may be explained on either of two suppositions: it may be 
assumed that the activity of the so-called “iodine free” thyroid is 
in reality due to iodine which is present in too small amounts to be 
detected when only small samples are available for analysis and that 
there is more iodine in the thyroids of the children and of the Alaskan 
bear than in those of the Maltese kid and of the aoudad. Or we 
may suppose that the iodine free thyreoglobulin has of itself some 
activity and that the greater activity of some of the “iodine free” 
thyroids is due to their containing a larger amount of this thyreoglob- 
ublin than do other thyroids. This suggestion would also explain 
why certain “iodine free” thyroids (children and Alaskan bear) are 
more active than thyroid (black leopard) containing a minimum 
amount of iodine. The effect of a large amount of iodine free thy- 
reoglobulin may be greater than that of a small amount of- iodine 
containing thyreoglobulin. In connection with this suggestion atten- 
tion may be called to the work of Oswald,® in which it is shown that 
the thyreoglobulin varies greatly in amount in different thyroids. 
Evidence will be presented in later parts of this bulletin that the 
physiological activity of the thyroid increases in direct proportion to 
the amount of iodine contained in proper combination. Some of 
the bearings (upon the physiology and pathology of the thyroid) of 
this view that iodine free thyroid has a certain degree of activity 
and that this is much increased by the presence, in proper combi- 
nation, of iodine have been discussed by one of the present writers 
in a previous paper. * 6 Another application of this conception may 
be referred to, although it is not our purpose to discuss fully in this 
connection these collateral subjects. It was stated in the historical 
review (p. 13) that one of the arguments advanced against the view 
a A. Oswald, Beitr. z. chem. Physiol, u. Path., Bmschwg., 1902, 2, p. 545. 
6 R. Hunt, J. Am. M. Ass., Chicago, 1907, 49, p. 1325. 
61494— Bull. 47—09 4 
