THE BIOCHEMISTKY OF THE THYROID GLAND 265 



Bergmann, Carlson and Jacobson, Yushchenko, 1911, Kendall, 1919), and 

 the theory of detoxication put forward by Blum (a) in 1898, and by Blum 

 and Griitzner in 1914, and other workers at various times, can no longer 

 be regarded as tenable. It is tempting, however, modifying Grobly's 

 idea, to link this with the metabolic activity of the secretion, and to con- 

 sider that under normal conditions iodin is gradually built up into the 

 iodothyroglobulin structure as it becomes available from the blood stream 

 (primarily, from the diet), the radical concerned being that of thyroxin, 

 derived from tryptophan, that iodothyroglobulin is the non-toxic storage 

 form of iodin, and that thyroxin is gradually liberated into the blood 

 stream by the breaking-up of iodothyroglobulin' within the gland, normally 

 at such a slow rate that no toxic effects are produced. 



It may be hoped that, at an early date, much fresh light will be shed 

 on the chemical problems of the thyroid. When we know the relationship 

 between iodothyroglobulin and thyroxin, the properties of thyroxin deriv- 

 atives containing other halogens, or with the halogen replaced by hydro- 

 gen, and the nature of the other organic iodin compounds which occur 

 amongst the cleavage products, it will be easier to propound theories which 

 will reasonably account for the chemical changes in the gland by which 

 the internal secretion is produced, and for the mechanism by which it pro- 

 duces its physiological and metabolic effects. 



