THE BIOCHEMISTEY OF THE THYEOID GLAND 247 



iodin. Numerous observations are available, based on one or other of these 

 methods; utilizing some of these, and critically examining others, it has 

 been possible to draw fairly definite conclusions as to the general distri- 

 bution of iodin in living tissue (Cameron (&) (c) (d), 1914-1915). 



Iodin is an invariable constituent of marine Algae (limits, dried tis- 

 sue, 0.001 to 0.7 per cent). Individual plants of the same species show 

 considerable differences; in the more complex Algge (as Nereocystis) it is 

 possible to show differences in different parts of the same plant. Land 

 plants contain much less iodin, though it is widely distributed. They 

 exhibit the same variations, showing a differentiation not only determined 

 by the species, but apparently by the plant cell also. All marine animals 

 contain iodin. As advances in evolution occur there is more differentiation 

 and probably less total iodin in the whole organism. The horny skeleton 

 of sponges and corals is rich in iodin. Much of the material which has 

 been found to contain marked quantities of iodin is in the nature of a 

 secretion on to the outer surface of the body (e.g. annelid worm tubes, 

 ascidian tests). In vertebrates the thyroid alone is of importance as an 

 iodin containing tissue. Normal thyroid (dried tissue) contains at least 

 0.01 per cent. Nearly all other mammalian tissue contains much less 

 than 0.001 per cent. Kendall recently found that a commercial desiccated 

 preparation of beef pituitary contained 0.0025 per cent (quoted by Hos- 

 kins, 1920) ; Bourcet (1900) found for rabbit hair 0.0018 per cent, and 

 fresh skin 0.0006 per cent, concluding that this indicated the chief chan- 

 nel of iodin elimination. In kidney tissue of the dogfish 0.003 per cent 

 has been observed. 



In the lower organisms there is evidence of a wide comparable distri- 

 bution of chlorin, bromin, and iodin (e.g., see Morner (a), 1907), but the 

 observations are insufficient to permit conclusions to be drawn as to the 

 relative functions of these halogens. All four halogens are present in 

 thyroid tissue; but only chlorin and iodin in appreciable amounts. Until 

 recently reliable methods for the estimation of small quantities of bromin 

 and fluorin were wanting. 



Chlorin is present in all vertebrate tissue in an ionised condition. 

 There is no definite evidence that it exists in mammals in organic combi- 

 nation. Its importance is usually considered to consist in its availability, 

 as the least toxic anion, to assist in the establishment of a definite osmotic 

 pressure in intra- and extra-cellular tissue fluids, its non-toxicity being an 

 inherited immunity from our marine ancestors. Its presence in the thy- 

 roid is therefore not significant. 



Labat found bromin present in the majority of human thyroids, the 

 maximum observed being 0.003 per cent, fresh tissue (0.01; dry tissue). 

 Most of the other tissues examined showed no distinct trace, although 

 human urine invariably contained 2 to 3 mg. per liter. The results indi- 

 cated a slight selective action of thyroid tissue- in retaining bromin. The 



