THE THYROID AND PARATHYROIDS 309 



element in the thyroid. It is now generally recognized that 

 the vertebrate thyroid invariably contains iodine. The per- 

 centage varies from O'Ol to 1*16 for dried glands. Other 

 mammalian tissues do not contain more than about O'OOl 

 per cent. 



There are now three reliable methods for the detection and 

 estimation of iodine : those of Bourcet, Hunter, and Kendall. 

 The two latter with 0*5 gramme of material will detect the 

 presence of O'OOl per cent, of iodine with accuracy. Utilizing 

 these methods, Cameron finds that iodine is an invariable 

 constituent of all marine Algae (O'OOl to 0*7 per cent, of dried 

 tissue). Land plants contain much less. All marine animals 

 contain iodine. In vertebrates this is found practically 

 entirely in the thyroid. Small amounts of the other halogens 

 are also found. 



The relative amount of thyroid tissue seems to increase as we 

 ascend the scale of evolution. But the iodine content does 

 not increase in a corresponding degree. It would appear that 

 in cattle both thyroid tissue and iodine content are greater in 

 the female than in the male. In regard to age, the maximum 

 iodine content in the human subject is found between 40 and 

 60 years. The lowest figures are obtained from subjects 

 under 15. According to Seidell and Fenger there is a seasonal 

 variation in the iodine content of sheep, pig, and ox thyroids. 

 Thus, the percentage between June and November is usually 

 from two to three times as great as that between December and 

 May. Diet appears to be insufficient to explain the seasonal 

 variations. They are probably due to metabolic changes due 

 to temperature. 



But the most important factor in the causation of variations 

 in the iodine content of the thyroid is undoubtedly diet. 

 The administration of iodine as iodides rapidly raises the 

 iodine content of the thyroid. From the time of Baumann 

 numerous observations have shown that the amount of iodine 

 in the thyroid depends on the amount of this element in the 

 normal diet. Thus the thyroids of the herbivora have more 

 iodine than those of the carnivora. Sheep fed largely on 

 seaweed show a large amount of iodine in the thyroid. In an 

 ordinary human diet the thyroid iodine is obtained from fish, 

 molluscs, milk, eggs, wine, etc. 



It is probable that the iodine of the thyroid is chiefly con- 



