THYROIDS AND PARATHYROIDS 



651 



Since the iodine is contained in the colloid as an iodine-protein 

 compound, the generalization may be made that in the thyroid 

 the iodine varies directly 

 with the amount of colloid, 

 and inversely with the de- 

 gree of hyperplasia. The ad- 

 ministration of any iodine- 

 containing substance to 

 animals with actively hyper- 

 plastic thyroids (goitres) re- 

 sults in a rapid storage of 

 iodine in the gland, and 

 quickly (in two to three 

 weeks in dogs) induces a 

 histological change, the end 

 stage of which is the so-called 

 colloid goitre (Fig. 204) . 



This is an involution to the 



, , , , . , , Fig. 204. Microphotograph of a Colloid 



normal histological Structure land (Goi tre) (Marine). The effect of ad- 



(Fig. 205), SO far as this is ministration of iodine is shown in the return 



possible in a gland which towards the normal structure from a pre- 



, ceding active hyperplasia, such as is shown 



has once undergone hyper- in Fi g 203 

 plasia. This storage of 



iodine in the thyroid is of unusual interest because it affords the 

 best opportunity for the quantitative study of an essential inor- 

 ganic constituent of the body. Iron is another such substance, 



but it is impossible to follow 

 its absorption quantitatively 

 with the same ease and 

 accuracy, since it is so widely 

 distributed. Iodine is con- 

 tained in the thyroid, and to 

 all intents and purposes in 

 the thyroid alone. And the 

 thyroid is so situated ana- 

 tomically that it is a simple 

 matter to remove a portion 

 of it for examination. The. 

 extraordinary affinity of the 

 thyroid for iodine and its 

 salts has-been demonstrated 

 both in vivo and in vitro. 

 There is little difference 

 whether the iodine salt is 

 injected into the circulation of the living animal, or perfused 

 through the excised surviving gland, so long as the gland is not 



Fig. 205. Microphotograph of Normal 

 Human Thyroid (Marine). 



