INTERNAL SECRETION 



of iodine associated with any degree of active hyperplasia (develop- 

 ing goitre) is always below the normal minimum, as shown by 



Marine in the dog, sheep, 

 man, and other mammals. 

 tAs active hyperplasia of the 

 Thyroid (goitre) (Fig. 203) 

 develops, the iodine content 

 of the gland, both relative 

 and absolute, decreases! until 

 in extreme degrees m. the 

 condition there may be no 

 demonstrable iodine present 

 at all. Since the iodine is 

 contained in the colloid as 

 an iodine-protein compound, 

 the generalization may be 

 made that in the thyroid the 



Fig. 204. Microphotograph of a Colloid 

 Gland (Goitre) (Marine). The effect of ad- 



iodine varies directly with 

 the amount of colloid, and 



ministration of iodine is shown in the return inversely with the degree of 

 towards the normal structure from a pre- hyperplasia. The admhlistra- 

 ceding active hyperplasia, such as is shown , * * . ,. , . . 



in Fig. 203. tion f an Y iodine-containing 



substance to animals with 



actively hyperplastic thyroids (goitres) 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 a reversion to 

 the normal histological struc- 

 ture (Fig. 205), so far as this 

 is possible in a gland which 

 has once undergone hyper- 

 plasia. |The physiological 

 influence of iodine on the 

 thyroid may be summed up 

 as follows : Iodine is abso- 

 lutely essential for the normal 

 activity of the gland. It 

 prevents spontaneous hyper- 

 plasia (goitre), and also the 

 compensatory hyperplasia 

 which follows partial re- 

 moval of the thyroid. It 

 exercises a curative effect 

 on active hyperplasias. The 

 physiological and therapeu- 

 tical activity of thyroid substance vaiies directly with the amouLt 

 of iodine in it in organic combination (thyroiodin)) 



