652 INTERNAL SECRETION ENDOCRINE GLANDS 



already saturated with iodine. The greatest quantitative absorption 

 takes place in those glands with the smallest iodine content, or, 

 what comes to the same thing, those glands with the most marked 

 hyperplasia. 



Thus, a dog's thyroid (in early hyperplasia) weighing 16 grammes, 

 and containing 0-54 milligramme of iodine per gramme of the dry 

 tissue was perfused for thirty minutes with a mixture of blood and 

 Ringer's solution to which was added 5 milligrammes of potassium 

 iodide. The gland was then washed out thoroughly with Ringer's 

 solution to remove remaining iodide. It was now found to contain 

 1-38 milligramme of iodine per gramme of dry tissue. Ten minutes 

 after the injection of 50 milligrammes of potassium iodide into the 

 femoral vein of a dog, from which one thyroid lobe had been removed 

 as a control, the remaining lobe was excised and all loose iodide re- 

 moved by thorough washing. The iodine in it was then determined and 

 found to be i -38 milligramme per gramme of the dry tissue, as com- 

 pared with o - 79 milligramme in the control lobe, an increase of 75 

 per cent. Only traces of iodine can be demonstrated in other organs 

 under such conditions, even when they have not been washed. Even 

 in five minutes, an absorption of iodine of practically the same magni- 

 tude has been shown to occur. So that the thyroid has the power of 

 taking up considerable amounts of iodine almost instantaneously. 

 While, however, the time required for the absorption of iodine by the 

 thyroid from the blood, and the binding of it in such a way that it 

 cannot be washed out, is so short as to be measured in minutes, the 

 time required for the elaboration of the iodine into the active iodine- 

 containing substance, which exhibits the specific action of the gland, 

 is much greater. In dogs, the earliest period at which an increase in 

 the amount of active iodine could be made out was eight hours after 

 the injection of an iodine salt into the circulation. 



Summary : The physiological significance of iodine in the thyroid 

 may be summed up as follows : Iodine is absolutely essential for the 

 normal activity of the gland. The amount in the gland at any given 

 time represents the store or reserve. Iodine prevents spontaneous hyper- 

 plasia (goitre) in all mammals, and also the compensatory hyperplasia 

 which follows partial removal of the thyroid. It exercises a curative 

 effect on active hyperplasias. It is present in an active and an in- 

 active form. The inactive form represents the newly acquired iodine, 

 which is usually a very small fraction of the total content, but may 

 undergo a great temporary increase after the administration of iodine. 

 Accordingly, the physiological and therapeutic activity of thyroid 

 substance appears in general to vary with the total amount of iodine 

 in it, although in reality with the amount which exists in the specific 

 compound. The nature of this compound is unknown. 



No other body has been shown to possess its specific activating effect 

 on metabolism. It is sometimes spoken of as the iodine-containing 

 hormone. Kendall has been able to separate it both from its protein 

 combination and from the inactive iodine by alkaline hydrolysis of the 

 thyroid, and to obtain it in a crystalline form containing about 60 per 

 cent., by weight, of iodine. 



