The Endocrine System 229 



the fact that if the iodine is replaced by hydrogen, the resulting mate- 

 rial fails to function as the hormone. 



When sufficient iodine is not supplied to an animal, a hypo condi- 

 tion may soon result. This condition is more prevalent in areas 

 where the soil is deficient in iodine. Since the ultimate source of all 

 our iodine is from the sea, areas remote from the sea are likely to have 

 an iodine-deficient soil unless in former geologic periods the sea covered 

 the area. Through much of the Middle West, the sea did cover the 

 land at one time, but subsequent glaciation and leaching removed this 

 valuable element. The so-called "goiter-belt" thus extends through 

 much of the Middle West. Other areas where iodine deficiencies exist 

 are the mountainous regions of Europe, much of the mountainous 

 area of Central America, the Andean plateau, and many others. 



Since about 1922, the simple expedient of adding iodine to common 

 table salt has alleviated much of the thyroid difficulty. The inclusion 

 of sea foods and better eating habits have also been of great im- 

 portance. 



Much of our present knowledge of thyroid functioning has been 

 accomplished by means of radioactive iodine. It has been well known 

 for many years that all cells contain traces of iodine, but that the 

 thyroid gland has a much larger amount. When radioactive iodine is 

 injected into an animal such as a rabbit within ten to fifteen minutes' 

 time the thyroid has its share of the iodine. This tagged element acts 

 as a perfectly normal element in the body except that its radiations 

 make it possible to detect the passage of the material from one site 

 to another. The affinity of the thyroid tissue for the iodine has also 

 been useful in the treatment of cancer of the thyroid. Radioactive iodine 

 is, of course, quickly picked up and carried to the site where it can do 

 its work most effectively. 



The Effects of Hypoactivity. — Obviously, an inadequate secre- 

 tion of iodothyroglobulin by the cells of the follicles will result in a gen- 

 eral lowering of the metabolism of the organism. This is accompanied 

 by many secondary effects including: slower heart beat, lower blood 

 pressure, a slow rate of respiration, increased deposition of fat, dry 

 skin, and general lack of vigor and alertness. Depending upon the 

 age and general condition of health of the individual, the exact disease 

 resulting from this hypofunctioning varies. 



Any enlargement of the thyroid gland is usually called a goiter. 

 This response may be either due to increased demands of the body for 



