380 ANIMAL BIOCHEMISTRY 



400 to 800 /i.g. daily. Therapeutic doses are very much higher, but 

 caution is indicated because an excess produces pathoh)gical changes 

 in the thyrf)id ghiud. 



In human beings a reduction in the rate of metabolism and enlarge- 

 ment of the thyroid gland are the most common symptoms of iodine 

 deficiency. A serious human disorder called cretinism is attributed 

 also to iodine deficiency, since it occurs with goiter and is much less 

 common with iodide supplements in diets. General retardation of 

 growth and mental development are typical, leading to dwarfed in- 

 dividuals with sharply limited mental capacity. Some response to ad- 

 ministration of iodine and dried thyroid tissue is often observed, but 

 recovery is never complete. Development of goiters is readily observed 

 and has been correlated all over the world with low iodine contents of 

 plants, soils, and water supplies. However, a few workers are coming 

 to believe that something more is involved than a simple lack of 

 dietary iodine. 



The problem of endemic goiters is complex because they occur also 

 in certain regions with iodine available in presumably adequate quan- 

 tity. Furthermore, the incidence of goiter rises and falls rapidly with- 

 out any known change in dietary iodine. For example, goiter was 

 unknown in the early days of the frontier town of Edmonton, Alberta. 

 Within one or two decades goiter became very prevalent and attracted 

 much attention. Near-by settlements were without goiter until some- 

 time later when it appeared in them and increased rapidly. The water 

 supply was probably not a factor, since the towns compared all ob- 

 tained their water from the Saskatchewan River. By about 1900, goiter 

 had almost completely disappeared from Edmonton and its environs, 

 but between 1910 and 1920 the disease again became widespread in 

 the city without known changes in the food and water supplies. 



During this last period goiter was very common in the Middle West 

 and Northwest, and large-scale studies showed that the disease could 

 be prevented in most individuals by supplements of iodide. Estab- 

 lished cases could be controlled and sometimes cured by iodine 

 therapy. As a result it is customary to add sodium iodide to salt 

 for human and animal consumption. Sodium chloride thus supple- 

 mented is described as iodized. Since iodide ion is slowly oxidized and 

 the iodine lost by evaporation, there is interest in using the more 

 stable sodium iodate. 



Although iodine deficiency can be experimentally produced, the 

 absolute requirement now appears to be much lower than was once 

 thought. Then how can goiters be explained? Some believe that nat- 

 ural thyroid antagonists occur in various plants like cabbage and 



