MINERAL METABOLISM 381 



luiiii])s. Others susijcct that dietary factors impede the absorption of 

 it)dide by the intestine. Still other workers suspect an epidemic or 

 endemic infectious disease which impairs the normal fimction of the 

 thyroid. Finally another opinion suggests that the intestinal flora may 

 come to contain organisms trapping the dietary iodine or otherwise 

 making it unavailable. AH four schools of thought concur in the value 

 of iodine supplementation as a means of overcoming the iodine defi- 

 ciency developed in the presence of adequate iodine. Obviously the 

 prolDlem is a complex one. 



Animals, like man, show diminution of metabolism when deprived 

 of iodine, after removal of the thyroid gland, or by feeding of anti- 

 thyroid (goitrogenic) compoiuids. Any of these treatments impedes 

 the growth of young animals. Suspected retardation of mental develop- 

 ment in human beings has not been universally found by different in- 

 vestigators but is probable in severe cases. In young animals and birds 

 the reproductive systems develop slowly and perhaps incompletely. 



Severe cases in animals are characterized by impairment of the 

 central nervous system, for example, by reduced sensitivity to carbon 

 dioxide and decreased rates of respiration. Apathy and decreased re- 

 sponses to stimuli are characteristic. Lack of sufficient thyroid hor- 

 mone affects the endocrine system, leading to adrenals of small size. 

 Skin becomes leathery and pale and loses flexibility. Hair is sparse and 

 grows slowly. Pigs are born without hair. Feathers grow poorly, molt- 

 ing is inhibited, and species plumage patterns do not develop. 



These symptoms develop in animals only on diets relatively much 

 lower in iodine than the levels leading to human goiters, another jniz- 

 zling factor. Feeds and forages from areas with little iodine in the 

 soil are very effectively supplemented with iodized salt. 1 his proce- 

 dure is not invariably successfid ^vith human beings. 



Iron 



About 0.005 per cent of the animal body is iron, and perhaps two- 

 thirds occurs in hemoglobin. Although present in small quantity com- 

 pared to calcium, sodium, chloride, and others, iron is essential to the 

 cells of animals. In addition to this role in hemoglobin, iron is a 

 component of myoglobin, a protein of muscle that functions as an 

 oxygen carrier. Myoglobin has the same prosthetic group as hemo- 

 globin, but the protein moiety is different. Myoglobin has a greater 

 affinity for oxygen than does hemoglobin and is believed to form a 

 complex with oxygen that serves as a reservoir to meet oxygen de- 

 mands for bursts of activity. 



Iron is also a component of a number of enzymes, mostly of the 



