MINERAL METABOLISM 



379 



ciKuhiLcd to ihc lh)r()icl i;l;tiul in the ihroai, wlicrc much of it is con- 

 centrated and stored. Probably every body cell contains a lidle iodine, 

 but 20 to 40 per cent ot the total occurs in the thyroid. Once in this 

 gland most of the iodide is oxidized to iodine, which initiates the 

 following reactions: 



lo 



CH2CHNH3+COO" 



CHsCHNH/cOO" 



tyrosine 



3-iodotyrosine 



CH2CHNH3+COO- 



3, 3', 5- triiodothyronine 



CHzCHNHg+COO" 



3,5-diodotyrosine 



Thyroglobulin 



protein 



^^-^CHoCHNHo+COO- 



13 

 alanine 



CHaCHNHg+COO" 



thyroxin 



Most of the iodine of the thyroid is stored as thyroglol^ulin, which 

 acts as a reservoir for tliyroxin and perhaps 3,3',5-triiodothyronine. 

 These last two compounds are hormones secreted by the thyroid gland 

 and distributed by the blood. Both are active (with 3,3',5-triiodothy- 

 ronine the more powerful) in controlling the general level of metabolic 

 activity by some unknown mechanism. A wide variety of enzymes has 

 been proposed as sites of the action, but there is little agreement be- 

 tween investigators, and it is rather unlikely that all the diverse the- 

 ories can be correct. Nevertheless, iodine deficiency, surgical removal 

 of the thyroid, and goiter cause a marked reduction in the basal rate 

 of metabolism. Iodine aiso occurs to a limited extent in a histidine 

 derivative for which tlie function is not known. 



Allowances are difficult to set and may vary with conditions. Esti- 

 mates for man range from 60 to 300 /a,g. daily, and values calculated for 

 chickens are 5 to 9, sheep 50 to 100, swine 80 to 160, and dairy cattle 



