Intake Levels. Iodine is widely distributed in nature in extremely 

 small amounts. The daily requirement for man and animals is of the 

 order of 2 to 4 Mg/kg body weight. Some approximate levels of occur- 

 rence are as follows: sea water, 17 to 50 ^g/liter; soils, GOO to 8000 Mg/kg; 

 crop plants, 2 to 500 Mg/kg on a dry basis; meat, 30 Mg/kg; milk and eggs, 

 10 to 400 Mg/kg; sea fish, 400 to 900 Mg/kg. These values serve only to 

 indicate the order of magnitude, since considerable variations occur. 

 The concentration in the thyroid is about 10,000 times that found in any 

 other organ. The body of adult man contains roughly 20 mg iodine, 

 10 mg being in the thyroid gland and the other 10 mg in the rest of the 

 body. Blood iodine ranges from 8 to 16 /ig/100 ml. 



The weight of the thyroid gland approximates 20 to 60 g for man, sheep, 

 swine, horses, and cattle. Other species have values as follows: hen, 

 0.5 g; rats, 15 mg; chicks, 10 mg. The concentration of iodine in the 

 gland ranges from 0.4 to 3 mg/g. Of particular importance is the fact 

 that the thyroid under normal conditions has a mechanism that prevents 

 the addition to its existing stores of more than 0.1 to 0.2 mg iodine per 

 gram of thyroid weight (I-l). This means that the dosage should be 

 Hmited to this mass of iodine in order to yield physiological results. 

 Fortunately the specific activity of the radioisotope preparations allows 

 compliance with this restriction (see Chap. 2 for possible physiological 

 effects of Ji'^i). 



Radioassay. Radioiodine has been a most widely used isotope for the 

 following reasons: (a) importance of iodine in the animal economy, (6) 

 low concentrations of iodine in normal metabolism which have made 

 chemical studies difficult in contrast to the ease of radiochemical proce- 

 dures, (c) ideal radiation characteristics and short half-life of P^', (d) pos- 

 sibilities of studying thyroid function in the live human being or animal 



