HORMONES 



299 



with certainty, but very likely involves addition of iodine to tyrosine 

 followed by self-condensation of the diiodotyrosine produced to form 

 thyroxine. Similar reactions, at any rate, take place quite readily out- 

 side the body. Thus, when proteins such as casein or egg albumin are 

 treated with iodine for several hours in the presence of a mild alkali 

 (sodium carbonate) , iodine becomes incorporated into the protein molecule 

 and thyroxine is formed. The amount of hormone formed is closely 

 correlated with the tyrosine content of the protein used; gelatin, which 

 does not contain tyrosine, fails to yield thyroxine under this treatment, 

 lodinated casein has been used extensively as a source of thyroid hormone, 

 for example, to feed dairy cows. Milk production is thereby increased, 

 but higher feed costs plus possible injury to the animals make the prac- 

 tice of doubtful economic value. 



Thyroid Disorders. Common goiter is an enlargement of the thyroid 

 resulting from low iodine intake. Where the iodine supply is not greatly 

 deficient, the enlarged gland is able to maintain normal function. The 

 only serious result is pressure on surrounding organs; for example, pres- 

 sure on the windpipe may be sufficient to cause difficulty in breathing. 

 The condition can usually be corrected by supplying adequate iodine (see 

 p. 192) , or in extreme cases by surgically removing a part of the gland. 



A more serious condition arises whenever the thyroid is damaged in any 

 way (for example, by an infection) so that it can no longer produce an 

 adequate supply of hormone. This may occur at any time from fetal 

 life onward. In adults the condition (called myxedema) is character- 

 ized by a general slowing down of metabolic activities, slow heart rate, 

 low blood pressure, easy fatigue, increased blood cholesterol levels, dimin- 

 ished urinary excretion of 17-kctosteroids, and decreased amounts of 

 "plasma-bound iodine." This plasma-bound iodine, which represents the 

 amount of circulating thyroid hormone, may drop from the normal range 

 of 4^6 to below 2 ;u,g. per 100 ml. of blood plasma. Replacement therapy 

 with thyroxine or whole thyroid substance (dried, ground animal thyroids) 

 is effective but must be continued throughout life. 



Thyroid deficiency in early life has all the above effects and, in addi- 

 tion, retards both mental and physical growth. The aff'ected individuals 

 often survive to adulthood, but are dwarfs and idiots. This condition is 

 called cretinism, and the sufferers from it, cretins. Treatment with 

 thyroid hormone is effective, but only if started before normal develop- 

 ment has been stunted. 



Overactivity of the thyroid (Grave's disease) is likewise most injurious 

 to health. Excessive production of the hormone leads to high basal 

 metabolism, excess energy production, restlessness, tremor of extremities, 

 excessive flushing and perspiration, loss of weight, low blood cholesterol, 

 high excretion of 17-ketosteroids, and frequently exopthalmos (protrud- 

 ing eyeballs). Psychic disturbances often accompany Grave's disease 

 and may be a factor in causing it. Treatment is designed, of course, to 



