404 - Multicellular Animals, Especially Man 



WITH THYROID 



Fig. 22-5. Mexican axolotls. In nature this amphib- 

 ian, which congenially lacks a thyroid gland, fails to 

 undergo metamorphosis; that is, it retains its larval 

 form (above). Below, metamorphosis, induced by thy- 

 roid treatment. 



results as to the role of the thyroid in de- 

 velopment. If tadpoles are continually fed 

 with minced sheep thyroid, they develop 

 with astonishing rapidity. In fact, such a tad- 

 pole becomes a full-fledged frog long before 

 it has a chance to grow to a suitable size; 

 and in some experiments these miniature 

 adults were scarcely larger than housefhes. 



In a newborn child a seriously underac- 

 tive thyroid leads to cretinism. This condi- 

 tion is now quite rate, but formerly cretins 

 were often encountered in the Alps and cer- 

 tain other districts where the iodine content 

 of the soil — and consequently of the local 

 produce — is exceptionally low. Lacking io- 

 dine, the thyroid is unable to synthesize 

 thyroxin, and gradually the thyroid tissues 

 lose their normal structural characteristics. 

 Typically the cretinous child is very small 

 and malformed, with swollen tongue and 

 puffy skin; and physical and mental develop- 

 ment are scarcely perceptible even after years. 

 The disease gives a very dramatic response 

 to thyroxin, however, if the hormone is given 

 in the early stages. 



In adults the commonest forms of hypo- 

 thyroidism are myxedema and endemic goi- 

 ter. Both of these conditions may arise from 



inadequate iodine in the diet, although prob- 

 ably there are other contributing factors, 

 including hereditary susceptibility. 



In myxedema the thyroid deficiency is rela- 

 tively great, but there is no appreciable swell- 

 ing of the thyroid tissue — which, in fact, may 

 become degenerate. Usually the myxedema- 

 tous patient displays a basal metabolism 

 which is at least 25 percent below normal. 

 The subject complains of cold, and, indeed, 

 with the metabolism at so low an ebb, the 

 bod\' temperature may fall as much as 4 

 degrees. Extreme physical and mental leth- 

 argy are common, and sometimes there is a 

 tendency toward obesity. Also the skin be- 

 comes puffy and doughy, due to a deposit of 

 mucoid material in the deeper layers. In 

 some cases, myxedema responds to the addi- 

 tion of iodine to the diet; but more generally 

 it is necessary to give the thyroid hormone. 



In endemic goiter the deficiency is rela- 

 tively nrild, so that the fall in basal metab- 

 olism is usually not more than 15 to 20 per- 

 cent. A swelling of the thyroid tissue forms 

 the goiter, which usually protrudes from the 

 neck. The swelling is due to the accumula- 

 tion of a colloid fluid in the thyroid tissue, 

 and consequently this type of goiter is also 

 called colloid goiter. The colloid in such 

 cases, however, is very low in its content o£ 

 thyroxin. 



Typically endemic goiter results from a 

 deficiency of iodine, and the swelling may 

 represent an attempt by the gland to com- 

 pensate for the lack of iodine. Endemic goi- 

 ter tends to be localized in certain regions. 

 Detroit, for example, occupies a localitv 

 where the soil lacks iodine; and in 1924, 36 

 percent of the school children of this city 

 displayed endemic goiter — at least in the in- 

 cipient stages. Seven years later, however, 

 subsequent to the compulsory addition ol 

 small quantities of potassium iodide (KI) to 

 the table salt ol the region, the incidence of 

 goiter in the school population of Detroit 

 had dropped to less than 3 percent. 



A swelling of the thyroid does not invari- 

 ably indicate that the gland is underactive. 



