THE ENDOCRINE SYSTEM 



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by the amount of heat given off, or calculated from the amount of 

 oxygen consumed, is decreased in thyroid deficiency and increased 

 when thyroid is administered or when the gland is overactive. Com- 

 plete removal of the thyroid glands from a mammal reduces its meta- 

 bolic rate to half of the normal vakie, and the body temperature 

 decreases slightly. Since foods are metabolized at a lower rate, they 

 tend to be stored and the animal becomes obese. Not only is the meta- 

 bolic rate of the intact animal decreased by thyroid deficiency, but 

 individual bits of tissue removed from the animal and incubated in 

 vitro show a decreased metabolic rate— decreased oxygen consumption 

 and decreased utilization of substrate molecules. The metabolism of 

 carbohydrates, fats, proteins, water and salts is affected, probably sec- 

 ondarily, by the amount of thyroid hormone present. 



Thyroid hormone, by its action on metabolic processes, has a 

 marked influence on growth and differentiation. Extirpating the thy- 

 roid of young animals causes decreased body growth, retarded mental 

 development, and delayed or decreased differentiation of gonads and 

 external genitalia. All of these changes are reversed by the administra- 

 tion of thyroxin. The metamorphosis of frog and salamander tadpoles 

 into adults is controlled by the thyroid. Removal of the larval thyroid 



Figure 30 3 The effect of thyroid feeding upon the tadpoles of Rana catesbtana. 

 A is the untreated control, which was killed at the end of the experiment. The 

 metamorphosed animal at the lower right (G) was killed two weeks after starting 

 the feeding of thyroid gland. The remaining animals {B to F) were removed from the 

 experiment at intervals during this period. Note the effect of thyroid substances on 

 the metamorphosis of the mouth, tail and paired appendages. (Turner: General 

 Endocrinology.) 



