358 THE VERTEBRATE BODY 



When there is a deficient secretion of thyroxin, hypothyroidism, there 

 will be a correspondingly low rate of metabolism. This causes a series 

 of symptoms that are about the reverse of those described for persons 

 with an overactive gland. There will be low heat production and the 

 person may feel chilly while those around him are comfortable or even 

 warm. Such persons may not have much of an appetite, yet, in some 

 cases, they may be overweight because much of the food that they eat is 

 not burned, but stored in the form of fat. They may exhibit a mental 

 sluggishness, lowered heart beat, and a decreased sex drive as a result 

 of the lowered rate of metabolism in the body organs involved. It is 

 quite simple to correct the abnormalities resulting from hypothyroidism. 

 Fortunately, thyroxin is highly resistant to the digestive juices and the 

 hormone may be taken by mouth. Powdered, dried thyroid glands of 

 slaughter house animals may be made into convenient oral tablets and 

 these will contain thyroxin to make up for the deficiency in the body. 



The symptoms just described apply to hypothyroidism which de- 

 velops after a person becomes an adult. If there is a deficiency from 

 birth the symptoms are much more serious because the decreased rate of 

 metabolism during the developmental period of life interferes with 

 growth and results in mental, physical, and sexual retardation. Persons 

 exhibiting this condition are known as cretins. A cretin thirty years old 

 may have intelligence no higher than a five-year-old child, he will be 

 dwarfed in stature, and probably sexually immature. If such persons 

 are given thyroid tablets as soon as the symptoms of cretinism appear 

 in childhood, they will be normal in every respect ; but if body growth 

 has been completed, improvement will be less marked. 



An enlargement of the thyroid gland causing it to swell out into the 

 neck sometimes occurs when the gland apparently tries to compensate 

 for abnormal function by increasing its size. This condition is known 

 as goiter. There seems to be a definite relation between the amount of 

 iodine in the diet and the incidence of goiter, for regions of the world 

 that are deficient in this mineral also have a high incidence of goiter. 

 Iodine is necessary for the synthesis of thyroxin, and undersecretion will 

 result if iodine is deficient. The use of iodized salt in such regions of 

 the United States has resulted in a decrease in the number of cases of 

 goiter. 



The parathyroid glands are four small endocrines, each about the size 

 of a pea, embedded in the thyroid glands. They secrete a hormone, para- 

 thormone, that regulates the calcium concentration of the blood. This 

 might not seem to be an important function until we learn that nerve and 

 muscle reactions require a certain concentration of calcium in their sur- 

 roundings for normal functioning. If we remove these glands from a 



