230 The Vertebrate Organ Systems 



the hormone, excessive secretion of the gland resulting in a hyper con- 

 dition, as a response to excess amounts of proteins, fat, and calcium 

 in the diet, or to an insufficient iodine supply for the elaboration of the 

 hormone. This last type is known as simple goiter which, until the 

 addition of iodine to table salt, was extremely common through the 

 "goiter belt" of the United States. For some reason, simple goiter 

 occurs more frequently among women than among men. 



If hypoactivity of the thyroid gland occurs during adulthood the 

 disease myxedema may result. In this disease, the epithelium of the 

 follicle is degenerate in appearance and the cavities are filled with 

 a colloid material deficient in iodine. The typical symptoms of hypo- 

 activity occur in myxedema : decreased basal metabolism, slow pulse, 

 low blood pressure, mental deterioration, and obesity. The skin feels 

 very cold and dry, and the individual has a subjective feeling of 

 being cold. Usually relief of this condition is obtained by administer- 

 ing desiccated thyroid material. 



This same hypoactivity can occur in the very young. Then the 

 condition is known as cretinism. In these individuals, there is failure 

 to develop both mentally and physically. Dwarfism may even result. 

 In this disease, as in myxedema, the thyroid is atrophied. It, too, 

 can be treated by carefully supervised thyroid medication. 



The Effects of Hyperactivity. — At times, the thyroid gland be- 

 comes overactive and produces an excessive amount of the hormone. 

 Occasionally, the gland is actually enlarged, but more usually it re- 

 mains more or less the normal size. If enlarged, the name exoph- 

 thalmic goiter is used. 



Fundamentally the deleterious effects of this oversecretion are due 

 to the increased rate of metabolism. This secondarily results in in- 

 creased heart rate, increased blood pressure, increased rate of respira- 

 tion, general nervous instability, and at times protruding eyeballs. 

 The cause of this condition has been variously explained, and per- 

 haps is due to an inherent disposition in the individual, with the dis- 

 ease itself being precipitated by a nervous shock such as an operation 

 or severe emotional stress. The usual treatment is the surgical removal 

 of a portion of the gland. Benefits are derived from the reduced amount 

 of hormone which is then produced. 



Thyroid Activity in Other Animals. — The activity of the thyroid 

 in many different animals has been a subject of great interest. It has 

 long been known that the addition of thyroxin to water will hasten 



