792 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 



THE THYROID GLAND 



Condition of the Gland 



In the crowded communities of the Great Lakes Basin of this conti- 

 nent, it has been found that in most animals the thyroid gland is more or 

 less abnormal. In Cleveland, for example, Marine has found this to be 

 the case in well over 90 per cent of the dogs brought to the laboratory. 36 

 The condition usually goes under the name of simple goiter, which in- 

 cludes all thyroid enlargements except those of exophthalmic goiter. 

 In man the goiter originates usually about the age of adolescence and 

 more frequently in girls than in boys. It may sometimes pass over into 

 the exophthalmic type. The exact pathological changes in the goitrous 

 gland vary with the species of animal and with the duration of the dis- 

 ease. In man, besides the cystic or colloid goiter an adenomatous type 

 is very common although rare in other animals. 



From the numerous observations that have been made on the glands of 

 domestic animals, it has been clearly established that the very earliest 

 sign of goiter is a diminution in the iodine content of the gland; fol- 

 lowed by an increase in the epithelial cells and in the blood supply and a 

 decrease in the colloid. Such hyperplasia may be induced in what re- 

 mains after removal of a large part of a normal gland (compensatory 

 hyperplasia), or if a similar operation be performed early in pregnancy, 

 the young when born will be found to have hyperplastic thyroids. A 

 certain degree of hyperplasia exists as an accompaniment of pregnancy, 

 and it can be produced in certain normal animals (particularly rats) by 

 placing them on an excessive meat diet. Important observations bearing on 

 this point have been made by Marine 37 on brook trout, in which it has been 

 found that the so-called carcinoma that develops when the fish kept in 

 hatcheries are fed with unsuitable food and overcrowded, is really a 

 typical hyperplasia. In its second stage this develops into what is known 

 as colloid goiter which is produced by a deposition of colloid material 

 between the rows of cells so as to cause an opening out again of the 

 vesicles (Fig. 195), with a consequent tendency to a reversion to the 

 normal histological structure, so far as this is possible. The vesicles in 

 such a gland are of enormous size, and the lining epithelium, low cubical, 

 or almost flat in shape. 



The outstanding characteristic feature of the colloid material is that 

 it contains iodine, which exists in combination with a nonprotein nitrog- 

 enous base, and is usually called iodothyrin. In the gland itself the 

 iodothyrin may be in combination with protein, forming iodothyro- 

 globulin. (E.G. Kendall 38 has recently succeeded in isolating a pure crys- 

 talline substance of perfectly constant composition and containing over 60 



