Nature of Autacoid Substances produced by Thyroid 41 



administration of thyroid. Moreover, it is undoubtedly the case that the 

 administration of adrenalin exercises a beneficial effect in some cases of 

 exophthalmic goitre : a fact which would be inexplicable on the theory 

 that the results of hyperthyroidism are simply due to excitation of the 

 suprarenals and the production of an excess of adrenalin. 



But although many of the effects of thyroid feeding and of hyperthy- 

 roidism otherwise produced can be explained by the presence of excitatory 

 or hormonic autacoids, it is not possible to explain the results produced by 

 thyroid deprivation, or hypothyroidism, upon the body generally, and certain 

 organs in particular, merely by assuming the absence of these excitatory 

 substances. For loss of the thyroid is followed by increased growth and 

 presumably increased activity of certain other organs This is the case 

 with the pituitary ; and although it has been suggested that this is an 

 instance of vicarious increase of size associated with the performance of 

 similar functions, the similarity of the functions of the two glands is far 

 from obvious. A more probable explanation is that besides a hormone 

 which excites some cells, the thyroid secretes an autacoid of chalonic nature 

 which restrains the activity of others : on removal of the thyroid this 

 restraint is abolished, and over-activity is the result. 



The thyroid autacoids affect the growth of the thymus gland, which 

 has been found to undergo an increase in size in foetal animals after 

 thyroid feeding of the pregnant mother (guinea-pigs) ; on the other hand, 

 the suprarenal capsules (cortex) and the thyroid itself are said to undergo 

 a diminution in weight under these circumstances. Hoskins, who furnishes 

 this observation, ascribes the effect upon the growth of the thymus to 

 stimulation by a hormone furnished by the thyroid ; if this is so, the 

 opposite results on suprarenal and thyroid should be ascribed to an autacoid 

 of opposite sign. To carry out this line of reasoning to its logical con- 

 clusion, the diminished growth of the skeleton and of the generative organs 

 which follows thyroid removal in young animals would be explained by 

 the absence of a hormone which promotes their development. We must, 

 however, be on our guard in assuming that every effect obtained is due to 

 a direct action upon the organ affected (see below). 



Interaction of the Thyroid with Other Organs 



With the Generative Organs. — The important influence exerted by the 

 thyroid secretion on nutrition generally is no doubt responsible for the fact 

 that hardly any organ in the body remains unaffected as a result of its 

 complete removal or atrophy. But the secreting organs with which it may 

 be regarded as being more specifically associated are few. Among them 

 are the generative glands. In the female the thyroid becomes enlarged 

 at puberty, during the menses, and during pregnancy. In the young 

 thyroidectomised subject the generative glands are only slowly or 'im- 

 perfectly developed, the resulting condition being characteristically one of 



