THYROIDS AND PARATHYROIDS 649 



When a small part of a thyroid is left, it may undergo great hyper- 

 trophy, and the same is true of the accessory thyroids. The ad- 

 ministration of extracts of the thyroid glands or the glands them- 

 selves by the mouth brings about a cure, permanent so long as the 

 thyroid treatment is continued, in cases of myxoedema in man, and 

 prevents the development of the symptoms in animals, or removes 

 them when they have appeared. The same is true of a compound 

 rich in iodine, the so-called thyroiodin, which has been extracted 

 from the organ. Under this treatment the total metabolism, which 

 in myxoedema is below the normal, is markedly increased. This is 

 partly due to an increase in the metabolism of protein. An increase 

 in the destruction of protein is also caused in normal persons and in 

 normal animals by feeding with thyroid or with thyroid prepara- 

 tions. The excretion of nitrogen, carbon dioxide, and phosphoric 

 acid, and the intake of oxygen, are augmented. But in spite of in- 

 creased appetite the body-weight falls off, and diarrhoea is often 

 caused. For these reasons the use of thyroid preparations to reduce 

 weight in cases of obesity, without evidence of thyroid insufficiency, 

 is a dangerous remedy. For while a fat man can very well spare a 

 great deal of his fat, he cannot spare much of his tissue-protein. 

 That the gland exerts in some way an important influence on the 

 metabolism of proteins is also indicated by other facts. The ques- 

 tion whether the thyroid or parathyroid is, in addition, concerned 

 in the carbo-hydrate metabolism is at present the subject of dis- 

 cussion, but the data are so contradictory that it would not be advis- 

 able to enter into the matter here. 



The ready response of the thyroid by hyperplasia or involution 

 to changes in the nutritive conditions is one of its most striking 

 characteristics, and further illustrates the significant role which it 

 plays in the chemical activities of the body. The thyroid swells 

 and shrinks almost as easily and under almost as great a variety 

 of conditions as the spleen. One of the most interesting of the 

 physiological changes is the hypertrophy and sometimes hyperplasia 

 of the gland, which is a normal accompaniment of menstruation 

 and pregnancy. This is a classical instance of a definite inter-rela- 

 tion of endocrine functions. Although known to the ancients in its 

 crudest manifestations, the underlying physiology of the condition 

 is as yet only slightly understood. A pathological change of great 

 interest, because of the careful manner in which it has been studied, 

 is the endemic goitre (sometimes erroneously termed ' carcinoma ') 

 of brook trout kept under artificial conditions in hatcheries. Marine 

 has shown that this depends upon overfeeding with unsuitable food 

 (such as livers of cattle, pigs, or sheep), overcrowding, and insuffi- 

 ciency of water-supply, and that the goitre can be readily cured or 

 prevented by changing the conditions in these respects. Similar 

 results have been obtained in mammals fed exclusively with meat. 



