CHEMICAL REGULATION OF METABOLISM 385 



The administration of an excess of thyroid tissue to animals or man 

 is accompanied by a very marked acceleration of metabolism. On a 

 normal mixed diet the total heat-output may be raised 100 per cent. 

 The effect of this enhanced metabolism is to cause a reduction of weight 

 due to loss of tissue, and especially of fat, and it is for this reason that 

 thyroid extract is the chief and only effective constituent of a variety 

 of Obesity-cures. Unfortunately, however, the nitrogenous output is 

 proportionately increased, so that the obese person loses not only fat, 

 but also tissue-protein, which he frequently can ill afford to spare. 

 Furthermore, distressing or even dangerous cardiac symptoms are 

 liable to supervene with overdoses of thyroid extract, or even with 

 moderate doses if the thyroid of the patient is normally active, so that 

 the unrestricted use of thyroid preparations by the public is attended 

 by serious danger. 



The stimulation of the destruction of nitrogenous tissue-constituents 

 which follows the administration of thyroid is extremely striking. 

 Thus, Rhode and Stockholm have found that in dogs receiving only 

 sugar as a diet, so that the nitrogenous output was minimal, the output 

 was increased fifty per cent, by so small an amount as 0.10 to 0.15 

 grams of dried thyroid tissue per kilogram body-weight of the 

 animals. Arguing chiefly from the fact that his crystalline active 

 fraction reacts with amino-acids, combining with the amino-group and 

 liberating carbonic acid, Kendall has advanced the view that the 

 thyroid secretion catalyzes the process of Deaminization of amino-acids. 

 The power of deaminizing amino-acids is known to be shared by all 

 the tissues and the stimulating effect of thyroid extract is likewise 

 common to all tissues. The question is an extremely difficult one to 

 decide, for when we recollect that the proteins of the tissues stand in a 

 relation of equilibrium to the reserve amino-acids which they contain, 

 and that these in turn are in equilibrium with the amino-acids circu- 

 lating in the blood it is evident that anything tending to break down 

 the amino-acids which have not yet become integral living tissue must 

 also indirectly lead to the breaking down of tissue-protein, and the 

 stimulation of endogenous catabolism. In support of Kendall's 

 theory, however, may be cited the facts that hyperthyroidism, as in 

 exophthalmic goiter, is aggravated by a high protein diet, and that 

 the effects of thyroidectomy are more serious in carnivorous than in 

 herbivorous animals. 



A remarkable effect of administration of thyroid tissue to mice is 

 the extraordinarily increased tolerance for Acetonitrile to which it 

 leads. Reid Hunt has found that if 0.1 milligrams of dried thyroid 

 tissue be administered to mice on ten successive days, they will with- 

 stand ten times the normal lethal dose of acetonitrile, administered 

 subcutaneously, and indeed he proposes this enhanced tolerance to a 

 specific substance as a test for the activity of various thyroid prepara- 

 tions. The significance of this effect is, however, uncertain because 

 it is not universal; in fact in such a closely allied animal as the rat, 

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